#
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            use core::unicode::{Case_Ignorable, Cased};
    422            match iter.skip_while(|&c| Case_Ignorable(c)).next() {
    423                Some(c) => Cased(c),
    424                None => false,
    425            }
    426        }
    427    }
    428
    429    /// Returns the uppercase equivalent of this string slice, as a new [`String`].
    430    ///
    431    /// 'Uppercase' is defined according to the terms of the Unicode Derived Core Property
    432    /// `Uppercase`.
    433    ///
    434    /// Since some characters can expand into multiple characters when changing
    435    /// the case, this function returns a [`String`] instead of modifying the
    436    /// parameter in-place.
    437    ///
    438    /// # Examples
    439    ///
    440    /// Basic usage:
    441    ///
    442    /// ```
    443    /// let s = "hello";
    444    ///
    445    /// assert_eq!("HELLO", s.to_uppercase());
    446    /// ```
    447    ///
    448    /// Scripts without case are not changed:
    449    ///
    450    /// ```
    451    /// let new_year = "农历新年";
    452    ///
    453    /// assert_eq!(new_year, new_year.to_uppercase());
    454    /// ```
    455    ///
    456    /// One character can become multiple:
    457    /// ```
    458    /// let s = "tschüß";
    459    ///
    460    /// assert_eq!("TSCHÜSS", s.to_uppercase());
    461    /// ```
    462    #[cfg(not(no_global_oom_handling))]
    463    #[rustc_allow_incoherent_impl]
    464    #[must_use = "this returns the uppercase string as a new String, \
    465                  without modifying the original"]
    466    #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
    467    pub fn to_uppercase(&self) -> String {
    468        let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_uppercase);
    469
    470        for c in rest.chars() {
    471            match conversions::to_upper(c) {
    472                [a, '\0', _] => s.push(a),
    473                [a, b, '\0'] => {
    474                    s.push(a);
    475                    s.push(b);
    476                }
    477                [a, b, c] => {
    478                    s.push(a);
    479                    s.push(b);
    480                    s.push(c);
    481                }
    482            }
    483        }
    484        s
    485    }
    486
    487    /// Converts a [`Box<str>`] into a [`String`] without copying or allocating.
    488    ///
    489    /// # Examples
    490    ///
    491    /// ```
    492    /// let string = String::from("birthday gift");
    493    /// let boxed_str = string.clone().into_boxed_str();
    494    ///
    495    /// assert_eq!(boxed_str.into_string(), string);
    496    /// ```
    497    #[stable(feature = "box_str", since = "1.4.0")]
    498    #[rustc_allow_incoherent_impl]
    499    #[must_use = "`self` will be dropped if the result is not used"]
    500    #[inline]
    501    pub fn into_string(self: Box<Self>) -> String {
    502        let slice = Box::<[u8]>::from(self);
    503        unsafe { String::from_utf8_unchecked(slice.into_vec()) }
    504    }
    505
    506    /// Creates a new [`String`] by repeating a string `n` times.
    507    ///
    508    /// # Panics
    509    ///
    510    /// This function will panic if the capacity would overflow.
    511    ///
    512    /// # Examples
    513    ///
    514    /// Basic usage:
    515    ///
    516    /// ```
    517    /// assert_eq!("abc".repeat(4), String::from("abcabcabcabc"));
    518    /// ```
    519    ///
    520    /// A panic upon overflow:
    521    ///
    522    /// ```should_panic
    523    /// // this will panic at runtime
    524    /// let huge = "0123456789abcdef".repeat(usize::MAX);
    525    /// ```
    526    #[cfg(not(no_global_oom_handling))]
    527    #[rustc_allow_incoherent_impl]
    528    #[must_use]
    529    #[stable(feature = "repeat_str", since = "1.16.0")]
    530    #[inline]
    531    pub fn repeat(&self, n: usize) -> String {
    532        unsafe { String::from_utf8_unchecked(self.as_bytes().repeat(n)) }
    533    }
    534
    535    /// Returns a copy of this string where each character is mapped to its
    536    /// ASCII upper case equivalent.
