map.rs - source
Description: Source of the Rust file `library/alloc/src/collections/btree/map.rs`.

#
alloc/collections/btree/
map.rs

    1use core::borrow::Borrow;
    2use core::cmp::Ordering;
    3use core::error::Error;
    4use core::fmt::{self, Debug};
    5use core::hash::{Hash, Hasher};
    6use core::iter::{FusedIterator, TrustedLen};
    7use core::marker::PhantomData;
    8use core::mem::{self, ManuallyDrop};
    9use core::ops::{Bound, Index, RangeBounds};
    10use core::ptr;
    11
    12use super::borrow::DormantMutRef;
    13use super::dedup_sorted_iter::DedupSortedIter;
    14use super::navigate::{LazyLeafRange, LeafRange};
    15use super::node::ForceResult::*;
    16use super::node::{self, Handle, NodeRef, Root, marker};
    17use super::search::SearchBound;
    18use super::search::SearchResult::*;
    19use super::set_val::SetValZST;
    20use crate::alloc::{Allocator, Global};
    21use crate::vec::Vec;
    22
    23mod entry;
    24
    25use Entry::*;
    26#[stable(feature = "rust1", since = "1.0.0")]
    27pub use entry::{Entry, OccupiedEntry, OccupiedError, VacantEntry};
    28
    29/// Minimum number of elements in a node that is not a root.
    30/// We might temporarily have fewer elements during methods.
    31pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
    32
    33// A tree in a `BTreeMap` is a tree in the `node` module with additional invariants:
    34// - Keys must appear in ascending order (according to the key's type).
    35// - Every non-leaf node contains at least 1 element (has at least 2 children).
    36// - Every non-root node contains at least MIN_LEN elements.
    37//
    38// An empty map is represented either by the absence of a root node or by a
    39// root node that is an empty leaf.
    40
    41/// An ordered map based on a [B-Tree].
    42///
    43/// Given a key type with a [total order], an ordered map stores its entries in key order.
    44/// That means that keys must be of a type that implements the [`Ord`] trait,
    45/// such that two keys can always be compared to determine their [`Ordering`].
    46/// Examples of keys with a total order are strings with lexicographical order,
    47/// and numbers with their natural order.
    48///
    49/// Iterators obtained from functions such as [`BTreeMap::iter`], [`BTreeMap::into_iter`], [`BTreeMap::values`], or
    50/// [`BTreeMap::keys`] produce their items in key order, and take worst-case logarithmic and
    51/// amortized constant time per item returned.
    52///
    53/// It is a logic error for a key to be modified in such a way that the key's ordering relative to
    54/// any other key, as determined by the [`Ord`] trait, changes while it is in the map. This is
    55/// normally only possible through [`Cell`], [`RefCell`], global state, I/O, or unsafe code.
    56/// The behavior resulting from such a logic error is not specified, but will be encapsulated to the
    57/// `BTreeMap` that observed the logic error and not result in undefined behavior. This could
    58/// include panics, incorrect results, aborts, memory leaks, and non-termination.
    59///
    60/// # Examples
    61///
    62/// ```
    63/// use std::collections::BTreeMap;
    64///
    65/// // type inference lets us omit an explicit type signature (which
    66/// // would be `BTreeMap<&str, &str>` in this example).
    67/// let mut movie_reviews = BTreeMap::new();
    68///
    69/// // review some movies.
    70/// movie_reviews.insert("Office Space",       "Deals with real issues in the workplace.");
    71/// movie_reviews.insert("Pulp Fiction",       "Masterpiece.");
    72/// movie_reviews.insert("The Godfather",      "Very enjoyable.");
    73/// movie_reviews.insert("The Blues Brothers", "Eye lyked it a lot.");
    74///
    75/// // check for a specific one.
    76/// if !movie_reviews.contains_key("Les Misérables") {
    77///     println!("We've got {} reviews, but Les Misérables ain't one.",
    78///              movie_reviews.len());
    79/// }
    80///
    81/// // oops, this review has a lot of spelling mistakes, let's delete it.
    82/// movie_reviews.remove("The Blues Brothers");
    83///
    84/// // look up the values associated with some keys.
    85/// let to_find = ["Up!", "Office Space"];
    86/// for movie in &to_find {
    87///     match movie_reviews.get(movie) {
    88///        Some(review) => println!("{movie}: {review}"),
    89///        None => println!("{movie} is unreviewed.")
    90///     }
    91/// }
    92///
    93/// // Look up the value for a key (will panic if the key is not found).
    94/// println!("Movie review: {}", movie_reviews["Office Space"]);
    95///
    96/// // iterate over everything.
    97/// for (movie, review) in &movie_reviews {
    98///     println!("{movie}: \"{review}\"");
    99/// }
    100/// ```
    101///
    102/// A `BTreeMap` with a known list of items can be initialized from an array:
    103///
    104/// ```
    105/// use std::collections::BTreeMap;
    106///
    107/// let solar_distance = BTreeMap::from([
    108///     ("Mercury", 0.4),
    109///     ("Venus", 0.7),
    110///     ("Earth", 1.0),
    111///     ("Mars", 1.5),
    112/// ]);
    113/// ```
    114///
    115/// ## `Entry` API
    116///
    117/// `BTreeMap` implements an [`Entry API`], which allows for complex
    118/// methods of getting, setting, updating and removing keys and their values:
    119///
    120/// [`Entry API`]: BTreeMap::entry
    121///
    122/// ```
    123/// use std::collections::BTreeMap;
    124///
    125/// // type inference lets us omit an explicit type signature (which
    126/// // would be `BTreeMap<&str, u8>` in this example).
    127/// let mut player_stats = BTreeMap::new();
    128///
    129/// fn random_stat_buff() -> u8 {
    130///     // could actually return some random value here - let's just return
    131///     // some fixed value for now
    132///     42
    133/// }
    134///
    135/// // insert a key only if it doesn't already exist
    136/// player_stats.entry("health").or_insert(100);
    137///
    138/// // insert a key using a function that provides a new value only if it
    139/// // doesn't already exist
    140/// player_stats.entry("defence").or_insert_with(random_stat_buff);
    141///
    142/// // update a key, guarding against the key possibly not being set
    143/// let stat = player_stats.entry("attack").or_insert(100);
    144/// *stat += random_stat_buff();
    145///
    146/// // modify an entry before an insert with in-place mutation
    147/// player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);
    148/// ```
    149///
    150/// # Background
    151///
    152/// A B-tree is (like) a [binary search tree], but adapted to the natural granularity that modern
    153/// machines like to consume data at. This means that each node contains an entire array of elements,
    154/// instead of just a single element.
    155///
    156/// B-Trees represent a fundamental compromise between cache-efficiency and actually minimizing
    157/// the amount of work performed in a search. In theory, a binary search tree (BST) is the optimal
    158/// choice for a sorted map, as a perfectly balanced BST performs the theoretical minimum number of
    159/// comparisons necessary to find an element (log<sub>2</sub>n). However, in practice the way this
    160/// is done is *very* inefficient for modern computer architectures. In particular, every element
    161/// is stored in its own individually heap-allocated node. This means that every single insertion
    162/// triggers a heap-allocation, and every comparison is a potential cache-miss due to the indirection.
    163/// Since both heap-allocations and cache-misses are notably expensive in practice, we are forced to,
    164/// at the very least, reconsider the BST strategy.
    165///
    166/// A B-Tree instead makes each node contain B-1 to 2B-1 elements in a contiguous array. By doing
    167/// this, we reduce the number of allocations by a factor of B, and improve cache efficiency in
    168/// searches. However, this does mean that searches will have to do *more* comparisons on average.
    169/// The precise number of comparisons depends on the node search strategy used. For optimal cache
    170/// efficiency, one could search the nodes linearly. For optimal comparisons, one could search
    171/// the node using binary search. As a compromise, one could also perform a linear search
    172/// that initially only checks every i<sup>th</sup> element for some choice of i.
    173///
    174/// Currently, our implementation simply performs naive linear search. This provides excellent
    175/// performance on *small* nodes of elements which are cheap to compare. However in the future we
    176/// would like to further explore choosing the optimal search strategy based on the choice of B,
    177/// and possibly other factors. Using linear search, searching for a random element is expected
    178/// to take B * log(n) comparisons, which is generally worse than a BST. In practice,
    179/// however, performance is excellent.
    180///
    181/// [B-Tree]: https://en.wikipedia.org/wiki/B-tree
    182/// [binary search tree]: https://en.wikipedia.org/wiki/Binary_search_tree
    183/// [total order]: https://en.wikipedia.org/wiki/Total_order
    184/// [`Cell`]: core::cell::Cell
    185/// [`RefCell`]: core::cell::RefCell
    186#[stable(feature = "rust1", since = "1.0.0")]
    187#[cfg_attr(not(test), rustc_diagnostic_item = "BTreeMap")]
    188#[rustc_insignificant_dtor]
    189pub struct BTreeMap<
    190    K,
    191    V,
    192    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
    193> {
    194    root: Option<Root<K, V>>,
    195    length: usize,
    196    /// `ManuallyDrop` to control drop order (needs to be dropped after all the nodes).
    197    // Although some of the accessory types store a copy of the allocator, the nodes do not.
    198    // Because allocations will remain live as long as any copy (like this one) of the allocator
    199    // is live, it's unnecessary to store the allocator in each node.
    200    pub(super) alloc: ManuallyDrop<A>,
    201    // For dropck; the `Box` avoids making the `Unpin` impl more strict than before
    202    _marker: PhantomData<crate::boxed::Box<(K, V), A>>,
    203}
    204
    205#[stable(feature = "btree_drop", since = "1.7.0")]
    206unsafe impl<#[may_dangle] K, #[may_dangle] V, A: Allocator + Clone> Drop for BTreeMap<K, V, A> {
    207    fn drop(&mut self) {
    208        drop(unsafe { ptr::read(self) }.into_iter())
    209    }
    210}
    211
    212// FIXME: This implementation is "wrong", but changing it would be a breaking change.
    213// (The bounds of the automatic `UnwindSafe` implementation have been like this since Rust 1.50.)
    214// Maybe we can fix it nonetheless with a crater run, or if the `UnwindSafe`
    215// traits are deprecated, or disarmed (no longer causing hard errors) in the future.
    216#[stable(feature = "btree_unwindsafe", since = "1.64.0")]
    217impl<K, V, A: Allocator + Clone> core::panic::UnwindSafe for BTreeMap<K, V, A>
    218where
    219    A: core::panic::UnwindSafe,
    220    K: core::panic::RefUnwindSafe,
    221    V: core::panic::RefUnwindSafe,
    222{
    223}
    224
    225#[stable(feature = "rust1", since = "1.0.0")]
    226impl<K: Clone, V: Clone, A: Allocator + Clone> Clone for BTreeMap<K, V, A> {
    227    fn clone(&self) -> BTreeMap<K, V, A> {
    228        fn clone_subtree<'a, K: Clone, V: Clone, A: Allocator + Clone>(
    229            node: NodeRef<marker::Immut<'a>, K, V, marker::LeafOrInternal>,
    230            alloc: A,
    231        ) -> BTreeMap<K, V, A>
    232        where
    233            K: 'a,
    234            V: 'a,
    235        {
    236            match node.force() {
    237                Leaf(leaf) => {
    238                    let mut out_tree = BTreeMap {
    239                        root: Some(Root::new(alloc.clone())),
    240                        length: 0,
    241                        alloc: ManuallyDrop::new(alloc),
    242                        _marker: PhantomData,
    243                    };
    244
    245                    {
    246                        let root = out_tree.root.as_mut().unwrap(); // unwrap succeeds because we just wrapped
    247                        let mut out_node = match root.borrow_mut().force() {
    248                            Leaf(leaf) => leaf,
    249                            Internal(_) => unreachable!(),
    250                        };
    251
    252                        let mut in_edge = leaf.first_edge();
    253                        while let Ok(kv) = in_edge.right_kv() {
    254                            let (k, v) = kv.into_kv();
    255                            in_edge = kv.right_edge();
    256
    257                            out_node.push(k.clone(), v.clone());
    258                            out_tree.length += 1;
    259                        }
    260                    }
    261
    262                    out_tree
    263                }
    264                Internal(internal) => {
    265                    let mut out_tree =
    266                        clone_subtree(internal.first_edge().descend(), alloc.clone());
    267
    268                    {
    269                        let out_root = out_tree.root.as_mut().unwrap();
    270                        let mut out_node = out_root.push_internal_level(alloc.clone());
    271                        let mut in_edge = internal.first_edge();
    272                        while let Ok(kv) = in_edge.right_kv() {
    273                            let (k, v) = kv.into_kv();
    274                            in_edge = kv.right_edge();
    275
    276                            let k = (*k).clone();
    277                            let v = (*v).clone();
    278                            let subtree = clone_subtree(in_edge.descend(), alloc.clone());
    279
    280                            // We can't destructure subtree directly
    281                            // because BTreeMap implements Drop
    282                            let (subroot, sublength) = unsafe {
    283                                let subtree = ManuallyDrop::new(subtree);
    284                                let root = ptr::read(&subtree.root);
    285                                let length = subtree.length;
    286                                (root, length)
    287                            };
    288
    289                            out_node.push(
    290                                k,
    291                                v,
    292                                subroot.unwrap_or_else(|| Root::new(alloc.clone())),
    293                            );
    294                            out_tree.length += 1 + sublength;
    295                        }
    296                    }
    297
    298                    out_tree
    299                }
    300            }
    301        }
    302
    303        if self.is_empty() {
    304            BTreeMap::new_in((*self.alloc).clone())
    305        } else {
    306            clone_subtree(self.root.as_ref().unwrap().reborrow(), (*self.alloc).clone()) // unwrap succeeds because not empty
    307        }
    308    }
    309}
    310
    311// Internal functionality for `BTreeSet`.
    312impl<K, A: Allocator + Clone> BTreeMap<K, SetValZST, A> {
    313    pub(super) fn replace(&mut self, key: K) -> Option<K>
    314    where
    315        K: Ord,
    316    {
    317        let (map, dormant_map) = DormantMutRef::new(self);
    318        let root_node =
    319            map.root.get_or_insert_with(|| Root::new((*map.alloc).clone())).borrow_mut();
    320        match root_node.search_tree::<K>(&key) {
    321            Found(mut kv) => Some(mem::replace(kv.key_mut(), key)),
    322            GoDown(handle) => {
    323                VacantEntry {
    324                    key,
    325                    handle: Some(handle),
    326                    dormant_map,
    327                    alloc: (*map.alloc).clone(),
    328                    _marker: PhantomData,
    329                }
    330                .insert(SetValZST);
    331                None
    332            }
    333        }
    334    }
    335
    336    pub(super) fn get_or_insert_with<Q: ?Sized, F>(&mut self, q: &Q, f: F) -> &K
    337    where
    338        K: Borrow<Q> + Ord,
    339        Q: Ord,
    340        F: FnOnce(&Q) -> K,
    341    {
    342        let (map, dormant_map) = DormantMutRef::new(self);
    343        let root_node =
    344            map.root.get_or_insert_with(|| Root::new((*map.alloc).clone())).borrow_mut();
    345        match root_node.search_tree(q) {
    346            Found(handle) => handle.into_kv_mut().0,
    347            GoDown(handle) => {
    348                let key = f(q);
    349                assert!(*key.borrow() == *q, "new value is not equal");
    350                VacantEntry {
    351                    key,
    352                    handle: Some(handle),
    353                    dormant_map,
    354                    alloc: (*map.alloc).clone(),
    355                    _marker: PhantomData,
    356                }
    357                .insert_entry(SetValZST)
    358                .into_key()
    359            }
    360        }
    361    }
    362}
    363
    364/// An iterator over the entries of a `BTreeMap`.
    365///
    366/// This `struct` is created by the [`iter`] method on [`BTreeMap`]. See its
    367/// documentation for more.
    368///
    369/// [`iter`]: BTreeMap::iter
    370#[must_use = "iterators are lazy and do nothing unless consumed"]
    371#[stable(feature = "rust1", since = "1.0.0")]
    372pub struct Iter<'a, K: 'a, V: 'a> {
    373    range: LazyLeafRange<marker::Immut<'a>, K, V>,
    374    length: usize,
    375}
    376
    377#[stable(feature = "collection_debug", since = "1.17.0")]
    378impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Iter<'_, K, V> {
    379    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    380        f.debug_list().entries(self.clone()).finish()
    381    }
    382}
    383
    384#[stable(feature = "default_iters", since = "1.70.0")]
    385impl<'a, K: 'a, V: 'a> Default for Iter<'a, K, V> {
    386    /// Creates an empty `btree_map::Iter`.
    387    ///
    388    /// ```
    389    /// # use std::collections::btree_map;
    390    /// let iter: btree_map::Iter<'_, u8, u8> = Default::default();
    391    /// assert_eq!(iter.len(), 0);
    392    /// ```
    393    fn default() -> Self {
    394        Iter { range: Default::default(), length: 0 }
    395    }
    396}
    397
    398/// A mutable iterator over the entries of a `BTreeMap`.
    399///
    400/// This `struct` is created by the [`iter_mut`] method on [`BTreeMap`]. See its
    401/// documentation for more.
    402///
    403/// [`iter_mut`]: BTreeMap::iter_mut
    404#[must_use = "iterators are lazy and do nothing unless consumed"]
    405#[stable(feature = "rust1", since = "1.0.0")]
    406pub struct IterMut<'a, K: 'a, V: 'a> {
    407    range: LazyLeafRange<marker::ValMut<'a>, K, V>,
    408    length: usize,
    409
    410    // Be invariant in `K` and `V`
    411    _marker: PhantomData<&'a mut (K, V)>,
    412}
    413
    414#[stable(feature = "collection_debug", since = "1.17.0")]
    415impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
    416    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    417        let range = Iter { range: self.range.reborrow(), length: self.length };
    418        f.debug_list().entries(range).finish()
    419    }
    420}
    421
    422#[stable(feature = "default_iters", since = "1.70.0")]
    423impl<'a, K: 'a, V: 'a> Default for IterMut<'a, K, V> {
    424    /// Creates an empty `btree_map::IterMut`.
