mod.rs - source
Description: Source of the Rust file `library/core/src/convert/mod.rs`.

#
core/convert/
mod.rs

    1//! Traits for conversions between types.
    2//!
    3//! The traits in this module provide a way to convert from one type to another type.
    4//! Each trait serves a different purpose:
    5//!
    6//! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions
    7//! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions
    8//! - Implement the [`From`] trait for consuming value-to-value conversions
    9//! - Implement the [`Into`] trait for consuming value-to-value conversions to types
    10//!   outside the current crate
    11//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],
    12//!   but should be implemented when the conversion can fail.
    13//!
    14//! The traits in this module are often used as trait bounds for generic functions such that to
    15//! arguments of multiple types are supported. See the documentation of each trait for examples.
    16//!
    17//! As a library author, you should always prefer implementing [`From<T>`][`From`] or
    18//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
    19//! as [`From`] and [`TryFrom`] provide greater flexibility and offer
    20//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
    21//! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
    22//! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
    23//! outside the current crate.
    24//!
    25//! # Generic Implementations
    26//!
    27//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
    28//!   (but not generally for all [dereferenceable types][core::ops::Deref])
    29//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
    30//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
    31//! - [`From`] and [`Into`] are reflexive, which means that all types can
    32//!   `into` themselves and `from` themselves
    33//!
    34//! See each trait for usage examples.
    35
    36#![stable(feature = "rust1", since = "1.0.0")]
    37
    38use crate::error::Error;
    39use crate::fmt;
    40use crate::hash::{Hash, Hasher};
    41use crate::marker::PointeeSized;
    42
    43mod num;
    44
    45#[unstable(feature = "convert_float_to_int", issue = "67057")]
    46pub use num::FloatToInt;
    47
    48/// The identity function.
    49///
    50/// Two things are important to note about this function:
    51///
    52/// - It is not always equivalent to a closure like `|x| x`, since the
    53///   closure may coerce `x` into a different type.
    54///
    55/// - It moves the input `x` passed to the function.
    56///
    57/// While it might seem strange to have a function that just returns back the
    58/// input, there are some interesting uses.
    59///
    60/// # Examples
    61///
    62/// Using `identity` to do nothing in a sequence of other, interesting,
    63/// functions:
    64///
    65/// ```rust
    66/// use std::convert::identity;
    67///
    68/// fn manipulation(x: u32) -> u32 {
    69///     // Let's pretend that adding one is an interesting function.
    70///     x + 1
    71/// }
    72///
    73/// let _arr = &[identity, manipulation];
    74/// ```
    75///
    76/// Using `identity` as a "do nothing" base case in a conditional:
    77///
    78/// ```rust
    79/// use std::convert::identity;
    80///
    81/// # let condition = true;
    82/// #
    83/// # fn manipulation(x: u32) -> u32 { x + 1 }
    84/// #
    85/// let do_stuff = if condition { manipulation } else { identity };
    86///
    87/// // Do more interesting stuff...
    88///
    89/// let _results = do_stuff(42);
    90/// ```
    91///
    92/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
    93///
    94/// ```rust
    95/// use std::convert::identity;
    96///
    97/// let iter = [Some(1), None, Some(3)].into_iter();
    98/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
    99/// assert_eq!(vec![1, 3], filtered);
    100/// ```
    101#[stable(feature = "convert_id", since = "1.33.0")]
    102#[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
    103#[inline(always)]
    104#[rustc_diagnostic_item = "convert_identity"]
    105pub const fn identity<T>(x: T) -> T {
    106    x
    107}
    108
    109/// Used to do a cheap reference-to-reference conversion.
    110///
    111/// This trait is similar to [`AsMut`] which is used for converting between mutable references.
    112/// If you need to do a costly conversion it is better to implement [`From`] with type
    113/// `&T` or write a custom function.
    114///
    115/// # Relation to `Borrow`
    116///
    117/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in a few aspects:
    118///
    119/// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
    120///   a reference or a value. (See also note on `AsRef`'s reflexibility below.)
    121/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a borrowed value are
    122///   equivalent to those of the owned value. For this reason, if you want to
    123///   borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
    124///
    125/// **Note: This trait must not fail**. If the conversion can fail, use a
    126/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
    127///
    128/// # Generic Implementations
    129///
    130/// `AsRef` auto-dereferences if the inner type is a reference or a mutable reference
    131/// (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`).
