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

#
alloc/
str.rs

    1//! Utilities for the `str` primitive type.
    2//!
    3//! *[See also the `str` primitive type](str).*
    4
    5#![stable(feature = "rust1", since = "1.0.0")]
    6// Many of the usings in this module are only used in the test configuration.
    7// It's cleaner to just turn off the unused_imports warning than to fix them.
    8#![allow(unused_imports)]
    9
    10use core::borrow::{Borrow, BorrowMut};
    11use core::iter::FusedIterator;
    12use core::mem::MaybeUninit;
    13#[stable(feature = "encode_utf16", since = "1.8.0")]
    14pub use core::str::EncodeUtf16;
    15#[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
    16pub use core::str::SplitAsciiWhitespace;
    17#[stable(feature = "split_inclusive", since = "1.51.0")]
    18pub use core::str::SplitInclusive;
    19#[stable(feature = "rust1", since = "1.0.0")]
    20pub use core::str::SplitWhitespace;
    21#[stable(feature = "rust1", since = "1.0.0")]
    22pub use core::str::pattern;
    23use core::str::pattern::{DoubleEndedSearcher, Pattern, ReverseSearcher, Searcher, Utf8Pattern};
    24#[stable(feature = "rust1", since = "1.0.0")]
    25pub use core::str::{Bytes, CharIndices, Chars, from_utf8, from_utf8_mut};
    26#[stable(feature = "str_escape", since = "1.34.0")]
    27pub use core::str::{EscapeDebug, EscapeDefault, EscapeUnicode};
    28#[stable(feature = "rust1", since = "1.0.0")]
    29pub use core::str::{FromStr, Utf8Error};
    30#[allow(deprecated)]
    31#[stable(feature = "rust1", since = "1.0.0")]
    32pub use core::str::{Lines, LinesAny};
    33#[stable(feature = "rust1", since = "1.0.0")]
    34pub use core::str::{MatchIndices, RMatchIndices};
    35#[stable(feature = "rust1", since = "1.0.0")]
    36pub use core::str::{Matches, RMatches};
    37#[stable(feature = "rust1", since = "1.0.0")]
    38pub use core::str::{ParseBoolError, from_utf8_unchecked, from_utf8_unchecked_mut};
    39#[stable(feature = "rust1", since = "1.0.0")]
    40pub use core::str::{RSplit, Split};
    41#[stable(feature = "rust1", since = "1.0.0")]
    42pub use core::str::{RSplitN, SplitN};
    43#[stable(feature = "rust1", since = "1.0.0")]
    44pub use core::str::{RSplitTerminator, SplitTerminator};
    45#[stable(feature = "utf8_chunks", since = "1.79.0")]
    46pub use core::str::{Utf8Chunk, Utf8Chunks};
    47#[unstable(feature = "str_from_raw_parts", issue = "119206")]
    48pub use core::str::{from_raw_parts, from_raw_parts_mut};
    49use core::unicode::conversions;
    50use core::{mem, ptr};
    51
    52use crate::borrow::ToOwned;
    53use crate::boxed::Box;
    54use crate::slice::{Concat, Join, SliceIndex};
    55use crate::string::String;
    56use crate::vec::Vec;
    57
    58/// Note: `str` in `Concat<str>` is not meaningful here.
    59/// This type parameter of the trait only exists to enable another impl.