    425    ///
    426    /// ```
    427    /// # use std::collections::btree_map;
    428    /// let iter: btree_map::IterMut<'_, u8, u8> = Default::default();
    429    /// assert_eq!(iter.len(), 0);
    430    /// ```
    431    fn default() -> Self {
    432        IterMut { range: Default::default(), length: 0, _marker: PhantomData {} }
    433    }
    434}
    435
    436/// An owning iterator over the entries of a `BTreeMap`, sorted by key.
    437///
    438/// This `struct` is created by the [`into_iter`] method on [`BTreeMap`]
    439/// (provided by the [`IntoIterator`] trait). See its documentation for more.
    440///
    441/// [`into_iter`]: IntoIterator::into_iter
    442#[stable(feature = "rust1", since = "1.0.0")]
    443#[rustc_insignificant_dtor]
    444pub struct IntoIter<
    445    K,
    446    V,
    447    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
    448> {
    449    range: LazyLeafRange<marker::Dying, K, V>,
    450    length: usize,
    451    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
    452    alloc: A,
    453}
    454
    455impl<K, V, A: Allocator + Clone> IntoIter<K, V, A> {
    456    /// Returns an iterator of references over the remaining items.
    457    #[inline]
    458    pub(super) fn iter(&self) -> Iter<'_, K, V> {
    459        Iter { range: self.range.reborrow(), length: self.length }
    460    }
    461}
    462
    463#[stable(feature = "collection_debug", since = "1.17.0")]
    464impl<K: Debug, V: Debug, A: Allocator + Clone> Debug for IntoIter<K, V, A> {
    465    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    466        f.debug_list().entries(self.iter()).finish()
    467    }
    468}
    469
    470#[stable(feature = "default_iters", since = "1.70.0")]
    471impl<K, V, A> Default for IntoIter<K, V, A>
    472where
    473    A: Allocator + Default + Clone,
    474{
    475    /// Creates an empty `btree_map::IntoIter`.
    476    ///
    477    /// ```
    478    /// # use std::collections::btree_map;
    479    /// let iter: btree_map::IntoIter<u8, u8> = Default::default();
    480    /// assert_eq!(iter.len(), 0);
    481    /// ```
    482    fn default() -> Self {
    483        IntoIter { range: Default::default(), length: 0, alloc: Default::default() }
    484    }
    485}
    486
    487/// An iterator over the keys of a `BTreeMap`.
    488///
    489/// This `struct` is created by the [`keys`] method on [`BTreeMap`]. See its
    490/// documentation for more.
    491///
    492/// [`keys`]: BTreeMap::keys
    493#[must_use = "iterators are lazy and do nothing unless consumed"]
    494#[stable(feature = "rust1", since = "1.0.0")]
    495pub struct Keys<'a, K, V> {
    496    inner: Iter<'a, K, V>,
    497}
    498
    499#[stable(feature = "collection_debug", since = "1.17.0")]
    500impl<K: fmt::Debug, V> fmt::Debug for Keys<'_, K, V> {
    501    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    502        f.debug_list().entries(self.clone()).finish()
    503    }
    504}
    505
    506/// An iterator over the values of a `BTreeMap`.
    507///
    508/// This `struct` is created by the [`values`] method on [`BTreeMap`]. See its
    509/// documentation for more.
    510///
    511/// [`values`]: BTreeMap::values
    512#[must_use = "iterators are lazy and do nothing unless consumed"]
    513#[stable(feature = "rust1", since = "1.0.0")]
    514pub struct Values<'a, K, V> {
    515    inner: Iter<'a, K, V>,
    516}
    517
    518#[stable(feature = "collection_debug", since = "1.17.0")]
    519impl<K, V: fmt::Debug> fmt::Debug for Values<'_, K, V> {
    520    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    521        f.debug_list().entries(self.clone()).finish()
    522    }
    523}
    524
    525/// A mutable iterator over the values of a `BTreeMap`.
    526///
    527/// This `struct` is created by the [`values_mut`] method on [`BTreeMap`]. See its
    528/// documentation for more.
    529///
    530/// [`values_mut`]: BTreeMap::values_mut
    531#[must_use = "iterators are lazy and do nothing unless consumed"]
    532#[stable(feature = "map_values_mut", since = "1.10.0")]
    533pub struct ValuesMut<'a, K, V> {
    534    inner: IterMut<'a, K, V>,
    535}
    536
    537#[stable(feature = "map_values_mut", since = "1.10.0")]
    538impl<K, V: fmt::Debug> fmt::Debug for ValuesMut<'_, K, V> {
    539    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    540        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
    541    }
    542}
    543
    544/// An owning iterator over the keys of a `BTreeMap`.
    545///
    546/// This `struct` is created by the [`into_keys`] method on [`BTreeMap`].
    547/// See its documentation for more.
    548///
    549/// [`into_keys`]: BTreeMap::into_keys
    550#[must_use = "iterators are lazy and do nothing unless consumed"]
    551#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    552pub struct IntoKeys<
    553    K,
    554    V,
    555    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
    556> {
    557    inner: IntoIter<K, V, A>,
    558}
    559
    560#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    561impl<K: fmt::Debug, V, A: Allocator + Clone> fmt::Debug for IntoKeys<K, V, A> {
    562    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    563        f.debug_list().entries(self.inner.iter().map(|(key, _)| key)).finish()
    564    }
    565}
    566
    567/// An owning iterator over the values of a `BTreeMap`.
    568///
    569/// This `struct` is created by the [`into_values`] method on [`BTreeMap`].
    570/// See its documentation for more.
    571///
    572/// [`into_values`]: BTreeMap::into_values
    573#[must_use = "iterators are lazy and do nothing unless consumed"]
    574#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    575pub struct IntoValues<
    576    K,
    577    V,
    578    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
    579> {
    580    inner: IntoIter<K, V, A>,
    581}
    582
    583#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    584impl<K, V: fmt::Debug, A: Allocator + Clone> fmt::Debug for IntoValues<K, V, A> {
    585    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    586        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
    587    }
    588}
    589
    590/// An iterator over a sub-range of entries in a `BTreeMap`.
    591///
    592/// This `struct` is created by the [`range`] method on [`BTreeMap`]. See its
    593/// documentation for more.
    594///
    595/// [`range`]: BTreeMap::range
    596#[must_use = "iterators are lazy and do nothing unless consumed"]
    597#[stable(feature = "btree_range", since = "1.17.0")]
    598pub struct Range<'a, K: 'a, V: 'a> {
    599    inner: LeafRange<marker::Immut<'a>, K, V>,
    600}
    601
    602#[stable(feature = "collection_debug", since = "1.17.0")]
    603impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Range<'_, K, V> {
    604    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    605        f.debug_list().entries(self.clone()).finish()
    606    }
    607}
    608
    609/// A mutable iterator over a sub-range of entries in a `BTreeMap`.
    610///
    611/// This `struct` is created by the [`range_mut`] method on [`BTreeMap`]. See its
    612/// documentation for more.
    613///
    614/// [`range_mut`]: BTreeMap::range_mut
    615#[must_use = "iterators are lazy and do nothing unless consumed"]
    616#[stable(feature = "btree_range", since = "1.17.0")]
    617pub struct RangeMut<'a, K: 'a, V: 'a> {
    618    inner: LeafRange<marker::ValMut<'a>, K, V>,
    619
    620    // Be invariant in `K` and `V`
    621    _marker: PhantomData<&'a mut (K, V)>,
    622}
    623
    624#[stable(feature = "collection_debug", since = "1.17.0")]
    625impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for RangeMut<'_, K, V> {
    626    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    627        let range = Range { inner: self.inner.reborrow() };
    628        f.debug_list().entries(range).finish()
    629    }
    630}
    631
    632impl<K, V> BTreeMap<K, V> {
    633    /// Makes a new, empty `BTreeMap`.
    634    ///
    635    /// Does not allocate anything on its own.
    636    ///
    637    /// # Examples
    638    ///
    639    /// ```
    640    /// use std::collections::BTreeMap;
    641    ///
    642    /// let mut map = BTreeMap::new();
    643    ///
    644    /// // entries can now be inserted into the empty map
    645    /// map.insert(1, "a");
    646    /// ```
    647    #[stable(feature = "rust1", since = "1.0.0")]
    648    #[rustc_const_stable(feature = "const_btree_new", since = "1.66.0")]
    649    #[inline]
    650    #[must_use]
    651    pub const fn new() -> BTreeMap<K, V> {
    652        BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(Global), _marker: PhantomData }
    653    }
    654}
    655
    656impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
    657    /// Clears the map, removing all elements.
    658    ///
    659    /// # Examples
    660    ///
    661    /// ```
    662    /// use std::collections::BTreeMap;
    663    ///
    664    /// let mut a = BTreeMap::new();
    665    /// a.insert(1, "a");
    666    /// a.clear();
    667    /// assert!(a.is_empty());
    668    /// ```
    669    #[stable(feature = "rust1", since = "1.0.0")]
    670    pub fn clear(&mut self) {
    671        // avoid moving the allocator
    672        drop(BTreeMap {
    673            root: mem::replace(&mut self.root, None),
    674            length: mem::replace(&mut self.length, 0),
    675            alloc: self.alloc.clone(),
    676            _marker: PhantomData,
    677        });
    678    }
    679
    680    /// Makes a new empty BTreeMap with a reasonable choice for B.
    681    ///
    682    /// # Examples
    683    ///
    684    /// ```
    685    /// # #![feature(allocator_api)]
    686    /// # #![feature(btreemap_alloc)]
    687    /// use std::collections::BTreeMap;
    688    /// use std::alloc::Global;
    689    ///
    690    /// let mut map = BTreeMap::new_in(Global);
    691    ///
    692    /// // entries can now be inserted into the empty map
    693    /// map.insert(1, "a");
    694    /// ```
    695    #[unstable(feature = "btreemap_alloc", issue = "32838")]
    696    pub const fn new_in(alloc: A) -> BTreeMap<K, V, A> {
    697        BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
    698    }
    699}
    700
    701impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
    702    /// Returns a reference to the value corresponding to the key.
    703    ///
    704    /// The key may be any borrowed form of the map's key type, but the ordering
    705    /// on the borrowed form *must* match the ordering on the key type.
    706    ///
    707    /// # Examples
    708    ///
    709    /// ```
    710    /// use std::collections::BTreeMap;
    711    ///
    712    /// let mut map = BTreeMap::new();
    713    /// map.insert(1, "a");
    714    /// assert_eq!(map.get(&1), Some(&"a"));
    715    /// assert_eq!(map.get(&2), None);
    716    /// ```
    717    #[stable(feature = "rust1", since = "1.0.0")]
    718    pub fn get<Q: ?Sized>(&self, key: &Q) -> Option<&V>
    719    where
    720        K: Borrow<Q> + Ord,
    721        Q: Ord,
    722    {
    723        let root_node = self.root.as_ref()?.reborrow();
    724        match root_node.search_tree(key) {
    725            Found(handle) => Some(handle.into_kv().1),
    726            GoDown(_) => None,
    727        }
    728    }
    729
    730    /// Returns the key-value pair corresponding to the supplied key. This is
    731    /// potentially useful:
    732    /// - for key types where non-identical keys can be considered equal;
    733    /// - for getting the `&K` stored key value from a borrowed `&Q` lookup key; or
    734    /// - for getting a reference to a key with the same lifetime as the collection.
    735    ///
    736    /// The supplied key may be any borrowed form of the map's key type, but the ordering
    737    /// on the borrowed form *must* match the ordering on the key type.
    738    ///
    739    /// # Examples
    740    ///
    741    /// ```
    742    /// use std::cmp::Ordering;
    743    /// use std::collections::BTreeMap;
    744    ///
    745    /// #[derive(Clone, Copy, Debug)]
    746    /// struct S {
    747    ///     id: u32,
    748    /// #   #[allow(unused)] // prevents a "field `name` is never read" error
    749    ///     name: &'static str, // ignored by equality and ordering operations
    750    /// }
    751    ///
    752    /// impl PartialEq for S {
    753    ///     fn eq(&self, other: &S) -> bool {
    754    ///         self.id == other.id
    755    ///     }
    756    /// }
    757    ///
    758    /// impl Eq for S {}
    759    ///
    760    /// impl PartialOrd for S {
    761    ///     fn partial_cmp(&self, other: &S) -> Option<Ordering> {
    762    ///         self.id.partial_cmp(&other.id)
    763    ///     }
    764    /// }
    765    ///
    766    /// impl Ord for S {
    767    ///     fn cmp(&self, other: &S) -> Ordering {
    768    ///         self.id.cmp(&other.id)
    769    ///     }
    770    /// }
    771    ///
    772    /// let j_a = S { id: 1, name: "Jessica" };
    773    /// let j_b = S { id: 1, name: "Jess" };
    774    /// let p = S { id: 2, name: "Paul" };
    775    /// assert_eq!(j_a, j_b);
    776    ///
    777    /// let mut map = BTreeMap::new();
    778    /// map.insert(j_a, "Paris");
    779    /// assert_eq!(map.get_key_value(&j_a), Some((&j_a, &"Paris")));
    780    /// assert_eq!(map.get_key_value(&j_b), Some((&j_a, &"Paris"))); // the notable case
    781    /// assert_eq!(map.get_key_value(&p), None);
    782    /// ```
    783    #[stable(feature = "map_get_key_value", since = "1.40.0")]
    784    pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
    785    where
    786        K: Borrow<Q> + Ord,
    787        Q: Ord,
    788    {
    789        let root_node = self.root.as_ref()?.reborrow();
    790        match root_node.search_tree(k) {
    791            Found(handle) => Some(handle.into_kv()),
    792            GoDown(_) => None,
    793        }
    794    }
    795
    796    /// Returns the first key-value pair in the map.
    797    /// The key in this pair is the minimum key in the map.
    798    ///
    799    /// # Examples
    800    ///
    801    /// ```
    802    /// use std::collections::BTreeMap;
    803    ///
    804    /// let mut map = BTreeMap::new();
    805    /// assert_eq!(map.first_key_value(), None);
    806    /// map.insert(1, "b");
    807    /// map.insert(2, "a");
    808    /// assert_eq!(map.first_key_value(), Some((&1, &"b")));
    809    /// ```
    810    #[stable(feature = "map_first_last", since = "1.66.0")]
    811    pub fn first_key_value(&self) -> Option<(&K, &V)>
    812    where
    813        K: Ord,
    814    {
    815        let root_node = self.root.as_ref()?.reborrow();
    816        root_node.first_leaf_edge().right_kv().ok().map(Handle::into_kv)
    817    }
    818
    819    /// Returns the first entry in the map for in-place manipulation.
    820    /// The key of this entry is the minimum key in the map.
    821    ///
    822    /// # Examples
    823    ///
    824    /// ```
    825    /// use std::collections::BTreeMap;
    826    ///
    827    /// let mut map = BTreeMap::new();
    828    /// map.insert(1, "a");
    829    /// map.insert(2, "b");
    830    /// if let Some(mut entry) = map.first_entry() {
    831    ///     if *entry.key() > 0 {
    832    ///         entry.insert("first");
    833    ///     }
    834    /// }
    835    /// assert_eq!(*map.get(&1).unwrap(), "first");
    836    /// assert_eq!(*map.get(&2).unwrap(), "b");
    837    /// ```
    838    #[stable(feature = "map_first_last", since = "1.66.0")]
    839    pub fn first_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
    840    where
    841        K: Ord,
    842    {
    843        let (map, dormant_map) = DormantMutRef::new(self);
    844        let root_node = map.root.as_mut()?.borrow_mut();
    845        let kv = root_node.first_leaf_edge().right_kv().ok()?;
    846        Some(OccupiedEntry {
    847            handle: kv.forget_node_type(),
    848            dormant_map,
    849            alloc: (*map.alloc).clone(),
    850            _marker: PhantomData,
    851        })
    852    }
    853
    854    /// Removes and returns the first element in the map.
    855    /// The key of this element is the minimum key that was in the map.
    856    ///
    857    /// # Examples
    858    ///
    859    /// Draining elements in ascending order, while keeping a usable map each iteration.
    860    ///
    861    /// ```
    862    /// use std::collections::BTreeMap;
    863    ///
    864    /// let mut map = BTreeMap::new();
    865    /// map.insert(1, "a");
    866    /// map.insert(2, "b");
    867    /// while let Some((key, _val)) = map.pop_first() {
    868    ///     assert!(map.iter().all(|(k, _v)| *k > key));
    869    /// }
    870    /// assert!(map.is_empty());
    871    /// ```
    872    #[stable(feature = "map_first_last", since = "1.66.0")]
    873    pub fn pop_first(&mut self) -> Option<(K, V)>
    874    where
    875        K: Ord,
    876    {
    877        self.first_entry().map(|entry| entry.remove_entry())
    878    }
    879
    880    /// Returns the last key-value pair in the map.
    881    /// The key in this pair is the maximum key in the map.
    882    ///
    883    /// # Examples
    884    ///
    885    /// ```
    886    /// use std::collections::BTreeMap;
    887    ///
    888    /// let mut map = BTreeMap::new();
    889    /// map.insert(1, "b");
    890    /// map.insert(2, "a");
    891    /// assert_eq!(map.last_key_value(), Some((&2, &"a")));
    892    /// ```
    893    #[stable(feature = "map_first_last", since = "1.66.0")]
    894    pub fn last_key_value(&self) -> Option<(&K, &V)>
    895    where
    896        K: Ord,
    897    {
    898        let root_node = self.root.as_ref()?.reborrow();
    899        root_node.last_leaf_edge().left_kv().ok().map(Handle::into_kv)
    900    }
    901
    902    /// Returns the last entry in the map for in-place manipulation.
    903    /// The key of this entry is the maximum key in the map.