    132///
    133/// Note that due to historic reasons, the above currently does not hold generally for all
    134/// [dereferenceable types], e.g. `foo.as_ref()` will *not* work the same as
    135/// `Box::new(foo).as_ref()`. Instead, many smart pointers provide an `as_ref` implementation which
    136/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
    137/// reference-to-reference conversion for that value). However, [`AsRef::as_ref`] should not be
    138/// used for the sole purpose of dereferencing; instead ['`Deref` coercion'] can be used:
    139///
    140/// [dereferenceable types]: core::ops::Deref
    141/// [pointed-to value]: core::ops::Deref::Target
    142/// ['`Deref` coercion']: core::ops::Deref#deref-coercion
    143///
    144/// ```
    145/// let x = Box::new(5i32);
    146/// // Avoid this:
    147/// // let y: &i32 = x.as_ref();
    148/// // Better just write:
    149/// let y: &i32 = &x;
    150/// ```
    151///
    152/// Types which implement [`Deref`] should consider implementing `AsRef<T>` as follows:
    153///
    154/// [`Deref`]: core::ops::Deref
    155///
    156/// ```
    157/// # use core::ops::Deref;
    158/// # struct SomeType;
    159/// # impl Deref for SomeType {
    160/// #     type Target = [u8];
    161/// #     fn deref(&self) -> &[u8] {
    162/// #         &[]
    163/// #     }
    164/// # }
    165/// impl<T> AsRef<T> for SomeType
    166/// where
    167///     T: ?Sized,
    168///     <SomeType as Deref>::Target: AsRef<T>,
    169/// {
    170///     fn as_ref(&self) -> &T {
    171///         self.deref().as_ref()
    172///     }
    173/// }
    174/// ```
    175///
    176/// # Reflexivity
    177///
    178/// Ideally, `AsRef` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsRef<T> for T`
    179/// with [`as_ref`] simply returning its argument unchanged.
    180/// Such a blanket implementation is currently *not* provided due to technical restrictions of
    181/// Rust's type system (it would be overlapping with another existing blanket implementation for
    182/// `&T where T: AsRef<U>` which allows `AsRef` to auto-dereference, see "Generic Implementations"
    183/// above).
    184///
    185/// [`as_ref`]: AsRef::as_ref
    186///
    187/// A trivial implementation of `AsRef<T> for T` must be added explicitly for a particular type `T`
    188/// where needed or desired. Note, however, that not all types from `std` contain such an
    189/// implementation, and those cannot be added by external code due to orphan rules.
    190///
    191/// # Examples
    192///
    193/// By using trait bounds we can accept arguments of different types as long as they can be
    194/// converted to the specified type `T`.
    195///
    196/// For example: By creating a generic function that takes an `AsRef<str>` we express that we
    197/// want to accept all references that can be converted to [`&str`] as an argument.
    198/// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
    199///
    200/// [`&str`]: primitive@str
    201/// [`Borrow`]: crate::borrow::Borrow
    202/// [`Eq`]: crate::cmp::Eq
    203/// [`Ord`]: crate::cmp::Ord
    204/// [`String`]: ../../std/string/struct.String.html
    205///
    206/// ```
    207/// fn is_hello<T: AsRef<str>>(s: T) {
    208///    assert_eq!("hello", s.as_ref());
    209/// }
    210///
    211/// let s = "hello";
    212/// is_hello(s);
    213///
    214/// let s = "hello".to_string();
    215/// is_hello(s);
    216/// ```
    217#[stable(feature = "rust1", since = "1.0.0")]
    218#[rustc_diagnostic_item = "AsRef"]
    219#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    220pub const trait AsRef<T: PointeeSized>: PointeeSized {
    221    /// Converts this type into a shared reference of the (usually inferred) input type.
    222    #[stable(feature = "rust1", since = "1.0.0")]
    223    fn as_ref(&self) -> &T;
    224}
    225
    226/// Used to do a cheap mutable-to-mutable reference conversion.
    227///
    228/// This trait is similar to [`AsRef`] but used for converting between mutable
    229/// references. If you need to do a costly conversion it is better to
    230/// implement [`From`] with type `&mut T` or write a custom function.
    231///
    232/// **Note: This trait must not fail**. If the conversion can fail, use a
    233/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
    234///
    235/// # Generic Implementations
    236///
    237/// `AsMut` auto-dereferences if the inner type is a mutable reference
    238/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo` or `&mut &mut Foo`).