    60#[cfg(not(no_global_oom_handling))]
    61#[unstable(feature = "slice_concat_ext", issue = "27747")]
    62impl<S: Borrow<str>> Concat<str> for [S] {
    63    type Output = String;
    64
    65    fn concat(slice: &Self) -> String {
    66        Join::join(slice, "")
    67    }
    68}
    69
    70#[cfg(not(no_global_oom_handling))]
    71#[unstable(feature = "slice_concat_ext", issue = "27747")]
    72impl<S: Borrow<str>> Join<&str> for [S] {
    73    type Output = String;
    74
    75    fn join(slice: &Self, sep: &str) -> String {
    76        unsafe { String::from_utf8_unchecked(join_generic_copy(slice, sep.as_bytes())) }
    77    }
    78}
    79
    80#[cfg(not(no_global_oom_handling))]
    81macro_rules! specialize_for_lengths {
    82    ($separator:expr, $target:expr, $iter:expr; $($num:expr),*) => {{
    83        let mut target = $target;
    84        let iter = $iter;
    85        let sep_bytes = $separator;
    86        match $separator.len() {
    87            $(
    88                // loops with hardcoded sizes run much faster
    89                // specialize the cases with small separator lengths
    90                $num => {
    91                    for s in iter {
    92                        copy_slice_and_advance!(target, sep_bytes);
    93                        let content_bytes = s.borrow().as_ref();
    94                        copy_slice_and_advance!(target, content_bytes);
    95                    }
    96                },
    97            )*
    98            _ => {
    99                // arbitrary non-zero size fallback
    100                for s in iter {
    101                    copy_slice_and_advance!(target, sep_bytes);
    102                    let content_bytes = s.borrow().as_ref();
    103                    copy_slice_and_advance!(target, content_bytes);
    104                }
    105            }
    106        }
    107        target
    108    }}
    109}
    110
    111#[cfg(not(no_global_oom_handling))]
    112macro_rules! copy_slice_and_advance {
    113    ($target:expr, $bytes:expr) => {
    114        let len = $bytes.len();
    115        let (head, tail) = { $target }.split_at_mut(len);
    116        head.copy_from_slice($bytes);
    117        $target = tail;
    118    };
    119}
    120
    121// Optimized join implementation that works for both Vec<T> (T: Copy) and String's inner vec
    122// Currently (2018-05-13) there is a bug with type inference and specialization (see issue #36262)
    123// For this reason SliceConcat<T> is not specialized for T: Copy and SliceConcat<str> is the
    124// only user of this function. It is left in place for the time when that is fixed.
    125//
    126// the bounds for String-join are S: Borrow<str> and for Vec-join Borrow<[T]>
    127// [T] and str both impl AsRef<[T]> for some T
    128// => s.borrow().as_ref() and we always have slices
    129#[cfg(not(no_global_oom_handling))]
    130fn join_generic_copy<B, T, S>(slice: &[S], sep: &[T]) -> Vec<T>
    131where
    132    T: Copy,
    133    B: AsRef<[T]> + ?Sized,
    134    S: Borrow<B>,
    135{
    136    let sep_len = sep.len();
    137    let mut iter = slice.iter();
    138
    139    // the first slice is the only one without a separator preceding it
    140    let first = match iter.next() {
    141        Some(first) => first,
    142        None => return vec![],
    143    };
    144
    145    // compute the exact total length of the joined Vec
    146    // if the `len` calculation overflows, we'll panic
    147    // we would have run out of memory anyway and the rest of the function requires
    148    // the entire Vec pre-allocated for safety
    149    let reserved_len = sep_len
    150        .checked_mul(iter.len())
    151        .and_then(|n| {
    152            slice.iter().map(|s| s.borrow().as_ref().len()).try_fold(n, usize::checked_add)
    153        })
    154        .expect("attempt to join into collection with len > usize::MAX");
    155
    156    // prepare an uninitialized buffer
    157    let mut result = Vec::with_capacity(reserved_len);
    158    debug_assert!(result.capacity() >= reserved_len);
    159
    160    result.extend_from_slice(first.borrow().as_ref());
    161
    162    unsafe {
    163        let pos = result.len();
    164        let target = result.spare_capacity_mut().get_unchecked_mut(..reserved_len - pos);
    165
    166        // Convert the separator and slices to slices of MaybeUninit
    167        // to simplify implementation in specialize_for_lengths
    168        let sep_uninit = core::slice::from_raw_parts(sep.as_ptr().cast(), sep.len());
    169        let iter_uninit = iter.map(|it| {
    170            let it = it.borrow().as_ref();
    171            core::slice::from_raw_parts(it.as_ptr().cast(), it.len())
    172        });
    173
    174        // copy separator and slices over without bounds checks
    175        // generate loops with hardcoded offsets for small separators
    176        // massive improvements possible (~ x2)
    177        let remain = specialize_for_lengths!(sep_uninit, target, iter_uninit; 0, 1, 2, 3, 4);
    178
    179        // A weird borrow implementation may return different
    180        // slices for the length calculation and the actual copy.
    181        // Make sure we don't expose uninitialized bytes to the caller.