    904    ///
    905    /// # Examples
    906    ///
    907    /// ```
    908    /// use std::collections::BTreeMap;
    909    ///
    910    /// let mut map = BTreeMap::new();
    911    /// map.insert(1, "a");
    912    /// map.insert(2, "b");
    913    /// if let Some(mut entry) = map.last_entry() {
    914    ///     if *entry.key() > 0 {
    915    ///         entry.insert("last");
    916    ///     }
    917    /// }
    918    /// assert_eq!(*map.get(&1).unwrap(), "a");
    919    /// assert_eq!(*map.get(&2).unwrap(), "last");
    920    /// ```
    921    #[stable(feature = "map_first_last", since = "1.66.0")]
    922    pub fn last_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
    923    where
    924        K: Ord,
    925    {
    926        let (map, dormant_map) = DormantMutRef::new(self);
    927        let root_node = map.root.as_mut()?.borrow_mut();
    928        let kv = root_node.last_leaf_edge().left_kv().ok()?;
    929        Some(OccupiedEntry {
    930            handle: kv.forget_node_type(),
    931            dormant_map,
    932            alloc: (*map.alloc).clone(),
    933            _marker: PhantomData,
    934        })
    935    }
    936
    937    /// Removes and returns the last element in the map.
    938    /// The key of this element is the maximum key that was in the map.
    939    ///
    940    /// # Examples
    941    ///
    942    /// Draining elements in descending order, while keeping a usable map each iteration.
    943    ///
    944    /// ```
    945    /// use std::collections::BTreeMap;
    946    ///
    947    /// let mut map = BTreeMap::new();
    948    /// map.insert(1, "a");
    949    /// map.insert(2, "b");
    950    /// while let Some((key, _val)) = map.pop_last() {
    951    ///     assert!(map.iter().all(|(k, _v)| *k < key));
    952    /// }
    953    /// assert!(map.is_empty());
    954    /// ```
    955    #[stable(feature = "map_first_last", since = "1.66.0")]
    956    pub fn pop_last(&mut self) -> Option<(K, V)>
    957    where
    958        K: Ord,
    959    {
    960        self.last_entry().map(|entry| entry.remove_entry())
    961    }
    962
    963    /// Returns `true` if the map contains a value for the specified key.
    964    ///
    965    /// The key may be any borrowed form of the map's key type, but the ordering
    966    /// on the borrowed form *must* match the ordering on the key type.
    967    ///
    968    /// # Examples
    969    ///
    970    /// ```
    971    /// use std::collections::BTreeMap;
    972    ///
    973    /// let mut map = BTreeMap::new();
    974    /// map.insert(1, "a");
    975    /// assert_eq!(map.contains_key(&1), true);
    976    /// assert_eq!(map.contains_key(&2), false);
    977    /// ```
    978    #[stable(feature = "rust1", since = "1.0.0")]
    979    #[cfg_attr(not(test), rustc_diagnostic_item = "btreemap_contains_key")]
    980    pub fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool
    981    where
    982        K: Borrow<Q> + Ord,
    983        Q: Ord,
    984    {
    985        self.get(key).is_some()
    986    }
    987
    988    /// Returns a mutable reference to the value corresponding to the key.
    989    ///
    990    /// The key may be any borrowed form of the map's key type, but the ordering
    991    /// on the borrowed form *must* match the ordering on the key type.
    992    ///
    993    /// # Examples
    994    ///
    995    /// ```
    996    /// use std::collections::BTreeMap;
    997    ///
    998    /// let mut map = BTreeMap::new();
    999    /// map.insert(1, "a");
    1000    /// if let Some(x) = map.get_mut(&1) {
    1001    ///     *x = "b";
    1002    /// }
    1003    /// assert_eq!(map[&1], "b");
    1004    /// ```
    1005    // See `get` for implementation notes, this is basically a copy-paste with mut's added
    1006    #[stable(feature = "rust1", since = "1.0.0")]
    1007    pub fn get_mut<Q: ?Sized>(&mut self, key: &Q) -> Option<&mut V>
    1008    where
    1009        K: Borrow<Q> + Ord,
    1010        Q: Ord,
    1011    {
    1012        let root_node = self.root.as_mut()?.borrow_mut();
    1013        match root_node.search_tree(key) {
    1014            Found(handle) => Some(handle.into_val_mut()),
    1015            GoDown(_) => None,
    1016        }
    1017    }
    1018
    1019    /// Inserts a key-value pair into the map.
    1020    ///
    1021    /// If the map did not have this key present, `None` is returned.
    1022    ///
    1023    /// If the map did have this key present, the value is updated, and the old
    1024    /// value is returned. The key is not updated, though; this matters for
    1025    /// types that can be `==` without being identical. See the [module-level
    1026    /// documentation] for more.
    1027    ///
    1028    /// [module-level documentation]: index.html#insert-and-complex-keys
    1029    ///
    1030    /// # Examples
    1031    ///
    1032    /// ```
    1033    /// use std::collections::BTreeMap;
    1034    ///
    1035    /// let mut map = BTreeMap::new();
    1036    /// assert_eq!(map.insert(37, "a"), None);
    1037    /// assert_eq!(map.is_empty(), false);
    1038    ///
    1039    /// map.insert(37, "b");
    1040    /// assert_eq!(map.insert(37, "c"), Some("b"));
    1041    /// assert_eq!(map[&37], "c");
    1042    /// ```
    1043    #[stable(feature = "rust1", since = "1.0.0")]
    1044    #[rustc_confusables("push", "put", "set")]
    1045    #[cfg_attr(not(test), rustc_diagnostic_item = "btreemap_insert")]
    1046    pub fn insert(&mut self, key: K, value: V) -> Option<V>
    1047    where
    1048        K: Ord,
    1049    {
    1050        match self.entry(key) {
    1051            Occupied(mut entry) => Some(entry.insert(value)),
    1052            Vacant(entry) => {
    1053                entry.insert(value);
    1054                None
    1055            }
    1056        }
    1057    }
    1058
    1059    /// Tries to insert a key-value pair into the map, and returns
    1060    /// a mutable reference to the value in the entry.
    1061    ///
    1062    /// If the map already had this key present, nothing is updated, and
    1063    /// an error containing the occupied entry and the value is returned.
    1064    ///
    1065    /// # Examples
    1066    ///
    1067    /// ```
    1068    /// #![feature(map_try_insert)]
    1069    ///
    1070    /// use std::collections::BTreeMap;
    1071    ///
    1072    /// let mut map = BTreeMap::new();
    1073    /// assert_eq!(map.try_insert(37, "a").unwrap(), &"a");
    1074    ///
    1075    /// let err = map.try_insert(37, "b").unwrap_err();
    1076    /// assert_eq!(err.entry.key(), &37);
    1077    /// assert_eq!(err.entry.get(), &"a");
    1078    /// assert_eq!(err.value, "b");
    1079    /// ```
    1080    #[unstable(feature = "map_try_insert", issue = "82766")]
    1081    pub fn try_insert(&mut self, key: K, value: V) -> Result<&mut V, OccupiedError<'_, K, V, A>>
    1082    where
    1083        K: Ord,
    1084    {
    1085        match self.entry(key) {
    1086            Occupied(entry) => Err(OccupiedError { entry, value }),
    1087            Vacant(entry) => Ok(entry.insert(value)),
    1088        }
    1089    }
    1090
    1091    /// Removes a key from the map, returning the value at the key if the key
    1092    /// was previously in the map.
    1093    ///
    1094    /// The key may be any borrowed form of the map's key type, but the ordering
    1095    /// on the borrowed form *must* match the ordering on the key type.
    1096    ///
    1097    /// # Examples
    1098    ///
    1099    /// ```
    1100    /// use std::collections::BTreeMap;
    1101    ///
    1102    /// let mut map = BTreeMap::new();
    1103    /// map.insert(1, "a");
    1104    /// assert_eq!(map.remove(&1), Some("a"));
    1105    /// assert_eq!(map.remove(&1), None);
    1106    /// ```
    1107    #[stable(feature = "rust1", since = "1.0.0")]
    1108    #[rustc_confusables("delete", "take")]
    1109    pub fn remove<Q: ?Sized>(&mut self, key: &Q) -> Option<V>
    1110    where
    1111        K: Borrow<Q> + Ord,
    1112        Q: Ord,
    1113    {
    1114        self.remove_entry(key).map(|(_, v)| v)
    1115    }
    1116
    1117    /// Removes a key from the map, returning the stored key and value if the key
    1118    /// was previously in the map.
    1119    ///
    1120    /// The key may be any borrowed form of the map's key type, but the ordering
    1121    /// on the borrowed form *must* match the ordering on the key type.
    1122    ///
    1123    /// # Examples
    1124    ///
    1125    /// ```
    1126    /// use std::collections::BTreeMap;
    1127    ///
    1128    /// let mut map = BTreeMap::new();
    1129    /// map.insert(1, "a");
    1130    /// assert_eq!(map.remove_entry(&1), Some((1, "a")));
    1131    /// assert_eq!(map.remove_entry(&1), None);
    1132    /// ```
    1133    #[stable(feature = "btreemap_remove_entry", since = "1.45.0")]
    1134    pub fn remove_entry<Q: ?Sized>(&mut self, key: &Q) -> Option<(K, V)>
    1135    where
    1136        K: Borrow<Q> + Ord,
    1137        Q: Ord,
    1138    {
    1139        let (map, dormant_map) = DormantMutRef::new(self);
    1140        let root_node = map.root.as_mut()?.borrow_mut();
    1141        match root_node.search_tree(key) {
    1142            Found(handle) => Some(
    1143                OccupiedEntry {
    1144                    handle,
    1145                    dormant_map,
    1146                    alloc: (*map.alloc).clone(),
    1147                    _marker: PhantomData,
    1148                }
    1149                .remove_entry(),
    1150            ),
    1151            GoDown(_) => None,
    1152        }
    1153    }
    1154
    1155    /// Retains only the elements specified by the predicate.
    1156    ///
    1157    /// In other words, remove all pairs `(k, v)` for which `f(&k, &mut v)` returns `false`.
    1158    /// The elements are visited in ascending key order.
    1159    ///
    1160    /// # Examples
    1161    ///
    1162    /// ```
    1163    /// use std::collections::BTreeMap;
    1164    ///
    1165    /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x*10)).collect();
    1166    /// // Keep only the elements with even-numbered keys.
    1167    /// map.retain(|&k, _| k % 2 == 0);
    1168    /// assert!(map.into_iter().eq(vec![(0, 0), (2, 20), (4, 40), (6, 60)]));
    1169    /// ```
    1170    #[inline]
    1171    #[stable(feature = "btree_retain", since = "1.53.0")]
    1172    pub fn retain<F>(&mut self, mut f: F)
    1173    where
    1174        K: Ord,
    1175        F: FnMut(&K, &mut V) -> bool,
    1176    {
    1177        self.extract_if(.., |k, v| !f(k, v)).for_each(drop);
    1178    }
    1179
    1180    /// Moves all elements from `other` into `self`, leaving `other` empty.
    1181    ///
    1182    /// If a key from `other` is already present in `self`, the respective
    1183    /// value from `self` will be overwritten with the respective value from `other`.
    1184    /// Similar to [`insert`], though, the key is not overwritten,
    1185    /// which matters for types that can be `==` without being identical.
    1186    ///
    1187    /// [`insert`]: BTreeMap::insert
    1188    ///
    1189    /// # Examples
    1190    ///
    1191    /// ```
    1192    /// use std::collections::BTreeMap;
    1193    ///
    1194    /// let mut a = BTreeMap::new();
    1195    /// a.insert(1, "a");
    1196    /// a.insert(2, "b");
    1197    /// a.insert(3, "c"); // Note: Key (3) also present in b.
    1198    ///
    1199    /// let mut b = BTreeMap::new();
    1200    /// b.insert(3, "d"); // Note: Key (3) also present in a.
    1201    /// b.insert(4, "e");
    1202    /// b.insert(5, "f");
    1203    ///
    1204    /// a.append(&mut b);
    1205    ///
    1206    /// assert_eq!(a.len(), 5);
    1207    /// assert_eq!(b.len(), 0);
    1208    ///
    1209    /// assert_eq!(a[&1], "a");
    1210    /// assert_eq!(a[&2], "b");
    1211    /// assert_eq!(a[&3], "d"); // Note: "c" has been overwritten.
    1212    /// assert_eq!(a[&4], "e");
    1213    /// assert_eq!(a[&5], "f");
    1214    /// ```
    1215    #[stable(feature = "btree_append", since = "1.11.0")]
    1216    pub fn append(&mut self, other: &mut Self)
    1217    where
    1218        K: Ord,
    1219        A: Clone,
    1220    {
    1221        // Do we have to append anything at all?
    1222        if other.is_empty() {
    1223            return;
    1224        }
    1225
    1226        // We can just swap `self` and `other` if `self` is empty.
    1227        if self.is_empty() {
    1228            mem::swap(self, other);
    1229            return;
    1230        }
    1231
    1232        let self_iter = mem::replace(self, Self::new_in((*self.alloc).clone())).into_iter();
    1233        let other_iter = mem::replace(other, Self::new_in((*self.alloc).clone())).into_iter();
    1234        let root = self.root.get_or_insert_with(|| Root::new((*self.alloc).clone()));
    1235        root.append_from_sorted_iters(
    1236            self_iter,
    1237            other_iter,
    1238            &mut self.length,
    1239            (*self.alloc).clone(),
    1240        )
    1241    }
    1242
    1243    /// Constructs a double-ended iterator over a sub-range of elements in the map.
    1244    /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
    1245    /// yield elements from min (inclusive) to max (exclusive).
    1246    /// The range may also be entered as `(Bound<T>, Bound<T>)`, so for example
    1247    /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
    1248    /// range from 4 to 10.
    1249    ///
    1250    /// # Panics
    1251    ///
    1252    /// Panics if range `start > end`.
    1253    /// Panics if range `start == end` and both bounds are `Excluded`.
    1254    ///
    1255    /// # Examples
    1256    ///
    1257    /// ```
    1258    /// use std::collections::BTreeMap;
    1259    /// use std::ops::Bound::Included;
    1260    ///
    1261    /// let mut map = BTreeMap::new();
    1262    /// map.insert(3, "a");
    1263    /// map.insert(5, "b");
    1264    /// map.insert(8, "c");
    1265    /// for (&key, &value) in map.range((Included(&4), Included(&8))) {
    1266    ///     println!("{key}: {value}");
    1267    /// }
    1268    /// assert_eq!(Some((&5, &"b")), map.range(4..).next());
    1269    /// ```
    1270    #[stable(feature = "btree_range", since = "1.17.0")]
    1271    pub fn range<T: ?Sized, R>(&self, range: R) -> Range<'_, K, V>
    1272    where
    1273        T: Ord,
    1274        K: Borrow<T> + Ord,
    1275        R: RangeBounds<T>,
    1276    {
    1277        if let Some(root) = &self.root {
    1278            Range { inner: root.reborrow().range_search(range) }
    1279        } else {
    1280            Range { inner: LeafRange::none() }
    1281        }
    1282    }
    1283
    1284    /// Constructs a mutable double-ended iterator over a sub-range of elements in the map.
    1285    /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
    1286    /// yield elements from min (inclusive) to max (exclusive).
    1287    /// The range may also be entered as `(Bound<T>, Bound<T>)`, so for example
    1288    /// `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive
    1289    /// range from 4 to 10.
    1290    ///
    1291    /// # Panics
    1292    ///
    1293    /// Panics if range `start > end`.
    1294    /// Panics if range `start == end` and both bounds are `Excluded`.
    1295    ///
    1296    /// # Examples
    1297    ///
    1298    /// ```
    1299    /// use std::collections::BTreeMap;
    1300    ///
    1301    /// let mut map: BTreeMap<&str, i32> =
    1302    ///     [("Alice", 0), ("Bob", 0), ("Carol", 0), ("Cheryl", 0)].into();
    1303    /// for (_, balance) in map.range_mut("B".."Cheryl") {
    1304    ///     *balance += 100;
    1305    /// }
    1306    /// for (name, balance) in &map {
    1307    ///     println!("{name} => {balance}");
    1308    /// }
    1309    /// ```
    1310    #[stable(feature = "btree_range", since = "1.17.0")]
    1311    pub fn range_mut<T: ?Sized, R>(&mut self, range: R) -> RangeMut<'_, K, V>
    1312    where
    1313        T: Ord,
    1314        K: Borrow<T> + Ord,
    1315        R: RangeBounds<T>,
    1316    {
    1317        if let Some(root) = &mut self.root {
    1318            RangeMut { inner: root.borrow_valmut().range_search(range), _marker: PhantomData }
    1319        } else {
    1320            RangeMut { inner: LeafRange::none(), _marker: PhantomData }
    1321        }
    1322    }
    1323
    1324    /// Gets the given key's corresponding entry in the map for in-place manipulation.
    1325    ///
    1326    /// # Examples
    1327    ///
    1328    /// ```
    1329    /// use std::collections::BTreeMap;
    1330    ///
    1331    /// let mut count: BTreeMap<&str, usize> = BTreeMap::new();
    1332    ///
    1333    /// // count the number of occurrences of letters in the vec
    1334    /// for x in ["a", "b", "a", "c", "a", "b"] {
    1335    ///     count.entry(x).and_modify(|curr| *curr += 1).or_insert(1);
    1336    /// }
    1337    ///
    1338    /// assert_eq!(count["a"], 3);
    1339    /// assert_eq!(count["b"], 2);
    1340    /// assert_eq!(count["c"], 1);
    1341    /// ```
    1342    #[stable(feature = "rust1", since = "1.0.0")]
    1343    pub fn entry(&mut self, key: K) -> Entry<'_, K, V, A>
    1344    where
    1345        K: Ord,
    1346    {
    1347        let (map, dormant_map) = DormantMutRef::new(self);
    1348        match map.root {
    1349            None => Vacant(VacantEntry {
    1350                key,
    1351                handle: None,
    1352                dormant_map,
    1353                alloc: (*map.alloc).clone(),
    1354                _marker: PhantomData,
    1355            }),
    1356            Some(ref mut root) => match root.borrow_mut().search_tree(&key) {
    1357                Found(handle) => Occupied(OccupiedEntry {
    1358                    handle,
    1359                    dormant_map,
    1360                    alloc: (*map.alloc).clone(),
    1361                    _marker: PhantomData,
    1362                }),
    1363                GoDown(handle) => Vacant(VacantEntry {
    1364                    key,
    1365                    handle: Some(handle),
    1366                    dormant_map,
    1367                    alloc: (*map.alloc).clone(),
    1368                    _marker: PhantomData,
    1369                }),
    1370            },
    1371        }
    1372    }
    1373
    1374    /// Splits the collection into two at the given key. Returns everything after the given key,
    1375    /// including the key. If the key is not present, the split will occur at the nearest
    1376    /// greater key, or return an empty map if no such key exists.