    537    ///
    538    /// ASCII letters 'a' to 'z' are mapped to 'A' to 'Z',
    539    /// but non-ASCII letters are unchanged.
    540    ///
    541    /// To uppercase the value in-place, use [`make_ascii_uppercase`].
    542    ///
    543    /// To uppercase ASCII characters in addition to non-ASCII characters, use
    544    /// [`to_uppercase`].
    545    ///
    546    /// # Examples
    547    ///
    548    /// ```
    549    /// let s = "Grüße, Jürgen ❤";
    550    ///
    551    /// assert_eq!("GRüßE, JüRGEN ❤", s.to_ascii_uppercase());
    552    /// ```
    553    ///
    554    /// [`make_ascii_uppercase`]: str::make_ascii_uppercase
    555    /// [`to_uppercase`]: #method.to_uppercase
    556    #[cfg(not(no_global_oom_handling))]
    557    #[rustc_allow_incoherent_impl]
    558    #[must_use = "to uppercase the value in-place, use `make_ascii_uppercase()`"]
    559    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    560    #[inline]
    561    pub fn to_ascii_uppercase(&self) -> String {
    562        let mut s = self.to_owned();
    563        s.make_ascii_uppercase();
    564        s
    565    }
    566
    567    /// Returns a copy of this string where each character is mapped to its
    568    /// ASCII lower case equivalent.
    569    ///
    570    /// ASCII letters 'A' to 'Z' are mapped to 'a' to 'z',
    571    /// but non-ASCII letters are unchanged.
    572    ///
    573    /// To lowercase the value in-place, use [`make_ascii_lowercase`].
    574    ///
    575    /// To lowercase ASCII characters in addition to non-ASCII characters, use
    576    /// [`to_lowercase`].
    577    ///
    578    /// # Examples
    579    ///
    580    /// ```
    581    /// let s = "Grüße, Jürgen ❤";
    582    ///
    583    /// assert_eq!("grüße, jürgen ❤", s.to_ascii_lowercase());
    584    /// ```
    585    ///
    586    /// [`make_ascii_lowercase`]: str::make_ascii_lowercase
    587    /// [`to_lowercase`]: #method.to_lowercase
    588    #[cfg(not(no_global_oom_handling))]
    589    #[rustc_allow_incoherent_impl]
    590    #[must_use = "to lowercase the value in-place, use `make_ascii_lowercase()`"]
    591    #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
    592    #[inline]
    593    pub fn to_ascii_lowercase(&self) -> String {
    594        let mut s = self.to_owned();
    595        s.make_ascii_lowercase();
    596        s
    597    }
    598}
    599
    600/// Converts a boxed slice of bytes to a boxed string slice without checking
    601/// that the string contains valid UTF-8.
    602///
    603/// # Safety
    604///
    605/// * The provided bytes must contain a valid UTF-8 sequence.
    606///
    607/// # Examples
    608///
    609/// ```
    610/// let smile_utf8 = Box::new([226, 152, 186]);
    611/// let smile = unsafe { std::str::from_boxed_utf8_unchecked(smile_utf8) };
    612///
    613/// assert_eq!("☺", &*smile);
    614/// ```
    615#[stable(feature = "str_box_extras", since = "1.20.0")]
    616#[must_use]
    617#[inline]
    618pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
    619    unsafe { Box::from_raw(Box::into_raw(v) as *mut str) }
    620}
    621
    622/// Converts leading ascii bytes in `s` by calling the `convert` function.
    623///
    624/// For better average performance, this happens in chunks of `2*size_of::<usize>()`.
    625///
    626/// Returns a tuple of the converted prefix and the remainder starting from
    627/// the first non-ascii character.