    182        let result_len = reserved_len - remain.len();
    183        result.set_len(result_len);
    184    }
    185    result
    186}
    187
    188#[stable(feature = "rust1", since = "1.0.0")]
    189impl Borrow<str> for String {
    190    #[inline]
    191    fn borrow(&self) -> &str {
    192        &self[..]
    193    }
    194}
    195
    196#[stable(feature = "string_borrow_mut", since = "1.36.0")]
    197impl BorrowMut<str> for String {
    198    #[inline]
    199    fn borrow_mut(&mut self) -> &mut str {
    200        &mut self[..]
    201    }
    202}
    203
    204#[cfg(not(no_global_oom_handling))]
    205#[stable(feature = "rust1", since = "1.0.0")]
    206impl ToOwned for str {
    207    type Owned = String;
    208
    209    #[inline]
    210    fn to_owned(&self) -> String {
    211        unsafe { String::from_utf8_unchecked(self.as_bytes().to_owned()) }
    212    }
    213
    214    #[inline]
    215    fn clone_into(&self, target: &mut String) {
    216        target.clear();
    217        target.push_str(self);
    218    }
    219}
    220
    221/// Methods for string slices.
    222impl str {
    223    /// Converts a `Box<str>` into a `Box<[u8]>` without copying or allocating.
    224    ///
    225    /// # Examples
    226    ///
    227    /// ```
    228    /// let s = "this is a string";
    229    /// let boxed_str = s.to_owned().into_boxed_str();
    230    /// let boxed_bytes = boxed_str.into_boxed_bytes();
    231    /// assert_eq!(*boxed_bytes, *s.as_bytes());
    232    /// ```
    233    #[rustc_allow_incoherent_impl]
    234    #[stable(feature = "str_box_extras", since = "1.20.0")]
    235    #[must_use = "`self` will be dropped if the result is not used"]
    236    #[inline]
    237    pub fn into_boxed_bytes(self: Box<Self>) -> Box<[u8]> {
    238        self.into()
    239    }
    240
    241    /// Replaces all matches of a pattern with another string.
    242    ///
    243    /// `replace` creates a new [`String`], and copies the data from this string slice into it.
    244    /// While doing so, it attempts to find matches of a pattern. If it finds any, it
    245    /// replaces them with the replacement string slice.
    246    ///
    247    /// # Examples
    248    ///
    249    /// ```
    250    /// let s = "this is old";
    251    ///
    252    /// assert_eq!("this is new", s.replace("old", "new"));
    253    /// assert_eq!("than an old", s.replace("is", "an"));
    254    /// ```
    255    ///
    256    /// When the pattern doesn't match, it returns this string slice as [`String`]:
    257    ///
    258    /// ```
    259    /// let s = "this is old";
    260    /// assert_eq!(s, s.replace("cookie monster", "little lamb"));
    261    /// ```
    262    #[cfg(not(no_global_oom_handling))]
    263    #[rustc_allow_incoherent_impl]
    264    #[must_use = "this returns the replaced string as a new allocation, \
    265                  without modifying the original"]
    266    #[stable(feature = "rust1", since = "1.0.0")]
    267    #[inline]
    268    pub fn replace<P: Pattern>(&self, from: P, to: &str) -> String {
    269        // Fast path for replacing a single ASCII character with another.
    270        if let Some(from_byte) = match from.as_utf8_pattern() {
    271            Some(Utf8Pattern::StringPattern([from_byte])) => Some(*from_byte),
    272            Some(Utf8Pattern::CharPattern(c)) => c.as_ascii().map(|ascii_char| ascii_char.to_u8()),
    273            _ => None,
    274        } {
    275            if let [to_byte] = to.as_bytes() {
    276                return unsafe { replace_ascii(self.as_bytes(), from_byte, *to_byte) };
    277            }
    278        }
    279        // Set result capacity to self.len() when from.len() <= to.len()
    280        let default_capacity = match from.as_utf8_pattern() {
    281            Some(Utf8Pattern::StringPattern(s)) if s.len() <= to.len() => self.len(),
    282            Some(Utf8Pattern::CharPattern(c)) if c.len_utf8() <= to.len() => self.len(),
    283            _ => 0,
    284        };
    285        let mut result = String::with_capacity(default_capacity);
    286        let mut last_end = 0;
    287        for (start, part) in self.match_indices(from) {
    288            result.push_str(unsafe { self.get_unchecked(last_end..start) });
    289            result.push_str(to);
    290            last_end = start + part.len();
    291        }
    292        result.push_str(unsafe { self.get_unchecked(last_end..self.len()) });
    293        result
    294    }
    295
    296    /// Replaces first N matches of a pattern with another string.