    1377    ///
    1378    /// # Examples
    1379    ///
    1380    /// ```
    1381    /// use std::collections::BTreeMap;
    1382    ///
    1383    /// let mut a = BTreeMap::new();
    1384    /// a.insert(1, "a");
    1385    /// a.insert(2, "b");
    1386    /// a.insert(3, "c");
    1387    /// a.insert(17, "d");
    1388    /// a.insert(41, "e");
    1389    ///
    1390    /// let b = a.split_off(&3);
    1391    ///
    1392    /// assert_eq!(a.len(), 2);
    1393    /// assert_eq!(b.len(), 3);
    1394    ///
    1395    /// assert_eq!(a[&1], "a");
    1396    /// assert_eq!(a[&2], "b");
    1397    ///
    1398    /// assert_eq!(b[&3], "c");
    1399    /// assert_eq!(b[&17], "d");
    1400    /// assert_eq!(b[&41], "e");
    1401    /// ```
    1402    #[stable(feature = "btree_split_off", since = "1.11.0")]
    1403    pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self
    1404    where
    1405        K: Borrow<Q> + Ord,
    1406        A: Clone,
    1407    {
    1408        if self.is_empty() {
    1409            return Self::new_in((*self.alloc).clone());
    1410        }
    1411
    1412        let total_num = self.len();
    1413        let left_root = self.root.as_mut().unwrap(); // unwrap succeeds because not empty
    1414
    1415        let right_root = left_root.split_off(key, (*self.alloc).clone());
    1416
    1417        let (new_left_len, right_len) = Root::calc_split_length(total_num, &left_root, &right_root);
    1418        self.length = new_left_len;
    1419
    1420        BTreeMap {
    1421            root: Some(right_root),
    1422            length: right_len,
    1423            alloc: self.alloc.clone(),
    1424            _marker: PhantomData,
    1425        }
    1426    }
    1427
    1428    /// Creates an iterator that visits elements (key-value pairs) in the specified range in
    1429    /// ascending key order and uses a closure to determine if an element
    1430    /// should be removed.
    1431    ///
    1432    /// If the closure returns `true`, the element is removed from the map and
    1433    /// yielded. If the closure returns `false`, or panics, the element remains
    1434    /// in the map and will not be yielded.
    1435    ///
    1436    /// The iterator also lets you mutate the value of each element in the
    1437    /// closure, regardless of whether you choose to keep or remove it.
    1438    ///
    1439    /// If the returned `ExtractIf` is not exhausted, e.g. because it is dropped without iterating
    1440    /// or the iteration short-circuits, then the remaining elements will be retained.
    1441    /// Use `extract_if().for_each(drop)` if you do not need the returned iterator,
    1442    /// or [`retain`] with a negated predicate if you also do not need to restrict the range.
    1443    ///
    1444    /// [`retain`]: BTreeMap::retain
    1445    ///
    1446    /// # Examples
    1447    ///
    1448    /// ```
    1449    /// use std::collections::BTreeMap;
    1450    ///
    1451    /// // Splitting a map into even and odd keys, reusing the original map:
    1452    /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    1453    /// let evens: BTreeMap<_, _> = map.extract_if(.., |k, _v| k % 2 == 0).collect();
    1454    /// let odds = map;
    1455    /// assert_eq!(evens.keys().copied().collect::<Vec<_>>(), [0, 2, 4, 6]);
    1456    /// assert_eq!(odds.keys().copied().collect::<Vec<_>>(), [1, 3, 5, 7]);
    1457    ///
    1458    /// // Splitting a map into low and high halves, reusing the original map:
    1459    /// let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    1460    /// let low: BTreeMap<_, _> = map.extract_if(0..4, |_k, _v| true).collect();
    1461    /// let high = map;
    1462    /// assert_eq!(low.keys().copied().collect::<Vec<_>>(), [0, 1, 2, 3]);
    1463    /// assert_eq!(high.keys().copied().collect::<Vec<_>>(), [4, 5, 6, 7]);
    1464    /// ```
    1465    #[stable(feature = "btree_extract_if", since = "1.91.0")]
    1466    pub fn extract_if<F, R>(&mut self, range: R, pred: F) -> ExtractIf<'_, K, V, R, F, A>
    1467    where
    1468        K: Ord,
    1469        R: RangeBounds<K>,
    1470        F: FnMut(&K, &mut V) -> bool,
    1471    {
    1472        let (inner, alloc) = self.extract_if_inner(range);
    1473        ExtractIf { pred, inner, alloc }
    1474    }
    1475
    1476    pub(super) fn extract_if_inner<R>(&mut self, range: R) -> (ExtractIfInner<'_, K, V, R>, A)
    1477    where
    1478        K: Ord,
    1479        R: RangeBounds<K>,
    1480    {
    1481        if let Some(root) = self.root.as_mut() {
    1482            let (root, dormant_root) = DormantMutRef::new(root);
    1483            let first = root.borrow_mut().lower_bound(SearchBound::from_range(range.start_bound()));
    1484            (
    1485                ExtractIfInner {
    1486                    length: &mut self.length,
    1487                    dormant_root: Some(dormant_root),
    1488                    cur_leaf_edge: Some(first),
    1489                    range,
    1490                },
    1491                (*self.alloc).clone(),
    1492            )
    1493        } else {
    1494            (
    1495                ExtractIfInner {
    1496                    length: &mut self.length,
    1497                    dormant_root: None,
    1498                    cur_leaf_edge: None,
    1499                    range,
    1500                },
    1501                (*self.alloc).clone(),
    1502            )
    1503        }
    1504    }
    1505
    1506    /// Creates a consuming iterator visiting all the keys, in sorted order.
    1507    /// The map cannot be used after calling this.
    1508    /// The iterator element type is `K`.
    1509    ///
    1510    /// # Examples
    1511    ///
    1512    /// ```
    1513    /// use std::collections::BTreeMap;
    1514    ///
    1515    /// let mut a = BTreeMap::new();
    1516    /// a.insert(2, "b");
    1517    /// a.insert(1, "a");
    1518    ///
    1519    /// let keys: Vec<i32> = a.into_keys().collect();
    1520    /// assert_eq!(keys, [1, 2]);
    1521    /// ```
    1522    #[inline]
    1523    #[stable(feature = "map_into_keys_values", since = "1.54.0")]
    1524    pub fn into_keys(self) -> IntoKeys<K, V, A> {
    1525        IntoKeys { inner: self.into_iter() }
    1526    }
    1527
    1528    /// Creates a consuming iterator visiting all the values, in order by key.
    1529    /// The map cannot be used after calling this.
    1530    /// The iterator element type is `V`.
    1531    ///
    1532    /// # Examples
    1533    ///
    1534    /// ```
    1535    /// use std::collections::BTreeMap;
    1536    ///
    1537    /// let mut a = BTreeMap::new();
    1538    /// a.insert(1, "hello");
    1539    /// a.insert(2, "goodbye");
    1540    ///
    1541    /// let values: Vec<&str> = a.into_values().collect();
    1542    /// assert_eq!(values, ["hello", "goodbye"]);
    1543    /// ```
    1544    #[inline]
    1545    #[stable(feature = "map_into_keys_values", since = "1.54.0")]
    1546    pub fn into_values(self) -> IntoValues<K, V, A> {
    1547        IntoValues { inner: self.into_iter() }
    1548    }
    1549
    1550    /// Makes a `BTreeMap` from a sorted iterator.
    1551    pub(crate) fn bulk_build_from_sorted_iter<I>(iter: I, alloc: A) -> Self
    1552    where
    1553        K: Ord,
    1554        I: IntoIterator<Item = (K, V)>,
    1555    {
    1556        let mut root = Root::new(alloc.clone());
    1557        let mut length = 0;
    1558        root.bulk_push(DedupSortedIter::new(iter.into_iter()), &mut length, alloc.clone());
    1559        BTreeMap { root: Some(root), length, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
    1560    }
    1561}
    1562
    1563#[stable(feature = "rust1", since = "1.0.0")]
    1564impl<'a, K, V, A: Allocator + Clone> IntoIterator for &'a BTreeMap<K, V, A> {
    1565    type Item = (&'a K, &'a V);
    1566    type IntoIter = Iter<'a, K, V>;
    1567
    1568    fn into_iter(self) -> Iter<'a, K, V> {
    1569        self.iter()
    1570    }
    1571}
    1572
    1573#[stable(feature = "rust1", since = "1.0.0")]
    1574impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
    1575    type Item = (&'a K, &'a V);
    1576
    1577    fn next(&mut self) -> Option<(&'a K, &'a V)> {
    1578        if self.length == 0 {
    1579            None
    1580        } else {
    1581            self.length -= 1;
    1582            Some(unsafe { self.range.next_unchecked() })
    1583        }
    1584    }
    1585
    1586    fn size_hint(&self) -> (usize, Option<usize>) {
    1587        (self.length, Some(self.length))
    1588    }
    1589
    1590    fn last(mut self) -> Option<(&'a K, &'a V)> {
    1591        self.next_back()
    1592    }
    1593
    1594    fn min(mut self) -> Option<(&'a K, &'a V)>
    1595    where
    1596        (&'a K, &'a V): Ord,
    1597    {
    1598        self.next()
    1599    }
    1600
    1601    fn max(mut self) -> Option<(&'a K, &'a V)>
    1602    where
    1603        (&'a K, &'a V): Ord,
    1604    {
    1605        self.next_back()
    1606    }
    1607}
    1608
    1609#[stable(feature = "fused", since = "1.26.0")]
    1610impl<K, V> FusedIterator for Iter<'_, K, V> {}
    1611
    1612#[stable(feature = "rust1", since = "1.0.0")]
    1613impl<'a, K: 'a, V: 'a> DoubleEndedIterator for Iter<'a, K, V> {
    1614    fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
    1615        if self.length == 0 {
    1616            None
    1617        } else {
    1618            self.length -= 1;
    1619            Some(unsafe { self.range.next_back_unchecked() })
    1620        }
    1621    }
    1622}
    1623
    1624#[stable(feature = "rust1", since = "1.0.0")]
    1625impl<K, V> ExactSizeIterator for Iter<'_, K, V> {
    1626    fn len(&self) -> usize {
    1627        self.length
    1628    }
    1629}
    1630
    1631#[unstable(feature = "trusted_len", issue = "37572")]
    1632unsafe impl<K, V> TrustedLen for Iter<'_, K, V> {}
    1633
    1634#[stable(feature = "rust1", since = "1.0.0")]
    1635impl<K, V> Clone for Iter<'_, K, V> {
    1636    fn clone(&self) -> Self {
    1637        Iter { range: self.range.clone(), length: self.length }
    1638    }
    1639}
    1640
    1641#[stable(feature = "rust1", since = "1.0.0")]
    1642impl<'a, K, V, A: Allocator + Clone> IntoIterator for &'a mut BTreeMap<K, V, A> {
    1643    type Item = (&'a K, &'a mut V);
    1644    type IntoIter = IterMut<'a, K, V>;
    1645
    1646    fn into_iter(self) -> IterMut<'a, K, V> {
    1647        self.iter_mut()
    1648    }
    1649}
    1650
    1651#[stable(feature = "rust1", since = "1.0.0")]
    1652impl<'a, K, V> Iterator for IterMut<'a, K, V> {
    1653    type Item = (&'a K, &'a mut V);
    1654
    1655    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
    1656        if self.length == 0 {
    1657            None
    1658        } else {
    1659            self.length -= 1;
    1660            Some(unsafe { self.range.next_unchecked() })
    1661        }
    1662    }
    1663
    1664    fn size_hint(&self) -> (usize, Option<usize>) {
    1665        (self.length, Some(self.length))
    1666    }
    1667
    1668    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
    1669        self.next_back()
    1670    }
    1671
    1672    fn min(mut self) -> Option<(&'a K, &'a mut V)>
    1673    where
    1674        (&'a K, &'a mut V): Ord,
    1675    {
    1676        self.next()
    1677    }
    1678
    1679    fn max(mut self) -> Option<(&'a K, &'a mut V)>
    1680    where
    1681        (&'a K, &'a mut V): Ord,
    1682    {
    1683        self.next_back()
    1684    }
    1685}
    1686
    1687#[stable(feature = "rust1", since = "1.0.0")]
    1688impl<'a, K, V> DoubleEndedIterator for IterMut<'a, K, V> {
    1689    fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
    1690        if self.length == 0 {
    1691            None
    1692        } else {
    1693            self.length -= 1;
    1694            Some(unsafe { self.range.next_back_unchecked() })
    1695        }
    1696    }
    1697}
    1698
    1699#[stable(feature = "rust1", since = "1.0.0")]
    1700impl<K, V> ExactSizeIterator for IterMut<'_, K, V> {
    1701    fn len(&self) -> usize {
    1702        self.length
    1703    }
    1704}
    1705
    1706#[unstable(feature = "trusted_len", issue = "37572")]
    1707unsafe impl<K, V> TrustedLen for IterMut<'_, K, V> {}
    1708
    1709#[stable(feature = "fused", since = "1.26.0")]
    1710impl<K, V> FusedIterator for IterMut<'_, K, V> {}
    1711
    1712impl<'a, K, V> IterMut<'a, K, V> {
    1713    /// Returns an iterator of references over the remaining items.
    1714    #[inline]
    1715    pub(super) fn iter(&self) -> Iter<'_, K, V> {
    1716        Iter { range: self.range.reborrow(), length: self.length }
    1717    }
    1718}
    1719
    1720#[stable(feature = "rust1", since = "1.0.0")]
    1721impl<K, V, A: Allocator + Clone> IntoIterator for BTreeMap<K, V, A> {
    1722    type Item = (K, V);
    1723    type IntoIter = IntoIter<K, V, A>;
    1724
    1725    /// Gets an owning iterator over the entries of the map, sorted by key.
    1726    fn into_iter(self) -> IntoIter<K, V, A> {
    1727        let mut me = ManuallyDrop::new(self);
    1728        if let Some(root) = me.root.take() {
    1729            let full_range = root.into_dying().full_range();
    1730
    1731            IntoIter {
    1732                range: full_range,
    1733                length: me.length,
    1734                alloc: unsafe { ManuallyDrop::take(&mut me.alloc) },
    1735            }
    1736        } else {
    1737            IntoIter {
    1738                range: LazyLeafRange::none(),
    1739                length: 0,
    1740                alloc: unsafe { ManuallyDrop::take(&mut me.alloc) },
    1741            }
    1742        }
    1743    }
    1744}
    1745
    1746#[stable(feature = "btree_drop", since = "1.7.0")]
    1747impl<K, V, A: Allocator + Clone> Drop for IntoIter<K, V, A> {
    1748    fn drop(&mut self) {
    1749        struct DropGuard<'a, K, V, A: Allocator + Clone>(&'a mut IntoIter<K, V, A>);
    1750
    1751        impl<'a, K, V, A: Allocator + Clone> Drop for DropGuard<'a, K, V, A> {
    1752            fn drop(&mut self) {
    1753                // Continue the same loop we perform below. This only runs when unwinding, so we
    1754                // don't have to care about panics this time (they'll abort).
    1755                while let Some(kv) = self.0.dying_next() {
    1756                    // SAFETY: we consume the dying handle immediately.
    1757                    unsafe { kv.drop_key_val() };
    1758                }
    1759            }
    1760        }
    1761
    1762        while let Some(kv) = self.dying_next() {
    1763            let guard = DropGuard(self);
    1764            // SAFETY: we don't touch the tree before consuming the dying handle.
    1765            unsafe { kv.drop_key_val() };
    1766            mem::forget(guard);
    1767        }
    1768    }
    1769}
    1770
    1771impl<K, V, A: Allocator + Clone> IntoIter<K, V, A> {
    1772    /// Core of a `next` method returning a dying KV handle,
    1773    /// invalidated by further calls to this function and some others.
    1774    fn dying_next(
    1775        &mut self,
    1776    ) -> Option<Handle<NodeRef<marker::Dying, K, V, marker::LeafOrInternal>, marker::KV>> {
    1777        if self.length == 0 {
    1778            self.range.deallocating_end(self.alloc.clone());
    1779            None
    1780        } else {
    1781            self.length -= 1;
    1782            Some(unsafe { self.range.deallocating_next_unchecked(self.alloc.clone()) })
    1783        }
    1784    }
    1785
    1786    /// Core of a `next_back` method returning a dying KV handle,
    1787    /// invalidated by further calls to this function and some others.
    1788    fn dying_next_back(
    1789        &mut self,
    1790    ) -> Option<Handle<NodeRef<marker::Dying, K, V, marker::LeafOrInternal>, marker::KV>> {
    1791        if self.length == 0 {
    1792            self.range.deallocating_end(self.alloc.clone());
    1793            None
    1794        } else {
    1795            self.length -= 1;
    1796            Some(unsafe { self.range.deallocating_next_back_unchecked(self.alloc.clone()) })
    1797        }
    1798    }
    1799}
    1800
    1801#[stable(feature = "rust1", since = "1.0.0")]
    1802impl<K, V, A: Allocator + Clone> Iterator for IntoIter<K, V, A> {
    1803    type Item = (K, V);
    1804
    1805    fn next(&mut self) -> Option<(K, V)> {
    1806        // SAFETY: we consume the dying handle immediately.