    628///
    629/// This function is only public so that it can be verified in a codegen test,
    630/// see `issue-123712-str-to-lower-autovectorization.rs`.
    631#[unstable(feature = "str_internals", issue = "none")]
    632#[doc(hidden)]
    633#[inline]
    634#[cfg(not(no_global_oom_handling))]
    635pub fn convert_while_ascii(s: &str, convert: fn(&u8) -> u8) -> (String, &str) {
    636    // Process the input in chunks of 16 bytes to enable auto-vectorization.
    637    // Previously the chunk size depended on the size of `usize`,
    638    // but on 32-bit platforms with sse or neon is also the better choice.
    639    // The only downside on other platforms would be a bit more loop-unrolling.
    640    const N: usize = 16;
    641
    642    let mut slice = s.as_bytes();
    643    let mut out = Vec::with_capacity(slice.len());
    644    let mut out_slice = out.spare_capacity_mut();
    645
    646    let mut ascii_prefix_len = 0_usize;
    647    let mut is_ascii = [false; N];
    648
    649    while slice.len() >= N {
    650        // SAFETY: checked in loop condition
    651        let chunk = unsafe { slice.get_unchecked(..N) };
    652        // SAFETY: out_slice has at least same length as input slice and gets sliced with the same offsets
    653        let out_chunk = unsafe { out_slice.get_unchecked_mut(..N) };
    654
    655        for j in 0..N {
    656            is_ascii[j] = chunk[j] <= 127;
    657        }
    658
    659        // Auto-vectorization for this check is a bit fragile, sum and comparing against the chunk
    660        // size gives the best result, specifically a pmovmsk instruction on x86.
    661        // See https://github.com/llvm/llvm-project/issues/96395 for why llvm currently does not
    662        // currently recognize other similar idioms.
    663        if is_ascii.iter().map(|x| *x as u8).sum::<u8>() as usize != N {
    664            break;
    665        }
    666
    667        for j in 0..N {
    668            out_chunk[j] = MaybeUninit::new(convert(&chunk[j]));
    669        }
    670
    671        ascii_prefix_len += N;
    672        slice = unsafe { slice.get_unchecked(N..) };
    673        out_slice = unsafe { out_slice.get_unchecked_mut(N..) };
    674    }
    675
    676    // handle the remainder as individual bytes
    677    while slice.len() > 0 {
    678        let byte = slice[0];
    679        if byte > 127 {
    680            break;
    681        }
    682        // SAFETY: out_slice has at least same length as input slice
    683        unsafe {
    684            *out_slice.get_unchecked_mut(0) = MaybeUninit::new(convert(&byte));
    685        }
    686        ascii_prefix_len += 1;
    687        slice = unsafe { slice.get_unchecked(1..) };
    688        out_slice = unsafe { out_slice.get_unchecked_mut(1..) };
    689    }
    690
    691    unsafe {
    692        // SAFETY: ascii_prefix_len bytes have been initialized above
    693        out.set_len(ascii_prefix_len);
    694
    695        // SAFETY: We have written only valid ascii to the output vec
    696        let ascii_string = String::from_utf8_unchecked(out);
    697
    698        // SAFETY: we know this is a valid char boundary
    699        // since we only skipped over leading ascii bytes
    700        let rest = core::str::from_utf8_unchecked(slice);
    701
    702        (ascii_string, rest)
    703    }
    704}
    705#[inline]
    706#[cfg(not(no_global_oom_handling))]
    707#[allow(dead_code)]
    708/// Faster implementation of string replacement for ASCII to ASCII cases.
    709/// Should produce fast vectorized code.
    710unsafe fn replace_ascii(utf8_bytes: &[u8], from: u8, to: u8) -> String {
    711    let result: Vec<u8> = utf8_bytes.iter().map(|b| if *b == from { to } else { *b }).collect();
    712    // SAFETY: We replaced ascii with ascii on valid utf8 strings.
    713    unsafe { String::from_utf8_unchecked(result) }
    714}