    297    ///
    298    /// `replacen` creates a new [`String`], and copies the data from this string slice into it.
    299    /// While doing so, it attempts to find matches of a pattern. If it finds any, it
    300    /// replaces them with the replacement string slice at most `count` times.
    301    ///
    302    /// # Examples
    303    ///
    304    /// ```
    305    /// let s = "foo foo 123 foo";
    306    /// assert_eq!("new new 123 foo", s.replacen("foo", "new", 2));
    307    /// assert_eq!("faa fao 123 foo", s.replacen('o', "a", 3));
    308    /// assert_eq!("foo foo new23 foo", s.replacen(char::is_numeric, "new", 1));
    309    /// ```
    310    ///
    311    /// When the pattern doesn't match, it returns this string slice as [`String`]:
    312    ///
    313    /// ```
    314    /// let s = "this is old";
    315    /// assert_eq!(s, s.replacen("cookie monster", "little lamb", 10));
    316    /// ```
    317    #[cfg(not(no_global_oom_handling))]
    318    #[rustc_allow_incoherent_impl]
    319    #[doc(alias = "replace_first")]
    320    #[must_use = "this returns the replaced string as a new allocation, \
    321                  without modifying the original"]
    322    #[stable(feature = "str_replacen", since = "1.16.0")]
    323    pub fn replacen<P: Pattern>(&self, pat: P, to: &str, count: usize) -> String {
    324        // Hope to reduce the times of re-allocation
    325        let mut result = String::with_capacity(32);
    326        let mut last_end = 0;
    327        for (start, part) in self.match_indices(pat).take(count) {
    328            result.push_str(unsafe { self.get_unchecked(last_end..start) });
    329            result.push_str(to);
    330            last_end = start + part.len();
    331        }
    332        result.push_str(unsafe { self.get_unchecked(last_end..self.len()) });
    333        result
    334    }
    335
    336    /// Returns the lowercase equivalent of this string slice, as a new [`String`].
    337    ///
    338    /// 'Lowercase' is defined according to the terms of the Unicode Derived Core Property
    339    /// `Lowercase`.
    340    ///
    341    /// Since some characters can expand into multiple characters when changing
    342    /// the case, this function returns a [`String`] instead of modifying the
    343    /// parameter in-place.
    344    ///
    345    /// # Examples
    346    ///
    347    /// Basic usage:
    348    ///
    349    /// ```
    350    /// let s = "HELLO";
    351    ///
    352    /// assert_eq!("hello", s.to_lowercase());
    353    /// ```
    354    ///
    355    /// A tricky example, with sigma:
    356    ///
    357    /// ```
    358    /// let sigma = "Σ";
    359    ///
    360    /// assert_eq!("σ", sigma.to_lowercase());
    361    ///
    362    /// // but at the end of a word, it's ς, not σ:
    363    /// let odysseus = "ὈΔΥΣΣΕΎΣ";
    364    ///
    365    /// assert_eq!("ὀδυσσεύς", odysseus.to_lowercase());
    366    /// ```
    367    ///
    368    /// Languages without case are not changed:
    369    ///
    370    /// ```
    371    /// let new_year = "农历新年";
    372    ///
    373    /// assert_eq!(new_year, new_year.to_lowercase());
    374    /// ```
    375    #[cfg(not(no_global_oom_handling))]
    376    #[rustc_allow_incoherent_impl]
    377    #[must_use = "this returns the lowercase string as a new String, \
    378                  without modifying the original"]
    379    #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
    380    pub fn to_lowercase(&self) -> String {
    381        let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_lowercase);
    382
    383        let prefix_len = s.len();
    384
    385        for (i, c) in rest.char_indices() {
    386            if c == 'Σ' {
    387                // Σ maps to σ, except at the end of a word where it maps to ς.