    1807        self.dying_next().map(unsafe { |kv| kv.into_key_val() })
    1808    }
    1809
    1810    fn size_hint(&self) -> (usize, Option<usize>) {
    1811        (self.length, Some(self.length))
    1812    }
    1813}
    1814
    1815#[stable(feature = "rust1", since = "1.0.0")]
    1816impl<K, V, A: Allocator + Clone> DoubleEndedIterator for IntoIter<K, V, A> {
    1817    fn next_back(&mut self) -> Option<(K, V)> {
    1818        // SAFETY: we consume the dying handle immediately.
    1819        self.dying_next_back().map(unsafe { |kv| kv.into_key_val() })
    1820    }
    1821}
    1822
    1823#[stable(feature = "rust1", since = "1.0.0")]
    1824impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoIter<K, V, A> {
    1825    fn len(&self) -> usize {
    1826        self.length
    1827    }
    1828}
    1829
    1830#[unstable(feature = "trusted_len", issue = "37572")]
    1831unsafe impl<K, V, A: Allocator + Clone> TrustedLen for IntoIter<K, V, A> {}
    1832
    1833#[stable(feature = "fused", since = "1.26.0")]
    1834impl<K, V, A: Allocator + Clone> FusedIterator for IntoIter<K, V, A> {}
    1835
    1836#[stable(feature = "rust1", since = "1.0.0")]
    1837impl<'a, K, V> Iterator for Keys<'a, K, V> {
    1838    type Item = &'a K;
    1839
    1840    fn next(&mut self) -> Option<&'a K> {
    1841        self.inner.next().map(|(k, _)| k)
    1842    }
    1843
    1844    fn size_hint(&self) -> (usize, Option<usize>) {
    1845        self.inner.size_hint()
    1846    }
    1847
    1848    fn last(mut self) -> Option<&'a K> {
    1849        self.next_back()
    1850    }
    1851
    1852    fn min(mut self) -> Option<&'a K>
    1853    where
    1854        &'a K: Ord,
    1855    {
    1856        self.next()
    1857    }
    1858
    1859    fn max(mut self) -> Option<&'a K>
    1860    where
    1861        &'a K: Ord,
    1862    {
    1863        self.next_back()
    1864    }
    1865}
    1866
    1867#[stable(feature = "rust1", since = "1.0.0")]
    1868impl<'a, K, V> DoubleEndedIterator for Keys<'a, K, V> {
    1869    fn next_back(&mut self) -> Option<&'a K> {
    1870        self.inner.next_back().map(|(k, _)| k)
    1871    }
    1872}
    1873
    1874#[stable(feature = "rust1", since = "1.0.0")]
    1875impl<K, V> ExactSizeIterator for Keys<'_, K, V> {
    1876    fn len(&self) -> usize {
    1877        self.inner.len()
    1878    }
    1879}
    1880
    1881#[unstable(feature = "trusted_len", issue = "37572")]
    1882unsafe impl<K, V> TrustedLen for Keys<'_, K, V> {}
    1883
    1884#[stable(feature = "fused", since = "1.26.0")]
    1885impl<K, V> FusedIterator for Keys<'_, K, V> {}
    1886
    1887#[stable(feature = "rust1", since = "1.0.0")]
    1888impl<K, V> Clone for Keys<'_, K, V> {
    1889    fn clone(&self) -> Self {
    1890        Keys { inner: self.inner.clone() }
    1891    }
    1892}
    1893
    1894#[stable(feature = "default_iters", since = "1.70.0")]
    1895impl<K, V> Default for Keys<'_, K, V> {
    1896    /// Creates an empty `btree_map::Keys`.
    1897    ///
    1898    /// ```
    1899    /// # use std::collections::btree_map;
    1900    /// let iter: btree_map::Keys<'_, u8, u8> = Default::default();
    1901    /// assert_eq!(iter.len(), 0);
    1902    /// ```
    1903    fn default() -> Self {
    1904        Keys { inner: Default::default() }
    1905    }
    1906}
    1907
    1908#[stable(feature = "rust1", since = "1.0.0")]
    1909impl<'a, K, V> Iterator for Values<'a, K, V> {
    1910    type Item = &'a V;
    1911
    1912    fn next(&mut self) -> Option<&'a V> {
    1913        self.inner.next().map(|(_, v)| v)
    1914    }
    1915
    1916    fn size_hint(&self) -> (usize, Option<usize>) {
    1917        self.inner.size_hint()
    1918    }
    1919
    1920    fn last(mut self) -> Option<&'a V> {
    1921        self.next_back()
    1922    }
    1923}
    1924
    1925#[stable(feature = "rust1", since = "1.0.0")]
    1926impl<'a, K, V> DoubleEndedIterator for Values<'a, K, V> {
    1927    fn next_back(&mut self) -> Option<&'a V> {
    1928        self.inner.next_back().map(|(_, v)| v)
    1929    }
    1930}
    1931
    1932#[stable(feature = "rust1", since = "1.0.0")]
    1933impl<K, V> ExactSizeIterator for Values<'_, K, V> {
    1934    fn len(&self) -> usize {
    1935        self.inner.len()
    1936    }
    1937}
    1938
    1939#[unstable(feature = "trusted_len", issue = "37572")]
    1940unsafe impl<K, V> TrustedLen for Values<'_, K, V> {}
    1941
    1942#[stable(feature = "fused", since = "1.26.0")]
    1943impl<K, V> FusedIterator for Values<'_, K, V> {}
    1944
    1945#[stable(feature = "rust1", since = "1.0.0")]
    1946impl<K, V> Clone for Values<'_, K, V> {
    1947    fn clone(&self) -> Self {
    1948        Values { inner: self.inner.clone() }
    1949    }
    1950}
    1951
    1952#[stable(feature = "default_iters", since = "1.70.0")]
    1953impl<K, V> Default for Values<'_, K, V> {
    1954    /// Creates an empty `btree_map::Values`.
    1955    ///
    1956    /// ```
    1957    /// # use std::collections::btree_map;
    1958    /// let iter: btree_map::Values<'_, u8, u8> = Default::default();
    1959    /// assert_eq!(iter.len(), 0);
    1960    /// ```
    1961    fn default() -> Self {
    1962        Values { inner: Default::default() }
    1963    }
    1964}
    1965
    1966/// An iterator produced by calling `extract_if` on BTreeMap.
    1967#[stable(feature = "btree_extract_if", since = "1.91.0")]
    1968#[must_use = "iterators are lazy and do nothing unless consumed; \
    1969    use `retain` or `extract_if().for_each(drop)` to remove and discard elements"]
    1970pub struct ExtractIf<
    1971    'a,
    1972    K,
    1973    V,
    1974    R,
    1975    F,
    1976    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
    1977> {
    1978    pred: F,
    1979    inner: ExtractIfInner<'a, K, V, R>,
    1980    /// The BTreeMap will outlive this IntoIter so we don't care about drop order for `alloc`.
    1981    alloc: A,
    1982}
    1983
    1984/// Most of the implementation of ExtractIf are generic over the type
    1985/// of the predicate, thus also serving for BTreeSet::ExtractIf.
    1986pub(super) struct ExtractIfInner<'a, K, V, R> {
    1987    /// Reference to the length field in the borrowed map, updated live.
    1988    length: &'a mut usize,
    1989    /// Buried reference to the root field in the borrowed map.
    1990    /// Wrapped in `Option` to allow drop handler to `take` it.
    1991    dormant_root: Option<DormantMutRef<'a, Root<K, V>>>,
    1992    /// Contains a leaf edge preceding the next element to be returned, or the last leaf edge.
    1993    /// Empty if the map has no root, if iteration went beyond the last leaf edge,
    1994    /// or if a panic occurred in the predicate.
    1995    cur_leaf_edge: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
    1996    /// Range over which iteration was requested.  We don't need the left side, but we
    1997    /// can't extract the right side without requiring K: Clone.
    1998    range: R,
    1999}
    2000
    2001#[stable(feature = "btree_extract_if", since = "1.91.0")]
    2002impl<K, V, R, F, A> fmt::Debug for ExtractIf<'_, K, V, R, F, A>
    2003where
    2004    K: fmt::Debug,
    2005    V: fmt::Debug,
    2006    A: Allocator + Clone,
    2007{
    2008    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    2009        f.debug_struct("ExtractIf").field("peek", &self.inner.peek()).finish_non_exhaustive()
    2010    }
    2011}
    2012
    2013#[stable(feature = "btree_extract_if", since = "1.91.0")]
    2014impl<K, V, R, F, A: Allocator + Clone> Iterator for ExtractIf<'_, K, V, R, F, A>
    2015where
    2016    K: PartialOrd,
    2017    R: RangeBounds<K>,
    2018    F: FnMut(&K, &mut V) -> bool,
    2019{
    2020    type Item = (K, V);
    2021
    2022    fn next(&mut self) -> Option<(K, V)> {
    2023        self.inner.next(&mut self.pred, self.alloc.clone())
    2024    }
    2025
    2026    fn size_hint(&self) -> (usize, Option<usize>) {
    2027        self.inner.size_hint()
    2028    }
    2029}
    2030
    2031impl<'a, K, V, R> ExtractIfInner<'a, K, V, R> {
    2032    /// Allow Debug implementations to predict the next element.
    2033    pub(super) fn peek(&self) -> Option<(&K, &V)> {
    2034        let edge = self.cur_leaf_edge.as_ref()?;
    2035        edge.reborrow().next_kv().ok().map(Handle::into_kv)
    2036    }
    2037
    2038    /// Implementation of a typical `ExtractIf::next` method, given the predicate.
    2039    pub(super) fn next<F, A: Allocator + Clone>(&mut self, pred: &mut F, alloc: A) -> Option<(K, V)>
    2040    where
    2041        K: PartialOrd,
    2042        R: RangeBounds<K>,
    2043        F: FnMut(&K, &mut V) -> bool,
    2044    {
    2045        while let Ok(mut kv) = self.cur_leaf_edge.take()?.next_kv() {
    2046            let (k, v) = kv.kv_mut();
    2047
    2048            // On creation, we navigated directly to the left bound, so we need only check the
    2049            // right bound here to decide whether to stop.
    2050            match self.range.end_bound() {
    2051                Bound::Included(ref end) if (*k).le(end) => (),
    2052                Bound::Excluded(ref end) if (*k).lt(end) => (),
    2053                Bound::Unbounded => (),
    2054                _ => return None,
    2055            }
    2056
    2057            if pred(k, v) {
    2058                *self.length -= 1;
    2059                let (kv, pos) = kv.remove_kv_tracking(
    2060                    || {
    2061                        // SAFETY: we will touch the root in a way that will not
    2062                        // invalidate the position returned.
    2063                        let root = unsafe { self.dormant_root.take().unwrap().awaken() };
    2064                        root.pop_internal_level(alloc.clone());
    2065                        self.dormant_root = Some(DormantMutRef::new(root).1);
    2066                    },
    2067                    alloc.clone(),
    2068                );
    2069                self.cur_leaf_edge = Some(pos);
    2070                return Some(kv);
    2071            }
    2072            self.cur_leaf_edge = Some(kv.next_leaf_edge());
    2073        }
    2074        None
    2075    }
    2076
    2077    /// Implementation of a typical `ExtractIf::size_hint` method.
    2078    pub(super) fn size_hint(&self) -> (usize, Option<usize>) {
    2079        // In most of the btree iterators, `self.length` is the number of elements
    2080        // yet to be visited. Here, it includes elements that were visited and that
    2081        // the predicate decided not to drain. Making this upper bound more tight
    2082        // during iteration would require an extra field.
    2083        (0, Some(*self.length))
    2084    }
    2085}
    2086
    2087#[stable(feature = "btree_extract_if", since = "1.91.0")]
    2088impl<K, V, R, F> FusedIterator for ExtractIf<'_, K, V, R, F>
    2089where
    2090    K: PartialOrd,
    2091    R: RangeBounds<K>,
    2092    F: FnMut(&K, &mut V) -> bool,
    2093{
    2094}
    2095
    2096#[stable(feature = "btree_range", since = "1.17.0")]
    2097impl<'a, K, V> Iterator for Range<'a, K, V> {
    2098    type Item = (&'a K, &'a V);
    2099
    2100    fn next(&mut self) -> Option<(&'a K, &'a V)> {
    2101        self.inner.next_checked()
    2102    }
    2103
    2104    fn last(mut self) -> Option<(&'a K, &'a V)> {
    2105        self.next_back()
    2106    }
    2107
    2108    fn min(mut self) -> Option<(&'a K, &'a V)>
    2109    where
    2110        (&'a K, &'a V): Ord,
    2111    {
    2112        self.next()
    2113    }
    2114
    2115    fn max(mut self) -> Option<(&'a K, &'a V)>
    2116    where
    2117        (&'a K, &'a V): Ord,
    2118    {
    2119        self.next_back()
    2120    }
    2121}
    2122
    2123#[stable(feature = "default_iters", since = "1.70.0")]
    2124impl<K, V> Default for Range<'_, K, V> {
    2125    /// Creates an empty `btree_map::Range`.
    2126    ///
    2127    /// ```
    2128    /// # use std::collections::btree_map;
    2129    /// let iter: btree_map::Range<'_, u8, u8> = Default::default();
    2130    /// assert_eq!(iter.count(), 0);
    2131    /// ```
    2132    fn default() -> Self {
    2133        Range { inner: Default::default() }
    2134    }
    2135}
    2136
    2137#[stable(feature = "default_iters_sequel", since = "1.82.0")]
    2138impl<K, V> Default for RangeMut<'_, K, V> {
    2139    /// Creates an empty `btree_map::RangeMut`.
    2140    ///
    2141    /// ```
    2142    /// # use std::collections::btree_map;
    2143    /// let iter: btree_map::RangeMut<'_, u8, u8> = Default::default();
    2144    /// assert_eq!(iter.count(), 0);
    2145    /// ```
    2146    fn default() -> Self {
    2147        RangeMut { inner: Default::default(), _marker: PhantomData }
    2148    }
    2149}
    2150
    2151#[stable(feature = "map_values_mut", since = "1.10.0")]
    2152impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
    2153    type Item = &'a mut V;
    2154
    2155    fn next(&mut self) -> Option<&'a mut V> {
    2156        self.inner.next().map(|(_, v)| v)
    2157    }
    2158
    2159    fn size_hint(&self) -> (usize, Option<usize>) {
    2160        self.inner.size_hint()
    2161    }
    2162
    2163    fn last(mut self) -> Option<&'a mut V> {
    2164        self.next_back()
    2165    }
    2166}
    2167
    2168#[stable(feature = "map_values_mut", since = "1.10.0")]
    2169impl<'a, K, V> DoubleEndedIterator for ValuesMut<'a, K, V> {
    2170    fn next_back(&mut self) -> Option<&'a mut V> {
    2171        self.inner.next_back().map(|(_, v)| v)
    2172    }
    2173}
    2174
    2175#[stable(feature = "map_values_mut", since = "1.10.0")]
    2176impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V> {
    2177    fn len(&self) -> usize {
    2178        self.inner.len()
    2179    }
    2180}
    2181
    2182#[unstable(feature = "trusted_len", issue = "37572")]
    2183unsafe impl<K, V> TrustedLen for ValuesMut<'_, K, V> {}
    2184
    2185#[stable(feature = "fused", since = "1.26.0")]
    2186impl<K, V> FusedIterator for ValuesMut<'_, K, V> {}
    2187
    2188#[stable(feature = "default_iters_sequel", since = "1.82.0")]
    2189impl<K, V> Default for ValuesMut<'_, K, V> {
    2190    /// Creates an empty `btree_map::ValuesMut`.
    2191    ///
    2192    /// ```
    2193    /// # use std::collections::btree_map;
    2194    /// let iter: btree_map::ValuesMut<'_, u8, u8> = Default::default();
    2195    /// assert_eq!(iter.count(), 0);
    2196    /// ```
    2197    fn default() -> Self {
    2198        ValuesMut { inner: Default::default() }
    2199    }
    2200}
    2201
    2202#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2203impl<K, V, A: Allocator + Clone> Iterator for IntoKeys<K, V, A> {
    2204    type Item = K;
    2205
    2206    fn next(&mut self) -> Option<K> {
    2207        self.inner.next().map(|(k, _)| k)
    2208    }
    2209
    2210    fn size_hint(&self) -> (usize, Option<usize>) {
    2211        self.inner.size_hint()
    2212    }
    2213
    2214    fn last(mut self) -> Option<K> {
    2215        self.next_back()
    2216    }
    2217
    2218    fn min(mut self) -> Option<K>
    2219    where
    2220        K: Ord,
    2221    {
    2222        self.next()
    2223    }
    2224
    2225    fn max(mut self) -> Option<K>
    2226    where
    2227        K: Ord,
    2228    {
    2229        self.next_back()
    2230    }
    2231}
    2232
    2233#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2234impl<K, V, A: Allocator + Clone> DoubleEndedIterator for IntoKeys<K, V, A> {
    2235    fn next_back(&mut self) -> Option<K> {
    2236        self.inner.next_back().map(|(k, _)| k)
    2237    }
    2238}
    2239
    2240#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2241impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoKeys<K, V, A> {
    2242    fn len(&self) -> usize {
    2243        self.inner.len()
    2244    }
    2245}
    2246
    2247#[unstable(feature = "trusted_len", issue = "37572")]
    2248unsafe impl<K, V, A: Allocator + Clone> TrustedLen for IntoKeys<K, V, A> {}
    2249
    2250#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2251impl<K, V, A: Allocator + Clone> FusedIterator for IntoKeys<K, V, A> {}
    2252
    2253#[stable(feature = "default_iters", since = "1.70.0")]
    2254impl<K, V, A> Default for IntoKeys<K, V, A>
    2255where
    2256    A: Allocator + Default + Clone,
    2257{
    2258    /// Creates an empty `btree_map::IntoKeys`.