    239///
    240/// Note that due to historic reasons, the above currently does not hold generally for all
    241/// [mutably dereferenceable types], e.g. `foo.as_mut()` will *not* work the same as
    242/// `Box::new(foo).as_mut()`. Instead, many smart pointers provide an `as_mut` implementation which
    243/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
    244/// reference-to-reference conversion for that value). However, [`AsMut::as_mut`] should not be
    245/// used for the sole purpose of mutable dereferencing; instead ['`Deref` coercion'] can be used:
    246///
    247/// [mutably dereferenceable types]: core::ops::DerefMut
    248/// [pointed-to value]: core::ops::Deref::Target
    249/// ['`Deref` coercion']: core::ops::DerefMut#mutable-deref-coercion
    250///
    251/// ```
    252/// let mut x = Box::new(5i32);
    253/// // Avoid this:
    254/// // let y: &mut i32 = x.as_mut();
    255/// // Better just write:
    256/// let y: &mut i32 = &mut x;
    257/// ```
    258///
    259/// Types which implement [`DerefMut`] should consider to add an implementation of `AsMut<T>` as
    260/// follows:
    261///
    262/// [`DerefMut`]: core::ops::DerefMut
    263///
    264/// ```
    265/// # use core::ops::{Deref, DerefMut};
    266/// # struct SomeType;
    267/// # impl Deref for SomeType {
    268/// #     type Target = [u8];
    269/// #     fn deref(&self) -> &[u8] {
    270/// #         &[]
    271/// #     }
    272/// # }
    273/// # impl DerefMut for SomeType {
    274/// #     fn deref_mut(&mut self) -> &mut [u8] {
    275/// #         &mut []
    276/// #     }
    277/// # }
    278/// impl<T> AsMut<T> for SomeType
    279/// where
    280///     <SomeType as Deref>::Target: AsMut<T>,
    281/// {
    282///     fn as_mut(&mut self) -> &mut T {
    283///         self.deref_mut().as_mut()
    284///     }
    285/// }
    286/// ```
    287///
    288/// # Reflexivity
    289///
    290/// Ideally, `AsMut` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsMut<T> for T`
    291/// with [`as_mut`] simply returning its argument unchanged.
    292/// Such a blanket implementation is currently *not* provided due to technical restrictions of
    293/// Rust's type system (it would be overlapping with another existing blanket implementation for
    294/// `&mut T where T: AsMut<U>` which allows `AsMut` to auto-dereference, see "Generic
    295/// Implementations" above).
    296///
    297/// [`as_mut`]: AsMut::as_mut
    298///
    299/// A trivial implementation of `AsMut<T> for T` must be added explicitly for a particular type `T`
    300/// where needed or desired. Note, however, that not all types from `std` contain such an
    301/// implementation, and those cannot be added by external code due to orphan rules.
    302///
    303/// # Examples
    304///
    305/// Using `AsMut` as trait bound for a generic function, we can accept all mutable references that
    306/// can be converted to type `&mut T`. Unlike [dereference], which has a single [target type],
    307/// there can be multiple implementations of `AsMut` for a type. In particular, `Vec<T>` implements
    308/// both `AsMut<Vec<T>>` and `AsMut<[T]>`.
    309///
    310/// In the following, the example functions `caesar` and `null_terminate` provide a generic
    311/// interface which works with any type that can be converted by cheap mutable-to-mutable conversion
    312/// into a byte slice (`[u8]`) or a byte vector (`Vec<u8>`), respectively.
    313///
    314/// [dereference]: core::ops::DerefMut
    315/// [target type]: core::ops::Deref::Target
    316///
    317/// ```
    318/// struct Document {
    319///     info: String,
    320///     content: Vec<u8>,
    321/// }
    322///
    323/// impl<T: ?Sized> AsMut<T> for Document
    324/// where
    325///     Vec<u8>: AsMut<T>,
    326/// {
    327///     fn as_mut(&mut self) -> &mut T {
    328///         self.content.as_mut()
    329///     }
    330/// }
    331///
    332/// fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
    333///     for byte in data.as_mut() {
    334///         *byte = byte.wrapping_add(key);
    335///     }
    336/// }
    337///
    338/// fn null_terminate<T: AsMut<Vec<u8>>>(data: &mut T) {
    339///     // Using a non-generic inner function, which contains most of the
    340///     // functionality, helps to minimize monomorphization overhead.
    341///     fn doit(data: &mut Vec<u8>) {
    342///         let len = data.len();
    343///         if len == 0 || data[len-1] != 0 {
    344///             data.push(0);
    345///         }
    346///     }
    347///     doit(data.as_mut());
    348/// }
    349///
    350/// fn main() {
    351///     let mut v: Vec<u8> = vec![1, 2, 3];
    352///     caesar(&mut v, 5);
    353///     assert_eq!(v, [6, 7, 8]);
    354///     null_terminate(&mut v);
    355///     assert_eq!(v, [6, 7, 8, 0]);
    356///     let mut doc = Document {
    357///         info: String::from("Example"),
    358///         content: vec![17, 19, 8],
    359///     };
    360///     caesar(&mut doc, 1);
    361///     assert_eq!(doc.content, [18, 20, 9]);
    362///     null_terminate(&mut doc);
    363///     assert_eq!(doc.content, [18, 20, 9, 0]);
    364/// }
    365/// ```
    366///
    367/// Note, however, that APIs don't need to be generic. In many cases taking a `&mut [u8]` or
    368/// `&mut Vec<u8>`, for example, is the better choice (callers need to pass the correct type then).