    388                // This is the only conditional (contextual) but language-independent mapping
    389                // in `SpecialCasing.txt`,
    390                // so hard-code it rather than have a generic "condition" mechanism.
    391                // See https://github.com/rust-lang/rust/issues/26035
    392                let sigma_lowercase = map_uppercase_sigma(self, prefix_len + i);
    393                s.push(sigma_lowercase);
    394            } else {
    395                match conversions::to_lower(c) {
    396                    [a, '\0', _] => s.push(a),
    397                    [a, b, '\0'] => {
    398                        s.push(a);
    399                        s.push(b);
    400                    }
    401                    [a, b, c] => {
    402                        s.push(a);
    403                        s.push(b);
    404                        s.push(c);
    405                    }
    406                }
    407            }
    408        }
    409        return s;
    410
    411        fn map_uppercase_sigma(from: &str, i: usize) -> char {
    412            // See https://www.unicode.org/versions/Unicode7.0.0/ch03.pdf#G33992
    413            // for the definition of `Final_Sigma`.
    414            debug_assert!('Σ'.len_utf8() == 2);
    415            let is_word_final = case_ignorable_then_cased(from[..i].chars().rev())
    416                && !case_ignorable_then_cased(from[i + 2..].chars());
    417            if is_word_final { 'ς' } else { 'σ' }
    418        }
    419
    420        fn case_ignorable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
    421            match iter.skip_while(|&c| c.is_case_ignorable()).next() {
    422                Some(c) => c.is_cased(),
    423                None => false,
    424            }
    425        }
    426    }
    427
    428    /// Returns the uppercase equivalent of this string slice, as a new [`String`].
    429    ///
    430    /// 'Uppercase' is defined according to the terms of the Unicode Derived Core Property
    431    /// `Uppercase`.
    432    ///
    433    /// Since some characters can expand into multiple characters when changing
    434    /// the case, this function returns a [`String`] instead of modifying the
    435    /// parameter in-place.
    436    ///
    437    /// # Examples
    438    ///
    439    /// Basic usage:
    440    ///
    441    /// ```
    442    /// let s = "hello";
    443    ///
    444    /// assert_eq!("HELLO", s.to_uppercase());
    445    /// ```
    446    ///
    447    /// Scripts without case are not changed:
    448    ///
    449    /// ```
    450    /// let new_year = "农历新年";
    451    ///
    452    /// assert_eq!(new_year, new_year.to_uppercase());
    453    /// ```
    454    ///
    455    /// One character can become multiple:
    456    /// ```
    457    /// let s = "tschüß";
    458    ///
    459    /// assert_eq!("TSCHÜSS", s.to_uppercase());
    460    /// ```
    461    #[cfg(not(no_global_oom_handling))]
    462    #[rustc_allow_incoherent_impl]
    463    #[must_use = "this returns the uppercase string as a new String, \
    464                  without modifying the original"]
    465    #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
    466    pub fn to_uppercase(&self) -> String {
    467        let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_uppercase);
    468
    469        for c in rest.chars() {
    470            match conversions::to_upper(c) {
    471                [a, '\0', _] => s.push(a),
    472                [a, b, '\0'] => {
    473                    s.push(a);
    474                    s.push(b);
    475                }
    476                [a, b, c] => {
    477                    s.push(a);
    478                    s.push(b);
    479                    s.push(c);
    480                }
    481            }
    482        }
    483        s
    484    }
    485
    486    /// Converts a [`Box<str>`] into a [`String`] without copying or allocating.
    487    ///
    488    /// # Examples
    489    ///
    490    /// ```
    491    /// let string = String::from("birthday gift");
    492    /// let boxed_str = string.clone().into_boxed_str();
    493    ///
    494    /// assert_eq!(boxed_str.into_string(), string);
    495    /// ```
    496    #[stable(feature = "box_str", since = "1.4.0")]
    497    #[rustc_allow_incoherent_impl]
    498    #[must_use = "`self` will be dropped if the result is not used"]
    499    #[inline]
    500    pub fn into_string(self: Box<Self>) -> String {
    501        let slice = Box::<[u8]>::from(self);
    502        unsafe { String::from_utf8_unchecked(slice.into_vec()) }
    503    }
    504
    505    /// Creates a new [`String`] by repeating a string `n` times.