    2259    ///
    2260    /// ```
    2261    /// # use std::collections::btree_map;
    2262    /// let iter: btree_map::IntoKeys<u8, u8> = Default::default();
    2263    /// assert_eq!(iter.len(), 0);
    2264    /// ```
    2265    fn default() -> Self {
    2266        IntoKeys { inner: Default::default() }
    2267    }
    2268}
    2269
    2270#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2271impl<K, V, A: Allocator + Clone> Iterator for IntoValues<K, V, A> {
    2272    type Item = V;
    2273
    2274    fn next(&mut self) -> Option<V> {
    2275        self.inner.next().map(|(_, v)| v)
    2276    }
    2277
    2278    fn size_hint(&self) -> (usize, Option<usize>) {
    2279        self.inner.size_hint()
    2280    }
    2281
    2282    fn last(mut self) -> Option<V> {
    2283        self.next_back()
    2284    }
    2285}
    2286
    2287#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2288impl<K, V, A: Allocator + Clone> DoubleEndedIterator for IntoValues<K, V, A> {
    2289    fn next_back(&mut self) -> Option<V> {
    2290        self.inner.next_back().map(|(_, v)| v)
    2291    }
    2292}
    2293
    2294#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2295impl<K, V, A: Allocator + Clone> ExactSizeIterator for IntoValues<K, V, A> {
    2296    fn len(&self) -> usize {
    2297        self.inner.len()
    2298    }
    2299}
    2300
    2301#[unstable(feature = "trusted_len", issue = "37572")]
    2302unsafe impl<K, V, A: Allocator + Clone> TrustedLen for IntoValues<K, V, A> {}
    2303
    2304#[stable(feature = "map_into_keys_values", since = "1.54.0")]
    2305impl<K, V, A: Allocator + Clone> FusedIterator for IntoValues<K, V, A> {}
    2306
    2307#[stable(feature = "default_iters", since = "1.70.0")]
    2308impl<K, V, A> Default for IntoValues<K, V, A>
    2309where
    2310    A: Allocator + Default + Clone,
    2311{
    2312    /// Creates an empty `btree_map::IntoValues`.
    2313    ///
    2314    /// ```
    2315    /// # use std::collections::btree_map;
    2316    /// let iter: btree_map::IntoValues<u8, u8> = Default::default();
    2317    /// assert_eq!(iter.len(), 0);
    2318    /// ```
    2319    fn default() -> Self {
    2320        IntoValues { inner: Default::default() }
    2321    }
    2322}
    2323
    2324#[stable(feature = "btree_range", since = "1.17.0")]
    2325impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
    2326    fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
    2327        self.inner.next_back_checked()
    2328    }
    2329}
    2330
    2331#[stable(feature = "fused", since = "1.26.0")]
    2332impl<K, V> FusedIterator for Range<'_, K, V> {}
    2333
    2334#[stable(feature = "btree_range", since = "1.17.0")]
    2335impl<K, V> Clone for Range<'_, K, V> {
    2336    fn clone(&self) -> Self {
    2337        Range { inner: self.inner.clone() }
    2338    }
    2339}
    2340
    2341#[stable(feature = "btree_range", since = "1.17.0")]
    2342impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
    2343    type Item = (&'a K, &'a mut V);
    2344
    2345    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
    2346        self.inner.next_checked()
    2347    }
    2348
    2349    fn last(mut self) -> Option<(&'a K, &'a mut V)> {
    2350        self.next_back()
    2351    }
    2352
    2353    fn min(mut self) -> Option<(&'a K, &'a mut V)>
    2354    where
    2355        (&'a K, &'a mut V): Ord,
    2356    {
    2357        self.next()
    2358    }
    2359
    2360    fn max(mut self) -> Option<(&'a K, &'a mut V)>
    2361    where
    2362        (&'a K, &'a mut V): Ord,
    2363    {
    2364        self.next_back()
    2365    }
    2366}
    2367
    2368#[stable(feature = "btree_range", since = "1.17.0")]
    2369impl<'a, K, V> DoubleEndedIterator for RangeMut<'a, K, V> {
    2370    fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
    2371        self.inner.next_back_checked()
    2372    }
    2373}
    2374
    2375#[stable(feature = "fused", since = "1.26.0")]
    2376impl<K, V> FusedIterator for RangeMut<'_, K, V> {}
    2377
    2378#[stable(feature = "rust1", since = "1.0.0")]
    2379impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V> {
    2380    /// Constructs a `BTreeMap<K, V>` from an iterator of key-value pairs.
    2381    ///
    2382    /// If the iterator produces any pairs with equal keys,
    2383    /// all but one of the corresponding values will be dropped.
    2384    fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> BTreeMap<K, V> {
    2385        let mut inputs: Vec<_> = iter.into_iter().collect();
    2386
    2387        if inputs.is_empty() {
    2388            return BTreeMap::new();
    2389        }
    2390
    2391        // use stable sort to preserve the insertion order.
    2392        inputs.sort_by(|a, b| a.0.cmp(&b.0));
    2393        BTreeMap::bulk_build_from_sorted_iter(inputs, Global)
    2394    }
    2395}
    2396
    2397#[stable(feature = "rust1", since = "1.0.0")]
    2398impl<K: Ord, V, A: Allocator + Clone> Extend<(K, V)> for BTreeMap<K, V, A> {
    2399    #[inline]
    2400    fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T) {
    2401        iter.into_iter().for_each(move |(k, v)| {
    2402            self.insert(k, v);
    2403        });
    2404    }
    2405
    2406    #[inline]
    2407    fn extend_one(&mut self, (k, v): (K, V)) {
    2408        self.insert(k, v);
    2409    }
    2410}
    2411
    2412#[stable(feature = "extend_ref", since = "1.2.0")]
    2413impl<'a, K: Ord + Copy, V: Copy, A: Allocator + Clone> Extend<(&'a K, &'a V)>
    2414    for BTreeMap<K, V, A>
    2415{
    2416    fn extend<I: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: I) {
    2417        self.extend(iter.into_iter().map(|(&key, &value)| (key, value)));
    2418    }
    2419
    2420    #[inline]
    2421    fn extend_one(&mut self, (&k, &v): (&'a K, &'a V)) {
    2422        self.insert(k, v);
    2423    }
    2424}
    2425
    2426#[stable(feature = "rust1", since = "1.0.0")]
    2427impl<K: Hash, V: Hash, A: Allocator + Clone> Hash for BTreeMap<K, V, A> {
    2428    fn hash<H: Hasher>(&self, state: &mut H) {
    2429        state.write_length_prefix(self.len());
    2430        for elt in self {
    2431            elt.hash(state);
    2432        }
    2433    }
    2434}
    2435
    2436#[stable(feature = "rust1", since = "1.0.0")]
    2437impl<K, V> Default for BTreeMap<K, V> {
    2438    /// Creates an empty `BTreeMap`.
    2439    fn default() -> BTreeMap<K, V> {
    2440        BTreeMap::new()
    2441    }
    2442}
    2443
    2444#[stable(feature = "rust1", since = "1.0.0")]
    2445impl<K: PartialEq, V: PartialEq, A: Allocator + Clone> PartialEq for BTreeMap<K, V, A> {
    2446    fn eq(&self, other: &BTreeMap<K, V, A>) -> bool {
    2447        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a == b)
    2448    }
    2449}
    2450
    2451#[stable(feature = "rust1", since = "1.0.0")]
    2452impl<K: Eq, V: Eq, A: Allocator + Clone> Eq for BTreeMap<K, V, A> {}
    2453
    2454#[stable(feature = "rust1", since = "1.0.0")]
    2455impl<K: PartialOrd, V: PartialOrd, A: Allocator + Clone> PartialOrd for BTreeMap<K, V, A> {
    2456    #[inline]
    2457    fn partial_cmp(&self, other: &BTreeMap<K, V, A>) -> Option<Ordering> {
    2458        self.iter().partial_cmp(other.iter())
    2459    }
    2460}
    2461
    2462#[stable(feature = "rust1", since = "1.0.0")]
    2463impl<K: Ord, V: Ord, A: Allocator + Clone> Ord for BTreeMap<K, V, A> {
    2464    #[inline]
    2465    fn cmp(&self, other: &BTreeMap<K, V, A>) -> Ordering {
    2466        self.iter().cmp(other.iter())
    2467    }
    2468}
    2469
    2470#[stable(feature = "rust1", since = "1.0.0")]
    2471impl<K: Debug, V: Debug, A: Allocator + Clone> Debug for BTreeMap<K, V, A> {
    2472    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    2473        f.debug_map().entries(self.iter()).finish()
    2474    }
    2475}
    2476
    2477#[stable(feature = "rust1", since = "1.0.0")]
    2478impl<K, Q: ?Sized, V, A: Allocator + Clone> Index<&Q> for BTreeMap<K, V, A>
    2479where
    2480    K: Borrow<Q> + Ord,
    2481    Q: Ord,
    2482{
    2483    type Output = V;
    2484
    2485    /// Returns a reference to the value corresponding to the supplied key.
    2486    ///
    2487    /// # Panics
    2488    ///
    2489    /// Panics if the key is not present in the `BTreeMap`.
    2490    #[inline]
    2491    fn index(&self, key: &Q) -> &V {
    2492        self.get(key).expect("no entry found for key")
    2493    }
    2494}
    2495
    2496#[stable(feature = "std_collections_from_array", since = "1.56.0")]
    2497impl<K: Ord, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V> {
    2498    /// Converts a `[(K, V); N]` into a `BTreeMap<K, V>`.
    2499    ///
    2500    /// If any entries in the array have equal keys,
    2501    /// all but one of the corresponding values will be dropped.
    2502    ///
    2503    /// ```
    2504    /// use std::collections::BTreeMap;
    2505    ///
    2506    /// let map1 = BTreeMap::from([(1, 2), (3, 4)]);
    2507    /// let map2: BTreeMap<_, _> = [(1, 2), (3, 4)].into();
    2508    /// assert_eq!(map1, map2);
    2509    /// ```
    2510    fn from(mut arr: [(K, V); N]) -> Self {
    2511        if N == 0 {
    2512            return BTreeMap::new();
    2513        }
    2514
    2515        // use stable sort to preserve the insertion order.
    2516        arr.sort_by(|a, b| a.0.cmp(&b.0));
    2517        BTreeMap::bulk_build_from_sorted_iter(arr, Global)
    2518    }
    2519}
    2520
    2521impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
    2522    /// Gets an iterator over the entries of the map, sorted by key.
    2523    ///
    2524    /// # Examples
    2525    ///
    2526    /// ```
    2527    /// use std::collections::BTreeMap;
    2528    ///
    2529    /// let mut map = BTreeMap::new();
    2530    /// map.insert(3, "c");
    2531    /// map.insert(2, "b");
    2532    /// map.insert(1, "a");
    2533    ///
    2534    /// for (key, value) in map.iter() {
    2535    ///     println!("{key}: {value}");
    2536    /// }
    2537    ///
    2538    /// let (first_key, first_value) = map.iter().next().unwrap();
    2539    /// assert_eq!((*first_key, *first_value), (1, "a"));
    2540    /// ```
    2541    #[stable(feature = "rust1", since = "1.0.0")]
    2542    pub fn iter(&self) -> Iter<'_, K, V> {
    2543        if let Some(root) = &self.root {
    2544            let full_range = root.reborrow().full_range();
    2545
    2546            Iter { range: full_range, length: self.length }
    2547        } else {
    2548            Iter { range: LazyLeafRange::none(), length: 0 }
    2549        }
    2550    }
    2551
    2552    /// Gets a mutable iterator over the entries of the map, sorted by key.
    2553    ///
    2554    /// # Examples
    2555    ///
    2556    /// ```
    2557    /// use std::collections::BTreeMap;
    2558    ///
    2559    /// let mut map = BTreeMap::from([
    2560    ///    ("a", 1),
    2561    ///    ("b", 2),
    2562    ///    ("c", 3),
    2563    /// ]);
    2564    ///
    2565    /// // add 10 to the value if the key isn't "a"
    2566    /// for (key, value) in map.iter_mut() {
    2567    ///     if key != &"a" {
    2568    ///         *value += 10;
    2569    ///     }
    2570    /// }
    2571    /// ```
    2572    #[stable(feature = "rust1", since = "1.0.0")]
    2573    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
    2574        if let Some(root) = &mut self.root {
    2575            let full_range = root.borrow_valmut().full_range();
    2576
    2577            IterMut { range: full_range, length: self.length, _marker: PhantomData }
    2578        } else {
    2579            IterMut { range: LazyLeafRange::none(), length: 0, _marker: PhantomData }
    2580        }
    2581    }
    2582
    2583    /// Gets an iterator over the keys of the map, in sorted order.
    2584    ///
    2585    /// # Examples
    2586    ///
    2587    /// ```
    2588    /// use std::collections::BTreeMap;
    2589    ///
    2590    /// let mut a = BTreeMap::new();
    2591    /// a.insert(2, "b");
    2592    /// a.insert(1, "a");
    2593    ///
    2594    /// let keys: Vec<_> = a.keys().cloned().collect();
    2595    /// assert_eq!(keys, [1, 2]);
    2596    /// ```
    2597    #[stable(feature = "rust1", since = "1.0.0")]
    2598    pub fn keys(&self) -> Keys<'_, K, V> {
    2599        Keys { inner: self.iter() }
    2600    }
    2601
    2602    /// Gets an iterator over the values of the map, in order by key.
    2603    ///
    2604    /// # Examples
    2605    ///
    2606    /// ```
    2607    /// use std::collections::BTreeMap;
    2608    ///
    2609    /// let mut a = BTreeMap::new();
    2610    /// a.insert(1, "hello");
    2611    /// a.insert(2, "goodbye");
    2612    ///
    2613    /// let values: Vec<&str> = a.values().cloned().collect();
    2614    /// assert_eq!(values, ["hello", "goodbye"]);
    2615    /// ```
    2616    #[stable(feature = "rust1", since = "1.0.0")]
    2617    pub fn values(&self) -> Values<'_, K, V> {
    2618        Values { inner: self.iter() }
    2619    }
    2620
    2621    /// Gets a mutable iterator over the values of the map, in order by key.
    2622    ///
    2623    /// # Examples
    2624    ///
    2625    /// ```
    2626    /// use std::collections::BTreeMap;
    2627    ///
    2628    /// let mut a = BTreeMap::new();
    2629    /// a.insert(1, String::from("hello"));
    2630    /// a.insert(2, String::from("goodbye"));
    2631    ///
    2632    /// for value in a.values_mut() {
    2633    ///     value.push_str("!");
    2634    /// }
    2635    ///
    2636    /// let values: Vec<String> = a.values().cloned().collect();
    2637    /// assert_eq!(values, [String::from("hello!"),
    2638    ///                     String::from("goodbye!")]);
    2639    /// ```
    2640    #[stable(feature = "map_values_mut", since = "1.10.0")]
    2641    pub fn values_mut(&mut self) -> ValuesMut<'_, K, V> {
    2642        ValuesMut { inner: self.iter_mut() }
    2643    }
    2644
    2645    /// Returns the number of elements in the map.
    2646    ///
    2647    /// # Examples
    2648    ///
    2649    /// ```
    2650    /// use std::collections::BTreeMap;
    2651    ///
    2652    /// let mut a = BTreeMap::new();
    2653    /// assert_eq!(a.len(), 0);
    2654    /// a.insert(1, "a");
    2655    /// assert_eq!(a.len(), 1);
    2656    /// ```
    2657    #[must_use]
    2658    #[stable(feature = "rust1", since = "1.0.0")]
    2659    #[rustc_const_unstable(
    2660        feature = "const_btree_len",
    2661        issue = "71835",
    2662        implied_by = "const_btree_new"
    2663    )]
    2664    #[rustc_confusables("length", "size")]
    2665    pub const fn len(&self) -> usize {
    2666        self.length
    2667    }
    2668
    2669    /// Returns `true` if the map contains no elements.
    2670    ///
    2671    /// # Examples
    2672    ///
    2673    /// ```
    2674    /// use std::collections::BTreeMap;
    2675    ///
    2676    /// let mut a = BTreeMap::new();
    2677    /// assert!(a.is_empty());
    2678    /// a.insert(1, "a");
    2679    /// assert!(!a.is_empty());
    2680    /// ```
    2681    #[must_use]
    2682    #[stable(feature = "rust1", since = "1.0.0")]
    2683    #[rustc_const_unstable(
    2684        feature = "const_btree_len",
    2685        issue = "71835",
    2686        implied_by = "const_btree_new"
    2687    )]
    2688    pub const fn is_empty(&self) -> bool {
    2689        self.len() == 0
    2690    }
    2691
    2692    /// Returns a [`Cursor`] pointing at the gap before the smallest key
    2693    /// greater than the given bound.
    2694    ///
    2695    /// Passing `Bound::Included(x)` will return a cursor pointing to the
    2696    /// gap before the smallest key greater than or equal to `x`.
    2697    ///
    2698    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
    2699    /// gap before the smallest key greater than `x`.
    2700    ///
    2701    /// Passing `Bound::Unbounded` will return a cursor pointing to the
    2702    /// gap before the smallest key in the map.
    2703    ///
    2704    /// # Examples
    2705    ///
    2706    /// ```
    2707    /// #![feature(btree_cursors)]
    2708    ///
    2709    /// use std::collections::BTreeMap;
    2710    /// use std::ops::Bound;
    2711    ///
    2712    /// let map = BTreeMap::from([
    2713    ///     (1, "a"),
    2714    ///     (2, "b"),
    2715    ///     (3, "c"),
    2716    ///     (4, "d"),
    2717    /// ]);
    2718    ///
    2719    /// let cursor = map.lower_bound(Bound::Included(&2));
    2720    /// assert_eq!(cursor.peek_prev(), Some((&1, &"a")));
    2721    /// assert_eq!(cursor.peek_next(), Some((&2, &"b")));
    2722    ///
    2723    /// let cursor = map.lower_bound(Bound::Excluded(&2));
    2724    /// assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
    2725    /// assert_eq!(cursor.peek_next(), Some((&3, &"c")));
    2726    ///
    2727    /// let cursor = map.lower_bound(Bound::Unbounded);
    2728    /// assert_eq!(cursor.peek_prev(), None);
    2729    /// assert_eq!(cursor.peek_next(), Some((&1, &"a")));
    2730    /// ```
    2731    #[unstable(feature = "btree_cursors", issue = "107540")]
    2732    pub fn lower_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
    2733    where
    2734        K: Borrow<Q> + Ord,
    2735        Q: Ord,
    2736    {
    2737        let root_node = match self.root.as_ref() {
    2738            None => return Cursor { current: None, root: None },
    2739            Some(root) => root.reborrow(),
    2740        };
    2741        let edge = root_node.lower_bound(SearchBound::from_range(bound));
    2742        Cursor { current: Some(edge), root: self.root.as_ref() }
    2743    }
    2744
    2745    /// Returns a [`CursorMut`] pointing at the gap before the smallest key
    2746    /// greater than the given bound.