    369#[stable(feature = "rust1", since = "1.0.0")]
    370#[rustc_diagnostic_item = "AsMut"]
    371#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    372pub const trait AsMut<T: PointeeSized>: PointeeSized {
    373    /// Converts this type into a mutable reference of the (usually inferred) input type.
    374    #[stable(feature = "rust1", since = "1.0.0")]
    375    fn as_mut(&mut self) -> &mut T;
    376}
    377
    378/// A value-to-value conversion that consumes the input value. The
    379/// opposite of [`From`].
    380///
    381/// One should avoid implementing [`Into`] and implement [`From`] instead.
    382/// Implementing [`From`] automatically provides one with an implementation of [`Into`]
    383/// thanks to the blanket implementation in the standard library.
    384///
    385/// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
    386/// to ensure that types that only implement [`Into`] can be used as well.
    387///
    388/// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
    389///
    390/// # Generic Implementations
    391///
    392/// - [`From`]`<T> for U` implies `Into<U> for T`
    393/// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
    394///
    395/// # Implementing [`Into`] for conversions to external types in old versions of Rust
    396///
    397/// Prior to Rust 1.41, if the destination type was not part of the current crate
    398/// then you couldn't implement [`From`] directly.
    399/// For example, take this code:
    400///
    401/// ```
    402/// # #![allow(non_local_definitions)]
    403/// struct Wrapper<T>(Vec<T>);
    404/// impl<T> From<Wrapper<T>> for Vec<T> {
    405///     fn from(w: Wrapper<T>) -> Vec<T> {
    406///         w.0
    407///     }
    408/// }
    409/// ```
    410/// This will fail to compile in older versions of the language because Rust's orphaning rules
    411/// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
    412///
    413/// ```
    414/// struct Wrapper<T>(Vec<T>);
    415/// impl<T> Into<Vec<T>> for Wrapper<T> {
    416///     fn into(self) -> Vec<T> {
    417///         self.0
    418///     }
    419/// }
    420/// ```
    421///
    422/// It is important to understand that [`Into`] does not provide a [`From`] implementation
    423/// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
    424/// and then fall back to [`Into`] if [`From`] can't be implemented.
    425///
    426/// # Examples
    427///
    428/// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
    429///
    430/// In order to express that we want a generic function to take all arguments that can be
    431/// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
    432/// For example: The function `is_hello` takes all arguments that can be converted into a
    433/// [`Vec`]`<`[`u8`]`>`.
    434///
    435/// ```
    436/// fn is_hello<T: Into<Vec<u8>>>(s: T) {
    437///    let bytes = b"hello".to_vec();
    438///    assert_eq!(bytes, s.into());
    439/// }
    440///
    441/// let s = "hello".to_string();
    442/// is_hello(s);
    443/// ```
    444///
    445/// [`String`]: ../../std/string/struct.String.html
    446/// [`Vec`]: ../../std/vec/struct.Vec.html
    447#[rustc_diagnostic_item = "Into"]
    448#[stable(feature = "rust1", since = "1.0.0")]
    449#[doc(search_unbox)]
    450#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    451pub const trait Into<T>: Sized {
    452    /// Converts this type into the (usually inferred) input type.
    453    #[must_use]
    454    #[stable(feature = "rust1", since = "1.0.0")]
    455    fn into(self) -> T;
    456}
    457
    458/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
    459/// [`Into`].
    460///
    461/// One should always prefer implementing `From` over [`Into`]
    462/// because implementing `From` automatically provides one with an implementation of [`Into`]
    463/// thanks to the blanket implementation in the standard library.
    464///
    465/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
    466/// outside the current crate.
    467/// `From` was not able to do these types of conversions in earlier versions because of Rust's
    468/// orphaning rules.
    469/// See [`Into`] for more details.
    470///
    471/// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
    472/// to ensure that types that only implement [`Into`] can be used as well.
    473///
    474/// The `From` trait is also very useful when performing error handling. When constructing a function
    475/// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
    476/// `From` simplifies error handling by allowing a function to return a single error type
    477/// that encapsulates multiple error types. See the "Examples" section and [the book][book] for more
    478/// details.
    479///
    480/// **Note: This trait must not fail**. The `From` trait is intended for perfect conversions.
    481/// If the conversion can fail or is not perfect, use [`TryFrom`].