    506    ///
    507    /// # Panics
    508    ///
    509    /// This function will panic if the capacity would overflow.
    510    ///
    511    /// # Examples
    512    ///
    513    /// Basic usage:
    514    ///
    515    /// ```
    516    /// assert_eq!("abc".repeat(4), String::from("abcabcabcabc"));
    517    /// ```
    518    ///
    519    /// A panic upon overflow:
    520    ///
    521    /// ```should_panic
    522    /// // this will panic at runtime
    523    /// let huge = "0123456789abcdef".repeat(usize::MAX);
    524    /// ```
    525    #[cfg(not(no_global_oom_handling))]
    526    #[rustc_allow_incoherent_impl]
    527    #[must_use]
    528    #[stable(feature = "repeat_str", since = "1.16.0")]
    529    #[inline]
    530    pub fn repeat(&self, n: usize) -> String {
    531        unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
    532    }
    533
    534    /// Returns a copy of this string where each character is mapped to its
    535    /// ASCII upper case equivalent.
    536    ///
    537    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
    538    /// but non-ASCII letters are unchanged.
    539    ///
    540    /// To uppercase the value in-place, use [`make_ascii_uppercase`].
    541    ///
    542    /// To uppercase ASCII characters in addition to non-ASCII characters, use
    543    /// [`to_uppercase`].
    544    ///
    545    /// # Examples
    546    ///
    547    /// ```
    548    /// let s = "Grüße, Jürgen ❤";
    549    ///
    550    /// assert_eq!("GRüßE, JüRGEN ❤", s.to_ascii_uppercase());
    551    /// ```
    552    ///
    553    /// [`make_ascii_uppercase`]: str::make_ascii_uppercase
    554    /// [`to_uppercase`]: #method.to_uppercase
    555    #[cfg(not(no_global_oom_handling))]
    556    #[rustc_allow_incoherent_impl]
    557    #[must_use = "to uppercase the value in-place, use `make_ascii_uppercase()`"]
    558    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    559    #[inline]
    560    pub fn to_ascii_uppercase(&self) -> String {
    561        let mut s = self.to_owned();
    562        s.make_ascii_uppercase();
    563        s
    564    }
    565
    566    /// Returns a copy of this string where each character is mapped to its
    567    /// ASCII lower case equivalent.
    568    ///
    569    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
    570    /// but non-ASCII letters are unchanged.
    571    ///
    572    /// To lowercase the value in-place, use [`make_ascii_lowercase`].
    573    ///
    574    /// To lowercase ASCII characters in addition to non-ASCII characters, use
    575    /// [`to_lowercase`].
    576    ///
    577    /// # Examples
    578    ///
    579    /// ```
    580    /// let s = "Grüße, Jürgen ❤";
    581    ///
    582    /// assert_eq!("grüße, jürgen ❤", s.to_ascii_lowercase());
    583    /// ```
    584    ///
    585    /// [`make_ascii_lowercase`]: str::make_ascii_lowercase
    586    /// [`to_lowercase`]: #method.to_lowercase
    587    #[cfg(not(no_global_oom_handling))]
    588    #[rustc_allow_incoherent_impl]
    589    #[must_use = "to lowercase the value in-place, use `make_ascii_lowercase()`"]
    590    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    591    #[inline]
    592    pub fn to_ascii_lowercase(&self) -> String {
    593        let mut s = self.to_owned();
    594        s.make_ascii_lowercase();
    595        s
    596    }
    597}
    598
    599/// Converts a boxed slice of bytes to a boxed string slice without checking
    600/// that the string contains valid UTF-8.
    601///
    602/// # Safety
    603///
    604/// * The provided bytes must contain a valid UTF-8 sequence.
    605///
    606/// # Examples
    607///
    608/// ```
    609/// let smile_utf8 = Box::new([226, 152, 186]);
    610/// let smile = unsafe { std::str::from_boxed_utf8_unchecked(smile_utf8) };
    611///
    612/// assert_eq!("☺", &*smile);
    613/// ```
    614#[stable(feature = "str_box_extras", since = "1.20.0")]
    615#[must_use]
    616#[inline]
    617pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
    618    unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }
    619}
    620
    621/// Converts leading ascii bytes in `s` by calling the `convert` function.