    2747    ///
    2748    /// Passing `Bound::Included(x)` will return a cursor pointing to the
    2749    /// gap before the smallest key greater than or equal to `x`.
    2750    ///
    2751    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
    2752    /// gap before the smallest key greater than `x`.
    2753    ///
    2754    /// Passing `Bound::Unbounded` will return a cursor pointing to the
    2755    /// gap before the smallest key in the map.
    2756    ///
    2757    /// # Examples
    2758    ///
    2759    /// ```
    2760    /// #![feature(btree_cursors)]
    2761    ///
    2762    /// use std::collections::BTreeMap;
    2763    /// use std::ops::Bound;
    2764    ///
    2765    /// let mut map = BTreeMap::from([
    2766    ///     (1, "a"),
    2767    ///     (2, "b"),
    2768    ///     (3, "c"),
    2769    ///     (4, "d"),
    2770    /// ]);
    2771    ///
    2772    /// let mut cursor = map.lower_bound_mut(Bound::Included(&2));
    2773    /// assert_eq!(cursor.peek_prev(), Some((&1, &mut "a")));
    2774    /// assert_eq!(cursor.peek_next(), Some((&2, &mut "b")));
    2775    ///
    2776    /// let mut cursor = map.lower_bound_mut(Bound::Excluded(&2));
    2777    /// assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
    2778    /// assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));
    2779    ///
    2780    /// let mut cursor = map.lower_bound_mut(Bound::Unbounded);
    2781    /// assert_eq!(cursor.peek_prev(), None);
    2782    /// assert_eq!(cursor.peek_next(), Some((&1, &mut "a")));
    2783    /// ```
    2784    #[unstable(feature = "btree_cursors", issue = "107540")]
    2785    pub fn lower_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
    2786    where
    2787        K: Borrow<Q> + Ord,
    2788        Q: Ord,
    2789    {
    2790        let (root, dormant_root) = DormantMutRef::new(&mut self.root);
    2791        let root_node = match root.as_mut() {
    2792            None => {
    2793                return CursorMut {
    2794                    inner: CursorMutKey {
    2795                        current: None,
    2796                        root: dormant_root,
    2797                        length: &mut self.length,
    2798                        alloc: &mut *self.alloc,
    2799                    },
    2800                };
    2801            }
    2802            Some(root) => root.borrow_mut(),
    2803        };
    2804        let edge = root_node.lower_bound(SearchBound::from_range(bound));
    2805        CursorMut {
    2806            inner: CursorMutKey {
    2807                current: Some(edge),
    2808                root: dormant_root,
    2809                length: &mut self.length,
    2810                alloc: &mut *self.alloc,
    2811            },
    2812        }
    2813    }
    2814
    2815    /// Returns a [`Cursor`] pointing at the gap after the greatest key
    2816    /// smaller than the given bound.
    2817    ///
    2818    /// Passing `Bound::Included(x)` will return a cursor pointing to the
    2819    /// gap after the greatest key smaller than or equal to `x`.
    2820    ///
    2821    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
    2822    /// gap after the greatest key smaller than `x`.
    2823    ///
    2824    /// Passing `Bound::Unbounded` will return a cursor pointing to the
    2825    /// gap after the greatest key in the map.
    2826    ///
    2827    /// # Examples
    2828    ///
    2829    /// ```
    2830    /// #![feature(btree_cursors)]
    2831    ///
    2832    /// use std::collections::BTreeMap;
    2833    /// use std::ops::Bound;
    2834    ///
    2835    /// let map = BTreeMap::from([
    2836    ///     (1, "a"),
    2837    ///     (2, "b"),
    2838    ///     (3, "c"),
    2839    ///     (4, "d"),
    2840    /// ]);
    2841    ///
    2842    /// let cursor = map.upper_bound(Bound::Included(&3));
    2843    /// assert_eq!(cursor.peek_prev(), Some((&3, &"c")));
    2844    /// assert_eq!(cursor.peek_next(), Some((&4, &"d")));
    2845    ///
    2846    /// let cursor = map.upper_bound(Bound::Excluded(&3));
    2847    /// assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
    2848    /// assert_eq!(cursor.peek_next(), Some((&3, &"c")));
    2849    ///
    2850    /// let cursor = map.upper_bound(Bound::Unbounded);
    2851    /// assert_eq!(cursor.peek_prev(), Some((&4, &"d")));
    2852    /// assert_eq!(cursor.peek_next(), None);
    2853    /// ```
    2854    #[unstable(feature = "btree_cursors", issue = "107540")]
    2855    pub fn upper_bound<Q: ?Sized>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
    2856    where
    2857        K: Borrow<Q> + Ord,
    2858        Q: Ord,
    2859    {
    2860        let root_node = match self.root.as_ref() {
    2861            None => return Cursor { current: None, root: None },
    2862            Some(root) => root.reborrow(),
    2863        };
    2864        let edge = root_node.upper_bound(SearchBound::from_range(bound));
    2865        Cursor { current: Some(edge), root: self.root.as_ref() }
    2866    }
    2867
    2868    /// Returns a [`CursorMut`] pointing at the gap after the greatest key
    2869    /// smaller than the given bound.
    2870    ///
    2871    /// Passing `Bound::Included(x)` will return a cursor pointing to the
    2872    /// gap after the greatest key smaller than or equal to `x`.
    2873    ///
    2874    /// Passing `Bound::Excluded(x)` will return a cursor pointing to the
    2875    /// gap after the greatest key smaller than `x`.
    2876    ///
    2877    /// Passing `Bound::Unbounded` will return a cursor pointing to the
    2878    /// gap after the greatest key in the map.
    2879    ///
    2880    /// # Examples
    2881    ///
    2882    /// ```
    2883    /// #![feature(btree_cursors)]
    2884    ///
    2885    /// use std::collections::BTreeMap;
    2886    /// use std::ops::Bound;
    2887    ///
    2888    /// let mut map = BTreeMap::from([
    2889    ///     (1, "a"),
    2890    ///     (2, "b"),
    2891    ///     (3, "c"),
    2892    ///     (4, "d"),
    2893    /// ]);
    2894    ///
    2895    /// let mut cursor = map.upper_bound_mut(Bound::Included(&3));
    2896    /// assert_eq!(cursor.peek_prev(), Some((&3, &mut "c")));
    2897    /// assert_eq!(cursor.peek_next(), Some((&4, &mut "d")));
    2898    ///
    2899    /// let mut cursor = map.upper_bound_mut(Bound::Excluded(&3));
    2900    /// assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
    2901    /// assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));
    2902    ///
    2903    /// let mut cursor = map.upper_bound_mut(Bound::Unbounded);
    2904    /// assert_eq!(cursor.peek_prev(), Some((&4, &mut "d")));
    2905    /// assert_eq!(cursor.peek_next(), None);
    2906    /// ```
    2907    #[unstable(feature = "btree_cursors", issue = "107540")]
    2908    pub fn upper_bound_mut<Q: ?Sized>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
    2909    where
    2910        K: Borrow<Q> + Ord,
    2911        Q: Ord,
    2912    {
    2913        let (root, dormant_root) = DormantMutRef::new(&mut self.root);
    2914        let root_node = match root.as_mut() {
    2915            None => {
    2916                return CursorMut {
    2917                    inner: CursorMutKey {
    2918                        current: None,
    2919                        root: dormant_root,
    2920                        length: &mut self.length,
    2921                        alloc: &mut *self.alloc,
    2922                    },
    2923                };
    2924            }
    2925            Some(root) => root.borrow_mut(),
    2926        };
    2927        let edge = root_node.upper_bound(SearchBound::from_range(bound));
    2928        CursorMut {
    2929            inner: CursorMutKey {
    2930                current: Some(edge),
    2931                root: dormant_root,
    2932                length: &mut self.length,
    2933                alloc: &mut *self.alloc,
    2934            },
    2935        }
    2936    }
    2937}
    2938
    2939/// A cursor over a `BTreeMap`.
    2940///
    2941/// A `Cursor` is like an iterator, except that it can freely seek back-and-forth.
    2942///
    2943/// Cursors always point to a gap between two elements in the map, and can
    2944/// operate on the two immediately adjacent elements.
    2945///
    2946/// A `Cursor` is created with the [`BTreeMap::lower_bound`] and [`BTreeMap::upper_bound`] methods.
    2947#[unstable(feature = "btree_cursors", issue = "107540")]
    2948pub struct Cursor<'a, K: 'a, V: 'a> {
    2949    // If current is None then it means the tree has not been allocated yet.
    2950    current: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
    2951    root: Option<&'a node::Root<K, V>>,
    2952}
    2953
    2954#[unstable(feature = "btree_cursors", issue = "107540")]
    2955impl<K, V> Clone for Cursor<'_, K, V> {
    2956    fn clone(&self) -> Self {
    2957        let Cursor { current, root } = *self;
    2958        Cursor { current, root }
    2959    }
    2960}
    2961
    2962#[unstable(feature = "btree_cursors", issue = "107540")]
    2963impl<K: Debug, V: Debug> Debug for Cursor<'_, K, V> {
    2964    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    2965        f.write_str("Cursor")
    2966    }
    2967}
    2968
    2969/// A cursor over a `BTreeMap` with editing operations.
    2970///
    2971/// A `Cursor` is like an iterator, except that it can freely seek back-and-forth, and can
    2972/// safely mutate the map during iteration. This is because the lifetime of its yielded
    2973/// references is tied to its own lifetime, instead of just the underlying map. This means
    2974/// cursors cannot yield multiple elements at once.
    2975///
    2976/// Cursors always point to a gap between two elements in the map, and can
    2977/// operate on the two immediately adjacent elements.
    2978///
    2979/// A `CursorMut` is created with the [`BTreeMap::lower_bound_mut`] and [`BTreeMap::upper_bound_mut`]
    2980/// methods.
    2981#[unstable(feature = "btree_cursors", issue = "107540")]
    2982pub struct CursorMut<
    2983    'a,
    2984    K: 'a,
    2985    V: 'a,
    2986    #[unstable(feature = "allocator_api", issue = "32838")] A = Global,
    2987> {
    2988    inner: CursorMutKey<'a, K, V, A>,
    2989}
    2990
    2991#[unstable(feature = "btree_cursors", issue = "107540")]
    2992impl<K: Debug, V: Debug, A> Debug for CursorMut<'_, K, V, A> {
    2993    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    2994        f.write_str("CursorMut")
    2995    }
    2996}
    2997
    2998/// A cursor over a `BTreeMap` with editing operations, and which allows
    2999/// mutating the key of elements.
    3000///
    3001/// A `Cursor` is like an iterator, except that it can freely seek back-and-forth, and can
    3002/// safely mutate the map during iteration. This is because the lifetime of its yielded
    3003/// references is tied to its own lifetime, instead of just the underlying map. This means
    3004/// cursors cannot yield multiple elements at once.
    3005///
    3006/// Cursors always point to a gap between two elements in the map, and can
    3007/// operate on the two immediately adjacent elements.
    3008///
    3009/// A `CursorMutKey` is created from a [`CursorMut`] with the
    3010/// [`CursorMut::with_mutable_key`] method.
    3011///
    3012/// # Safety
    3013///
    3014/// Since this cursor allows mutating keys, you must ensure that the `BTreeMap`
    3015/// invariants are maintained. Specifically:
    3016///
    3017/// * The key of the newly inserted element must be unique in the tree.
    3018/// * All keys in the tree must remain in sorted order.
    3019#[unstable(feature = "btree_cursors", issue = "107540")]
    3020pub struct CursorMutKey<
    3021    'a,
    3022    K: 'a,
    3023    V: 'a,
    3024    #[unstable(feature = "allocator_api", issue = "32838")] A = Global,
    3025> {
    3026    // If current is None then it means the tree has not been allocated yet.
    3027    current: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
    3028    root: DormantMutRef<'a, Option<node::Root<K, V>>>,
    3029    length: &'a mut usize,
    3030    alloc: &'a mut A,
    3031}
    3032
    3033#[unstable(feature = "btree_cursors", issue = "107540")]
    3034impl<K: Debug, V: Debug, A> Debug for CursorMutKey<'_, K, V, A> {
    3035    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    3036        f.write_str("CursorMutKey")
    3037    }
    3038}
    3039
    3040impl<'a, K, V> Cursor<'a, K, V> {
    3041    /// Advances the cursor to the next gap, returning the key and value of the
    3042    /// element that it moved over.
    3043    ///
    3044    /// If the cursor is already at the end of the map then `None` is returned
    3045    /// and the cursor is not moved.
    3046    #[unstable(feature = "btree_cursors", issue = "107540")]
    3047    pub fn next(&mut self) -> Option<(&'a K, &'a V)> {
    3048        let current = self.current.take()?;
    3049        match current.next_kv() {
    3050            Ok(kv) => {
    3051                let result = kv.into_kv();
    3052                self.current = Some(kv.next_leaf_edge());
    3053                Some(result)
    3054            }
    3055            Err(root) => {
    3056                self.current = Some(root.last_leaf_edge());
    3057                None
    3058            }
    3059        }
    3060    }
    3061
    3062    /// Advances the cursor to the previous gap, returning the key and value of
    3063    /// the element that it moved over.
    3064    ///
    3065    /// If the cursor is already at the start of the map then `None` is returned
    3066    /// and the cursor is not moved.
    3067    #[unstable(feature = "btree_cursors", issue = "107540")]
    3068    pub fn prev(&mut self) -> Option<(&'a K, &'a V)> {
    3069        let current = self.current.take()?;
    3070        match current.next_back_kv() {
    3071            Ok(kv) => {
    3072                let result = kv.into_kv();
    3073                self.current = Some(kv.next_back_leaf_edge());
    3074                Some(result)
    3075            }
    3076            Err(root) => {
    3077                self.current = Some(root.first_leaf_edge());
    3078                None
    3079            }
    3080        }
    3081    }
    3082
    3083    /// Returns a reference to the key and value of the next element without
    3084    /// moving the cursor.
    3085    ///
    3086    /// If the cursor is at the end of the map then `None` is returned.
    3087    #[unstable(feature = "btree_cursors", issue = "107540")]
    3088    pub fn peek_next(&self) -> Option<(&'a K, &'a V)> {
    3089        self.clone().next()
    3090    }
    3091
    3092    /// Returns a reference to the key and value of the previous element
    3093    /// without moving the cursor.
    3094    ///
    3095    /// If the cursor is at the start of the map then `None` is returned.
    3096    #[unstable(feature = "btree_cursors", issue = "107540")]
    3097    pub fn peek_prev(&self) -> Option<(&'a K, &'a V)> {
    3098        self.clone().prev()
    3099    }
    3100}
    3101
    3102impl<'a, K, V, A> CursorMut<'a, K, V, A> {
    3103    /// Advances the cursor to the next gap, returning the key and value of the
    3104    /// element that it moved over.
    3105    ///
    3106    /// If the cursor is already at the end of the map then `None` is returned
    3107    /// and the cursor is not moved.
    3108    #[unstable(feature = "btree_cursors", issue = "107540")]
    3109    pub fn next(&mut self) -> Option<(&K, &mut V)> {
    3110        let (k, v) = self.inner.next()?;
    3111        Some((&*k, v))
    3112    }
    3113
    3114    /// Advances the cursor to the previous gap, returning the key and value of
    3115    /// the element that it moved over.
    3116    ///
    3117    /// If the cursor is already at the start of the map then `None` is returned
    3118    /// and the cursor is not moved.
    3119    #[unstable(feature = "btree_cursors", issue = "107540")]
    3120    pub fn prev(&mut self) -> Option<(&K, &mut V)> {
    3121        let (k, v) = self.inner.prev()?;
    3122        Some((&*k, v))
    3123    }
    3124
    3125    /// Returns a reference to the key and value of the next element without
    3126    /// moving the cursor.
    3127    ///
    3128    /// If the cursor is at the end of the map then `None` is returned.
    3129    #[unstable(feature = "btree_cursors", issue = "107540")]
    3130    pub fn peek_next(&mut self) -> Option<(&K, &mut V)> {
    3131        let (k, v) = self.inner.peek_next()?;
    3132        Some((&*k, v))
    3133    }
    3134
    3135    /// Returns a reference to the key and value of the previous element
    3136    /// without moving the cursor.
    3137    ///
    3138    /// If the cursor is at the start of the map then `None` is returned.
    3139    #[unstable(feature = "btree_cursors", issue = "107540")]
    3140    pub fn peek_prev(&mut self) -> Option<(&K, &mut V)> {
    3141        let (k, v) = self.inner.peek_prev()?;
    3142        Some((&*k, v))
    3143    }
    3144
    3145    /// Returns a read-only cursor pointing to the same location as the
    3146    /// `CursorMut`.
    3147    ///
    3148    /// The lifetime of the returned `Cursor` is bound to that of the
    3149    /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
    3150    /// `CursorMut` is frozen for the lifetime of the `Cursor`.
    3151    #[unstable(feature = "btree_cursors", issue = "107540")]
    3152    pub fn as_cursor(&self) -> Cursor<'_, K, V> {
    3153        self.inner.as_cursor()
    3154    }
    3155
    3156    /// Converts the cursor into a [`CursorMutKey`], which allows mutating
    3157    /// the key of elements in the tree.
    3158    ///
    3159    /// # Safety
    3160    ///
    3161    /// Since this cursor allows mutating keys, you must ensure that the `BTreeMap`
    3162    /// invariants are maintained. Specifically:
    3163    ///
    3164    /// * The key of the newly inserted element must be unique in the tree.
    3165    /// * All keys in the tree must remain in sorted order.