    482///
    483/// # Generic Implementations
    484///
    485/// - `From<T> for U` implies [`Into`]`<U> for T`
    486/// - `From` is reflexive, which means that `From<T> for T` is implemented
    487///
    488/// # When to implement `From`
    489///
    490/// While there's no technical restrictions on which conversions can be done using
    491/// a `From` implementation, the general expectation is that the conversions
    492/// should typically be restricted as follows:
    493///
    494/// * The conversion is *infallible*: if the conversion can fail, use [`TryFrom`]
    495///   instead; don't provide a `From` impl that panics.
    496///
    497/// * The conversion is *lossless*: semantically, it should not lose or discard
    498///   information. For example, `i32: From<u16>` exists, where the original
    499///   value can be recovered using `u16: TryFrom<i32>`.  And `String: From<&str>`
    500///   exists, where you can get something equivalent to the original value via
    501///   `Deref`.  But `From` cannot be used to convert from `u32` to `u16`, since
    502///   that cannot succeed in a lossless way.  (There's some wiggle room here for
    503///   information not considered semantically relevant.  For example,
    504///   `Box<[T]>: From<Vec<T>>` exists even though it might not preserve capacity,
    505///   like how two vectors can be equal despite differing capacities.)
    506///
    507/// * The conversion is *value-preserving*: the conceptual kind and meaning of
    508///   the resulting value is the same, even though the Rust type and technical
    509///   representation might be different.  For example `-1_i8 as u8` is *lossless*,
    510///   since `as` casting back can recover the original value, but that conversion
    511///   is *not* available via `From` because `-1` and `255` are different conceptual
    512///   values (despite being identical bit patterns technically).  But
    513///   `f32: From<i16>` *is* available because `1_i16` and `1.0_f32` are conceptually
    514///   the same real number (despite having very different bit patterns technically).
    515///   `String: From<char>` is available because they're both *text*, but
    516///   `String: From<u32>` is *not* available, since `1` (a number) and `"1"`
    517///   (text) are too different.  (Converting values to text is instead covered
    518///   by the [`Display`](crate::fmt::Display) trait.)
    519///
    520/// * The conversion is *obvious*: it's the only reasonable conversion between
    521///   the two types.  Otherwise it's better to have it be a named method or
    522///   constructor, like how [`str::as_bytes`] is a method and how integers have
    523///   methods like [`u32::from_ne_bytes`], [`u32::from_le_bytes`], and
    524///   [`u32::from_be_bytes`], none of which are `From` implementations.  Whereas
    525///   there's only one reasonable way to wrap an [`Ipv6Addr`](crate::net::Ipv6Addr)
    526///   into an [`IpAddr`](crate::net::IpAddr), thus `IpAddr: From<Ipv6Addr>` exists.
    527///
    528/// # Examples
    529///
    530/// [`String`] implements `From<&str>`:
    531///
    532/// An explicit conversion from a `&str` to a String is done as follows:
    533///
    534/// ```
    535/// let string = "hello".to_string();
    536/// let other_string = String::from("hello");
    537///
    538/// assert_eq!(string, other_string);
    539/// ```
    540///
    541/// While performing error handling it is often useful to implement `From` for your own error type.
    542/// By converting underlying error types to our own custom error type that encapsulates the
    543/// underlying error type, we can return a single error type without losing information on the
    544/// underlying cause. The '?' operator automatically converts the underlying error type to our
    545/// custom error type with `From::from`.
    546///
    547/// ```
    548/// use std::fs;
    549/// use std::io;
    550/// use std::num;
    551///
    552/// enum CliError {
    553///     IoError(io::Error),
    554///     ParseError(num::ParseIntError),
    555/// }
    556///
    557/// impl From<io::Error> for CliError {
    558///     fn from(error: io::Error) -> Self {
    559///         CliError::IoError(error)
    560///     }
    561/// }
    562///
    563/// impl From<num::ParseIntError> for CliError {
    564///     fn from(error: num::ParseIntError) -> Self {
    565///         CliError::ParseError(error)
    566///     }
    567/// }
    568///
    569/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
    570///     let mut contents = fs::read_to_string(&file_name)?;
    571///     let num: i32 = contents.trim().parse()?;
    572///     Ok(num)
    573/// }
    574/// ```
    575///
    576/// [`String`]: ../../std/string/struct.String.html
    577/// [`from`]: From::from
    578/// [book]: ../../book/ch09-00-error-handling.html
    579#[rustc_diagnostic_item = "From"]
    580#[stable(feature = "rust1", since = "1.0.0")]
    581#[rustc_on_unimplemented(on(
    582    all(Self = "&str", T = "alloc::string::String"),
    583    note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
    584))]
    585#[doc(search_unbox)]
    586#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    587pub const trait From<T>: Sized {
    588    /// Converts to this type from the input type.