    622///
    623/// For better average performance, this happens in chunks of `2*size_of::<usize>()`.
    624///
    625/// Returns a tuple of the converted prefix and the remainder starting from
    626/// the first non-ascii character.
    627///
    628/// This function is only public so that it can be verified in a codegen test,
    629/// see `issue-123712-str-to-lower-autovectorization.rs`.
    630#[unstable(feature = "str_internals", issue = "none")]
    631#[doc(hidden)]
    632#[inline]
    633#[cfg(not(no_global_oom_handling))]
    634pub fn convert_while_ascii(s: &str, convert: fn(&u8) -> u8) -> (String, &str) {
    635    // Process the input in chunks of 16 bytes to enable auto-vectorization.
    636    // Previously the chunk size depended on the size of `usize`,
    637    // but on 32-bit platforms with sse or neon is also the better choice.
    638    // The only downside on other platforms would be a bit more loop-unrolling.
    639    const N: usize = 16;
    640
    641    let mut slice = s.as_bytes();
    642    let mut out = Vec::with_capacity(slice.len());
    643    let mut out_slice = out.spare_capacity_mut();
    644
    645    let mut ascii_prefix_len = 0_usize;
    646    let mut is_ascii = [false; N];
    647
    648    while slice.len() >= N {
    649        // SAFETY: checked in loop condition
    650        let chunk = unsafe { slice.get_unchecked(..N) };
    651        // SAFETY: out_slice has at least same length as input slice and gets sliced with the same offsets
    652        let out_chunk = unsafe { out_slice.get_unchecked_mut(..N) };
    653
    654        for j in 0..N {
    655            is_ascii[j] = chunk[j] <= 127;
    656        }
    657
    658        // Auto-vectorization for this check is a bit fragile, sum and comparing against the chunk
    659        // size gives the best result, specifically a pmovmsk instruction on x86.
    660        // See https://github.com/llvm/llvm-project/issues/96395 for why llvm currently does not
    661        // currently recognize other similar idioms.
    662        if is_ascii.iter().map(|x| *x as u8).sum::<u8>() as usize != N {
    663            break;
    664        }
    665
    666        for j in 0..N {
    667            out_chunk[j] = MaybeUninit::new(convert(&chunk[j]));
    668        }
    669
    670        ascii_prefix_len += N;
    671        slice = unsafe { slice.get_unchecked(N..) };
    672        out_slice = unsafe { out_slice.get_unchecked_mut(N..) };
    673    }
    674
    675    // handle the remainder as individual bytes
    676    while slice.len() > 0 {
    677        let byte = slice[0];
    678        if byte > 127 {
    679            break;
    680        }
    681        // SAFETY: out_slice has at least same length as input slice
    682        unsafe {
    683            *out_slice.get_unchecked_mut(0) = MaybeUninit::new(convert(&byte));
    684        }
    685        ascii_prefix_len += 1;
    686        slice = unsafe { slice.get_unchecked(1..) };
    687        out_slice = unsafe { out_slice.get_unchecked_mut(1..) };
    688    }
    689
    690    unsafe {
    691        // SAFETY: ascii_prefix_len bytes have been initialized above
    692        out.set_len(ascii_prefix_len);
    693
    694        // SAFETY: We have written only valid ascii to the output vec
    695        let ascii_string = String::from_utf8_unchecked(out);
    696
    697        // SAFETY: we know this is a valid char boundary
    698        // since we only skipped over leading ascii bytes
    699        let rest = core::str::from_utf8_unchecked(slice);
    700
    701        (ascii_string, rest)
    702    }
    703}
    704#[inline]
    705#[cfg(not(no_global_oom_handling))]
    706#[allow(dead_code)]
    707/// Faster implementation of string replacement for ASCII to ASCII cases.
    708/// Should produce fast vectorized code.
    709unsafe fn replace_ascii(utf8_bytes: &[u8], from: u8, to: u8) -> String {
    710    let result: Vec<u8> = utf8_bytes.iter().map(|b| if *b == from { to } else { *b }).collect();
    711    // SAFETY: We replaced ascii with ascii on valid utf8 strings.
    712    unsafe { String::from_utf8_unchecked(result) }
    713}