    3166    #[unstable(feature = "btree_cursors", issue = "107540")]
    3167    pub unsafe fn with_mutable_key(self) -> CursorMutKey<'a, K, V, A> {
    3168        self.inner
    3169    }
    3170}
    3171
    3172impl<'a, K, V, A> CursorMutKey<'a, K, V, A> {
    3173    /// Advances the cursor to the next gap, returning the key and value of the
    3174    /// element that it moved over.
    3175    ///
    3176    /// If the cursor is already at the end of the map then `None` is returned
    3177    /// and the cursor is not moved.
    3178    #[unstable(feature = "btree_cursors", issue = "107540")]
    3179    pub fn next(&mut self) -> Option<(&mut K, &mut V)> {
    3180        let current = self.current.take()?;
    3181        match current.next_kv() {
    3182            Ok(mut kv) => {
    3183                // SAFETY: The key/value pointers remain valid even after the
    3184                // cursor is moved forward. The lifetimes then prevent any
    3185                // further access to the cursor.
    3186                let (k, v) = unsafe { kv.reborrow_mut().into_kv_mut() };
    3187                let (k, v) = (k as *mut _, v as *mut _);
    3188                self.current = Some(kv.next_leaf_edge());
    3189                Some(unsafe { (&mut *k, &mut *v) })
    3190            }
    3191            Err(root) => {
    3192                self.current = Some(root.last_leaf_edge());
    3193                None
    3194            }
    3195        }
    3196    }
    3197
    3198    /// Advances the cursor to the previous gap, returning the key and value of
    3199    /// the element that it moved over.
    3200    ///
    3201    /// If the cursor is already at the start of the map then `None` is returned
    3202    /// and the cursor is not moved.
    3203    #[unstable(feature = "btree_cursors", issue = "107540")]
    3204    pub fn prev(&mut self) -> Option<(&mut K, &mut V)> {
    3205        let current = self.current.take()?;
    3206        match current.next_back_kv() {
    3207            Ok(mut kv) => {
    3208                // SAFETY: The key/value pointers remain valid even after the
    3209                // cursor is moved forward. The lifetimes then prevent any
    3210                // further access to the cursor.
    3211                let (k, v) = unsafe { kv.reborrow_mut().into_kv_mut() };
    3212                let (k, v) = (k as *mut _, v as *mut _);
    3213                self.current = Some(kv.next_back_leaf_edge());
    3214                Some(unsafe { (&mut *k, &mut *v) })
    3215            }
    3216            Err(root) => {
    3217                self.current = Some(root.first_leaf_edge());
    3218                None
    3219            }
    3220        }
    3221    }
    3222
    3223    /// Returns a reference to the key and value of the next element without
    3224    /// moving the cursor.
    3225    ///
    3226    /// If the cursor is at the end of the map then `None` is returned.
    3227    #[unstable(feature = "btree_cursors", issue = "107540")]
    3228    pub fn peek_next(&mut self) -> Option<(&mut K, &mut V)> {
    3229        let current = self.current.as_mut()?;
    3230        // SAFETY: We're not using this to mutate the tree.
    3231        let kv = unsafe { current.reborrow_mut() }.next_kv().ok()?.into_kv_mut();
    3232        Some(kv)
    3233    }
    3234
    3235    /// Returns a reference to the key and value of the previous element
    3236    /// without moving the cursor.
    3237    ///
    3238    /// If the cursor is at the start of the map then `None` is returned.
    3239    #[unstable(feature = "btree_cursors", issue = "107540")]
    3240    pub fn peek_prev(&mut self) -> Option<(&mut K, &mut V)> {
    3241        let current = self.current.as_mut()?;
    3242        // SAFETY: We're not using this to mutate the tree.
    3243        let kv = unsafe { current.reborrow_mut() }.next_back_kv().ok()?.into_kv_mut();
    3244        Some(kv)
    3245    }
    3246
    3247    /// Returns a read-only cursor pointing to the same location as the
    3248    /// `CursorMutKey`.
    3249    ///
    3250    /// The lifetime of the returned `Cursor` is bound to that of the
    3251    /// `CursorMutKey`, which means it cannot outlive the `CursorMutKey` and that the
    3252    /// `CursorMutKey` is frozen for the lifetime of the `Cursor`.
    3253    #[unstable(feature = "btree_cursors", issue = "107540")]
    3254    pub fn as_cursor(&self) -> Cursor<'_, K, V> {
    3255        Cursor {
    3256            // SAFETY: The tree is immutable while the cursor exists.
    3257            root: unsafe { self.root.reborrow_shared().as_ref() },
    3258            current: self.current.as_ref().map(|current| current.reborrow()),
    3259        }
    3260    }
    3261}
    3262
    3263// Now the tree editing operations
    3264impl<'a, K: Ord, V, A: Allocator + Clone> CursorMutKey<'a, K, V, A> {
    3265    /// Inserts a new key-value pair into the map in the gap that the
    3266    /// cursor is currently pointing to.
    3267    ///
    3268    /// After the insertion the cursor will be pointing at the gap before the
    3269    /// newly inserted element.
    3270    ///
    3271    /// # Safety
    3272    ///
    3273    /// You must ensure that the `BTreeMap` invariants are maintained.
    3274    /// Specifically:
    3275    ///
    3276    /// * The key of the newly inserted element must be unique in the tree.
    3277    /// * All keys in the tree must remain in sorted order.
    3278    #[unstable(feature = "btree_cursors", issue = "107540")]
    3279    pub unsafe fn insert_after_unchecked(&mut self, key: K, value: V) {
    3280        let edge = match self.current.take() {
    3281            None => {
    3282                // Tree is empty, allocate a new root.
    3283                // SAFETY: We have no other reference to the tree.
    3284                let root = unsafe { self.root.reborrow() };
    3285                debug_assert!(root.is_none());
    3286                let mut node = NodeRef::new_leaf(self.alloc.clone());
    3287                // SAFETY: We don't touch the root while the handle is alive.
    3288                let handle = unsafe { node.borrow_mut().push_with_handle(key, value) };
    3289                *root = Some(node.forget_type());
    3290                *self.length += 1;
    3291                self.current = Some(handle.left_edge());
    3292                return;
    3293            }
    3294            Some(current) => current,
    3295        };
    3296
    3297        let handle = edge.insert_recursing(key, value, self.alloc.clone(), |ins| {
    3298            drop(ins.left);
    3299            // SAFETY: The handle to the newly inserted value is always on a
    3300            // leaf node, so adding a new root node doesn't invalidate it.
    3301            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
    3302            root.push_internal_level(self.alloc.clone()).push(ins.kv.0, ins.kv.1, ins.right)
    3303        });
    3304        self.current = Some(handle.left_edge());
    3305        *self.length += 1;
    3306    }
    3307
    3308    /// Inserts a new key-value pair into the map in the gap that the
    3309    /// cursor is currently pointing to.
    3310    ///
    3311    /// After the insertion the cursor will be pointing at the gap after the
    3312    /// newly inserted element.
    3313    ///
    3314    /// # Safety
    3315    ///
    3316    /// You must ensure that the `BTreeMap` invariants are maintained.
    3317    /// Specifically:
    3318    ///
    3319    /// * The key of the newly inserted element must be unique in the tree.
    3320    /// * All keys in the tree must remain in sorted order.
    3321    #[unstable(feature = "btree_cursors", issue = "107540")]
    3322    pub unsafe fn insert_before_unchecked(&mut self, key: K, value: V) {
    3323        let edge = match self.current.take() {
    3324            None => {
    3325                // SAFETY: We have no other reference to the tree.
    3326                match unsafe { self.root.reborrow() } {
    3327                    root @ None => {
    3328                        // Tree is empty, allocate a new root.
    3329                        let mut node = NodeRef::new_leaf(self.alloc.clone());
    3330                        // SAFETY: We don't touch the root while the handle is alive.
    3331                        let handle = unsafe { node.borrow_mut().push_with_handle(key, value) };
    3332                        *root = Some(node.forget_type());
    3333                        *self.length += 1;
    3334                        self.current = Some(handle.right_edge());
    3335                        return;
    3336                    }
    3337                    Some(root) => root.borrow_mut().last_leaf_edge(),
    3338                }
    3339            }
    3340            Some(current) => current,
    3341        };
    3342
    3343        let handle = edge.insert_recursing(key, value, self.alloc.clone(), |ins| {
    3344            drop(ins.left);
    3345            // SAFETY: The handle to the newly inserted value is always on a
    3346            // leaf node, so adding a new root node doesn't invalidate it.
    3347            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
    3348            root.push_internal_level(self.alloc.clone()).push(ins.kv.0, ins.kv.1, ins.right)
    3349        });
    3350        self.current = Some(handle.right_edge());
    3351        *self.length += 1;
    3352    }
    3353
    3354    /// Inserts a new key-value pair into the map in the gap that the
    3355    /// cursor is currently pointing to.
    3356    ///
    3357    /// After the insertion the cursor will be pointing at the gap before the
    3358    /// newly inserted element.
    3359    ///
    3360    /// If the inserted key is not greater than the key before the cursor
    3361    /// (if any), or if it not less than the key after the cursor (if any),
    3362    /// then an [`UnorderedKeyError`] is returned since this would
    3363    /// invalidate the [`Ord`] invariant between the keys of the map.
    3364    #[unstable(feature = "btree_cursors", issue = "107540")]
    3365    pub fn insert_after(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
    3366        if let Some((prev, _)) = self.peek_prev() {
    3367            if &key <= prev {
    3368                return Err(UnorderedKeyError {});
    3369            }
    3370        }
    3371        if let Some((next, _)) = self.peek_next() {
    3372            if &key >= next {
    3373                return Err(UnorderedKeyError {});
    3374            }
    3375        }
    3376        unsafe {
    3377            self.insert_after_unchecked(key, value);
    3378        }
    3379        Ok(())
    3380    }
    3381
    3382    /// Inserts a new key-value pair into the map in the gap that the
    3383    /// cursor is currently pointing to.
    3384    ///
    3385    /// After the insertion the cursor will be pointing at the gap after the
    3386    /// newly inserted element.
    3387    ///
    3388    /// If the inserted key is not greater than the key before the cursor
    3389    /// (if any), or if it not less than the key after the cursor (if any),
    3390    /// then an [`UnorderedKeyError`] is returned since this would
    3391    /// invalidate the [`Ord`] invariant between the keys of the map.
    3392    #[unstable(feature = "btree_cursors", issue = "107540")]
    3393    pub fn insert_before(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
    3394        if let Some((prev, _)) = self.peek_prev() {
    3395            if &key <= prev {
    3396                return Err(UnorderedKeyError {});
    3397            }
    3398        }
    3399        if let Some((next, _)) = self.peek_next() {
    3400            if &key >= next {
    3401                return Err(UnorderedKeyError {});
    3402            }
    3403        }
    3404        unsafe {
    3405            self.insert_before_unchecked(key, value);
    3406        }
    3407        Ok(())
    3408    }
    3409
    3410    /// Removes the next element from the `BTreeMap`.
    3411    ///
    3412    /// The element that was removed is returned. The cursor position is
    3413    /// unchanged (before the removed element).
    3414    #[unstable(feature = "btree_cursors", issue = "107540")]
    3415    pub fn remove_next(&mut self) -> Option<(K, V)> {
    3416        let current = self.current.take()?;
    3417        if current.reborrow().next_kv().is_err() {
    3418            self.current = Some(current);
    3419            return None;
    3420        }
    3421        let mut emptied_internal_root = false;
    3422        let (kv, pos) = current
    3423            .next_kv()
    3424            // This should be unwrap(), but that doesn't work because NodeRef
    3425            // doesn't implement Debug. The condition is checked above.
    3426            .ok()?
    3427            .remove_kv_tracking(|| emptied_internal_root = true, self.alloc.clone());
    3428        self.current = Some(pos);
    3429        *self.length -= 1;
    3430        if emptied_internal_root {
    3431            // SAFETY: This is safe since current does not point within the now
    3432            // empty root node.
    3433            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
    3434            root.pop_internal_level(self.alloc.clone());
    3435        }
    3436        Some(kv)
    3437    }
    3438
    3439    /// Removes the preceding element from the `BTreeMap`.
    3440    ///
    3441    /// The element that was removed is returned. The cursor position is
    3442    /// unchanged (after the removed element).
    3443    #[unstable(feature = "btree_cursors", issue = "107540")]
    3444    pub fn remove_prev(&mut self) -> Option<(K, V)> {
    3445        let current = self.current.take()?;
    3446        if current.reborrow().next_back_kv().is_err() {
    3447            self.current = Some(current);
    3448            return None;
    3449        }
    3450        let mut emptied_internal_root = false;
    3451        let (kv, pos) = current
    3452            .next_back_kv()
    3453            // This should be unwrap(), but that doesn't work because NodeRef
    3454            // doesn't implement Debug. The condition is checked above.
    3455            .ok()?
    3456            .remove_kv_tracking(|| emptied_internal_root = true, self.alloc.clone());
    3457        self.current = Some(pos);
    3458        *self.length -= 1;
    3459        if emptied_internal_root {
    3460            // SAFETY: This is safe since current does not point within the now
    3461            // empty root node.
    3462            let root = unsafe { self.root.reborrow().as_mut().unwrap() };
    3463            root.pop_internal_level(self.alloc.clone());
    3464        }
    3465        Some(kv)
    3466    }
    3467}
    3468
    3469impl<'a, K: Ord, V, A: Allocator + Clone> CursorMut<'a, K, V, A> {
    3470    /// Inserts a new key-value pair into the map in the gap that the
    3471    /// cursor is currently pointing to.
    3472    ///
    3473    /// After the insertion the cursor will be pointing at the gap after the
    3474    /// newly inserted element.
    3475    ///
    3476    /// # Safety
    3477    ///
    3478    /// You must ensure that the `BTreeMap` invariants are maintained.
    3479    /// Specifically:
    3480    ///
    3481    /// * The key of the newly inserted element must be unique in the tree.
    3482    /// * All keys in the tree must remain in sorted order.
    3483    #[unstable(feature = "btree_cursors", issue = "107540")]
    3484    pub unsafe fn insert_after_unchecked(&mut self, key: K, value: V) {
    3485        unsafe { self.inner.insert_after_unchecked(key, value) }
    3486    }
    3487
    3488    /// Inserts a new key-value pair into the map in the gap that the
    3489    /// cursor is currently pointing to.
    3490    ///
    3491    /// After the insertion the cursor will be pointing at the gap after the
    3492    /// newly inserted element.
    3493    ///
    3494    /// # Safety
    3495    ///
    3496    /// You must ensure that the `BTreeMap` invariants are maintained.
    3497    /// Specifically:
    3498    ///
    3499    /// * The key of the newly inserted element must be unique in the tree.
    3500    /// * All keys in the tree must remain in sorted order.
    3501    #[unstable(feature = "btree_cursors", issue = "107540")]
    3502    pub unsafe fn insert_before_unchecked(&mut self, key: K, value: V) {
    3503        unsafe { self.inner.insert_before_unchecked(key, value) }
    3504    }
    3505
    3506    /// Inserts a new key-value pair into the map in the gap that the
    3507    /// cursor is currently pointing to.
    3508    ///
    3509    /// After the insertion the cursor will be pointing at the gap before the
    3510    /// newly inserted element.
    3511    ///
    3512    /// If the inserted key is not greater than the key before the cursor
    3513    /// (if any), or if it not less than the key after the cursor (if any),
    3514    /// then an [`UnorderedKeyError`] is returned since this would
    3515    /// invalidate the [`Ord`] invariant between the keys of the map.
    3516    #[unstable(feature = "btree_cursors", issue = "107540")]
    3517    pub fn insert_after(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
    3518        self.inner.insert_after(key, value)
    3519    }
    3520
    3521    /// Inserts a new key-value pair into the map in the gap that the
    3522    /// cursor is currently pointing to.
    3523    ///
    3524    /// After the insertion the cursor will be pointing at the gap after the
    3525    /// newly inserted element.
    3526    ///
    3527    /// If the inserted key is not greater than the key before the cursor
    3528    /// (if any), or if it not less than the key after the cursor (if any),
    3529    /// then an [`UnorderedKeyError`] is returned since this would
    3530    /// invalidate the [`Ord`] invariant between the keys of the map.
    3531    #[unstable(feature = "btree_cursors", issue = "107540")]
    3532    pub fn insert_before(&mut self, key: K, value: V) -> Result<(), UnorderedKeyError> {
    3533        self.inner.insert_before(key, value)
    3534    }
    3535
    3536    /// Removes the next element from the `BTreeMap`.
    3537    ///
    3538    /// The element that was removed is returned. The cursor position is
    3539    /// unchanged (before the removed element).
    3540    #[unstable(feature = "btree_cursors", issue = "107540")]
    3541    pub fn remove_next(&mut self) -> Option<(K, V)> {
    3542        self.inner.remove_next()
    3543    }
    3544
    3545    /// Removes the preceding element from the `BTreeMap`.
    3546    ///
    3547    /// The element that was removed is returned. The cursor position is
    3548    /// unchanged (after the removed element).
    3549    #[unstable(feature = "btree_cursors", issue = "107540")]
    3550    pub fn remove_prev(&mut self) -> Option<(K, V)> {
    3551        self.inner.remove_prev()
    3552    }
    3553}
    3554
    3555/// Error type returned by [`CursorMut::insert_before`] and
    3556/// [`CursorMut::insert_after`] if the key being inserted is not properly
    3557/// ordered with regards to adjacent keys.
    3558#[derive(Clone, PartialEq, Eq, Debug)]
    3559#[unstable(feature = "btree_cursors", issue = "107540")]
    3560pub struct UnorderedKeyError {}
    3561
    3562#[unstable(feature = "btree_cursors", issue = "107540")]
    3563impl fmt::Display for UnorderedKeyError {
    3564    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    3565        write!(f, "key is not properly ordered relative to neighbors")
    3566    }
    3567}
    3568
    3569#[unstable(feature = "btree_cursors", issue = "107540")]
    3570impl Error for UnorderedKeyError {}
    3571
    3572#[cfg(test)]
    3573mod tests;