    589    #[rustc_diagnostic_item = "from_fn"]
    590    #[must_use]
    591    #[stable(feature = "rust1", since = "1.0.0")]
    592    fn from(value: T) -> Self;
    593}
    594
    595/// An attempted conversion that consumes `self`, which may or may not be
    596/// expensive.
    597///
    598/// Library authors should usually not directly implement this trait,
    599/// but should prefer implementing the [`TryFrom`] trait, which offers
    600/// greater flexibility and provides an equivalent `TryInto`
    601/// implementation for free, thanks to a blanket implementation in the
    602/// standard library. For more information on this, see the
    603/// documentation for [`Into`].
    604///
    605/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
    606/// to ensure that types that only implement [`TryInto`] can be used as well.
    607///
    608/// # Implementing `TryInto`
    609///
    610/// This suffers the same restrictions and reasoning as implementing
    611/// [`Into`], see there for details.
    612#[rustc_diagnostic_item = "TryInto"]
    613#[stable(feature = "try_from", since = "1.34.0")]
    614#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    615pub const trait TryInto<T>: Sized {
    616    /// The type returned in the event of a conversion error.
    617    #[stable(feature = "try_from", since = "1.34.0")]
    618    type Error;
    619
    620    /// Performs the conversion.
    621    #[stable(feature = "try_from", since = "1.34.0")]
    622    fn try_into(self) -> Result<T, Self::Error>;
    623}
    624
    625/// Simple and safe type conversions that may fail in a controlled
    626/// way under some circumstances. It is the reciprocal of [`TryInto`].
    627///
    628/// This is useful when you are doing a type conversion that may
    629/// trivially succeed but may also need special handling.
    630/// For example, there is no way to convert an [`i64`] into an [`i32`]
    631/// using the [`From`] trait, because an [`i64`] may contain a value
    632/// that an [`i32`] cannot represent and so the conversion would lose data.
    633/// This might be handled by truncating the [`i64`] to an [`i32`] or by
    634/// simply returning [`i32::MAX`], or by some other method.  The [`From`]
    635/// trait is intended for perfect conversions, so the `TryFrom` trait
    636/// informs the programmer when a type conversion could go bad and lets
    637/// them decide how to handle it.
    638///
    639/// # Generic Implementations
    640///
    641/// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
    642/// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
    643/// is implemented and cannot fail -- the associated `Error` type for
    644/// calling `T::try_from()` on a value of type `T` is [`Infallible`].
    645/// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
    646/// equivalent.
    647///
    648/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
    649/// to ensure that types that only implement [`TryInto`] can be used as well.
    650///
    651/// `TryFrom<T>` can be implemented as follows:
    652///
    653/// ```
    654/// struct GreaterThanZero(i32);
    655///
    656/// impl TryFrom<i32> for GreaterThanZero {
    657///     type Error = &'static str;
    658///
    659///     fn try_from(value: i32) -> Result<Self, Self::Error> {
    660///         if value <= 0 {
    661///             Err("GreaterThanZero only accepts values greater than zero!")
    662///         } else {
    663///             Ok(GreaterThanZero(value))
    664///         }
    665///     }
    666/// }
    667/// ```
    668///
    669/// # Examples
    670///
    671/// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
    672///
    673/// ```
    674/// let big_number = 1_000_000_000_000i64;
    675/// // Silently truncates `big_number`, requires detecting
    676/// // and handling the truncation after the fact.
    677/// let smaller_number = big_number as i32;
    678/// assert_eq!(smaller_number, -727379968);
    679///
    680/// // Returns an error because `big_number` is too big to
    681/// // fit in an `i32`.
    682/// let try_smaller_number = i32::try_from(big_number);
    683/// assert!(try_smaller_number.is_err());
    684///
    685/// // Returns `Ok(3)`.
    686/// let try_successful_smaller_number = i32::try_from(3);
    687/// assert!(try_successful_smaller_number.is_ok());
    688/// ```
    689///
    690/// [`try_from`]: TryFrom::try_from
    691#[rustc_diagnostic_item = "TryFrom"]
    692#[stable(feature = "try_from", since = "1.34.0")]
    693#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    694pub const trait TryFrom<T>: Sized {
    695    /// The type returned in the event of a conversion error.
    696    #[stable(feature = "try_from", since = "1.34.0")]
    697    type Error;
    698
    699    /// Performs the conversion.
    700    #[stable(feature = "try_from", since = "1.34.0")]
    701    #[rustc_diagnostic_item = "try_from_fn"]
    702    fn try_from(value: T) -> Result<Self, Self::Error>;
    703}
    704
    705////////////////////////////////////////////////////////////////////////////////
    706// GENERIC IMPLS
    707////////////////////////////////////////////////////////////////////////////////
    708
    709// As lifts over &
    710#[stable(feature = "rust1", since = "1.0.0")]
    711#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    712impl<T: PointeeSized, U: PointeeSized> const AsRef<U> for &T
    713where
    714    T: [const] AsRef<U>,
    715{
    716    #[inline]
    717    fn as_ref(&self) -> &U {
    718        <T as AsRef<U>>::as_ref(*self)
    719    }
    720}
    721
    722// As lifts over &mut
    723#[stable(feature = "rust1", since = "1.0.0")]
    724#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    725impl<T: PointeeSized, U: PointeeSized> const AsRef<U> for &mut T
    726where
    727    T: [const] AsRef<U>,
    728{
    729    #[inline]
    730    fn as_ref(&self) -> &U {
    731        <T as AsRef<U>>::as_ref(*self)
    732    }
    733}
    734
    735// FIXME (#45742): replace the above impls for &/&mut with the following more general one:
    736// // As lifts over Deref
    737// impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
    738//     fn as_ref(&self) -> &U {
    739//         self.deref().as_ref()
    740//     }
    741// }
    742
    743// AsMut lifts over &mut
    744#[stable(feature = "rust1", since = "1.0.0")]
    745#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    746impl<T: PointeeSized, U: PointeeSized> const AsMut<U> for &mut T
    747where
    748    T: [const] AsMut<U>,
    749{
    750    #[inline]
    751    fn as_mut(&mut self) -> &mut U {
    752        (*self).as_mut()
    753    }
    754}
    755
    756// FIXME (#45742): replace the above impl for &mut with the following more general one:
    757// // AsMut lifts over DerefMut
    758// impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
    759//     fn as_mut(&mut self) -> &mut U {
    760//         self.deref_mut().as_mut()
    761//     }
    762// }
    763
    764// From implies Into
    765#[stable(feature = "rust1", since = "1.0.0")]
    766#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    767impl<T, U> const Into<U> for T
    768where
    769    U: [const] From<T>,
    770{
    771    /// Calls `U::from(self)`.
    772    ///
    773    /// That is, this conversion is whatever the implementation of
    774    /// <code>[From]&lt;T&gt; for U</code> chooses to do.
    775    #[inline]
    776    #[track_caller]
    777    fn into(self) -> U {
    778        U::from(self)
    779    }
    780}
    781
    782// From (and thus Into) is reflexive
    783#[stable(feature = "rust1", since = "1.0.0")]
    784#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    785impl<T> const From<T> for T {
    786    /// Returns the argument unchanged.
    787    #[inline(always)]
    788    fn from(t: T) -> T {
    789        t
    790    }
    791}
    792
    793/// **Stability note:** This impl does not yet exist, but we are
    794/// "reserving space" to add it in the future. See
    795/// [rust-lang/rust#64715][#64715] for details.
    796///
    797/// [#64715]: https://github.com/rust-lang/rust/issues/64715
    798#[stable(feature = "convert_infallible", since = "1.34.0")]
    799#[rustc_reservation_impl = "permitting this impl would forbid us from adding \
    800                            `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
    801#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    802impl<T> const From<!> for T {
    803    fn from(t: !) -> T {
    804        t
    805    }
    806}
    807
    808// TryFrom implies TryInto
    809#[stable(feature = "try_from", since = "1.34.0")]
    810#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    811impl<T, U> const TryInto<U> for T
    812where
    813    U: [const] TryFrom<T>,
    814{
    815    type Error = U::Error;
    816
    817    #[inline]
    818    fn try_into(self) -> Result<U, U::Error> {
    819        U::try_from(self)
    820    }
    821}
    822
    823// Infallible conversions are semantically equivalent to fallible conversions
    824// with an uninhabited error type.
    825#[stable(feature = "try_from", since = "1.34.0")]
    826#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    827impl<T, U> const TryFrom<U> for T
    828where
    829    U: [const] Into<T>,
    830{
    831    type Error = Infallible;
    832
    833    #[inline]
    834    fn try_from(value: U) -> Result<Self, Self::Error> {
    835        Ok(U::into(value))
    836    }
    837}
    838
    839////////////////////////////////////////////////////////////////////////////////
    840// CONCRETE IMPLS
    841////////////////////////////////////////////////////////////////////////////////
    842
    843#[stable(feature = "rust1", since = "1.0.0")]
    844#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    845impl<T> const AsRef<[T]> for [T] {
    846    #[inline(always)]
    847    fn as_ref(&self) -> &[T] {
    848        self
    849    }
    850}
    851
    852#[stable(feature = "rust1", since = "1.0.0")]
    853#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    854impl<T> const AsMut<[T]> for [T] {
    855    #[inline(always)]
    856    fn as_mut(&mut self) -> &mut [T] {
    857        self
    858    }
    859}
    860
    861#[stable(feature = "rust1", since = "1.0.0")]
    862#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    863impl const AsRef<str> for str {
    864    #[inline(always)]
    865    fn as_ref(&self) -> &str {
    866        self
    867    }
    868}
    869
    870#[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
    871#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    872impl const AsMut<str> for str {
    873    #[inline(always)]
    874    fn as_mut(&mut self) -> &mut str {
    875        self
    876    }
    877}
    878
    879////////////////////////////////////////////////////////////////////////////////
    880// THE NO-ERROR ERROR TYPE
    881////////////////////////////////////////////////////////////////////////////////
    882
    883/// The error type for errors that can never happen.
    884///
    885/// Since this enum has no variant, a value of this type can never actually exist.
    886/// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
    887/// to indicate that the result is always [`Ok`].
    888///
    889/// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
    890/// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
    891///
    892/// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
    893/// impl<T, U> TryFrom<U> for T where U: Into<T> {
    894///     type Error = Infallible;
    895///
    896///     fn try_from(value: U) -> Result<Self, Infallible> {
    897///         Ok(U::into(value))  // Never returns `Err`
    898///     }
    899/// }
    900/// ```
    901///
    902/// # Future compatibility
    903///
    904/// This enum has the same role as [the `!` “never” type][never],
    905/// which is unstable in this version of Rust.
    906/// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
    907///
    908/// ```ignore (illustrates future std change)
    909/// pub type Infallible = !;
    910/// ```
    911///
    912/// … and eventually deprecate `Infallible`.
    913///
    914/// However there is one case where `!` syntax can be used
    915/// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
    916/// Specifically, it is possible to have implementations for two different function pointer types:
    917///
    918/// ```
    919/// trait MyTrait {}
    920/// impl MyTrait for fn() -> ! {}
    921/// impl MyTrait for fn() -> std::convert::Infallible {}
    922/// ```
    923///
    924/// With `Infallible` being an enum, this code is valid.
    925/// However when `Infallible` becomes an alias for the never type,
    926/// the two `impl`s will start to overlap
    927/// and therefore will be disallowed by the language’s trait coherence rules.
    928#[stable(feature = "convert_infallible", since = "1.34.0")]
    929#[derive(Copy)]
    930pub enum Infallible {}
    931
    932#[stable(feature = "convert_infallible", since = "1.34.0")]
    933#[rustc_const_unstable(feature = "const_clone", issue = "142757")]
    934impl const Clone for Infallible {
    935    fn clone(&self) -> Infallible {
    936        match *self {}
    937    }
    938}
    939
    940#[stable(feature = "convert_infallible", since = "1.34.0")]
    941impl fmt::Debug for Infallible {
    942    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
    943        match *self {}
    944    }
    945}
    946
    947#[stable(feature = "convert_infallible", since = "1.34.0")]
    948impl fmt::Display for Infallible {
    949    fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
    950        match *self {}
    951    }
    952}
    953
    954#[stable(feature = "str_parse_error2", since = "1.8.0")]
    955impl Error for Infallible {}
    956
    957#[stable(feature = "convert_infallible", since = "1.34.0")]
    958#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
    959impl const PartialEq for Infallible {
    960    fn eq(&self, _: &Infallible) -> bool {
    961        match *self {}
    962    }
    963}
    964
    965#[stable(feature = "convert_infallible", since = "1.34.0")]
    966#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
    967impl const Eq for Infallible {}
    968
    969#[stable(feature = "convert_infallible", since = "1.34.0")]
    970#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
    971impl const PartialOrd for Infallible {
    972    fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
    973        match *self {}
    974    }
    975}
    976
    977#[stable(feature = "convert_infallible", since = "1.34.0")]
    978#[rustc_const_unstable(feature = "const_cmp", issue = "143800")]
    979impl const Ord for Infallible {
    980    fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
    981        match *self {}
    982    }
    983}
    984
    985#[stable(feature = "convert_infallible", since = "1.34.0")]
    986#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
    987impl const From<!> for Infallible {
    988    #[inline]
    989    fn from(x: !) -> Self {
    990        x
    991    }
    992}
    993
    994#[stable(feature = "convert_infallible_hash", since = "1.44.0")]
    995impl Hash for Infallible {
    996    fn hash<H: Hasher>(&self, _: &mut H) {
    997        match *self {}
    998    }
    999}