core/array/
mod.rs

1//! Utilities for the array primitive type.
2//!
3//! *[See also the array primitive type](array).*
4
5#![stable(feature = "core_array", since = "1.35.0")]
6
7use crate::borrow::{Borrow, BorrowMut};
8use crate::cmp::Ordering;
9use crate::convert::Infallible;
10use crate::error::Error;
11use crate::fmt;
12use crate::hash::{self, Hash};
13use crate::intrinsics::transmute_unchecked;
14use crate::iter::{UncheckedIterator, repeat_n};
15use crate::mem::{self, MaybeUninit};
16use crate::ops::{
17    ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
18};
19use crate::ptr::{null, null_mut};
20use crate::slice::{Iter, IterMut};
21
22mod ascii;
23mod drain;
24mod equality;
25mod iter;
26
27pub(crate) use drain::drain_array_with;
28#[stable(feature = "array_value_iter", since = "1.51.0")]
29pub use iter::IntoIter;
30
31/// Creates an array of type `[T; N]` by repeatedly cloning a value.
32///
33/// This is the same as `[val; N]`, but it also works for types that do not
34/// implement [`Copy`].
35///
36/// The provided value will be used as an element of the resulting array and
37/// will be cloned N - 1 times to fill up the rest. If N is zero, the value
38/// will be dropped.
39///
40/// # Example
41///
42/// Creating multiple copies of a `String`:
43/// ```rust
44/// #![feature(array_repeat)]
45///
46/// use std::array;
47///
48/// let string = "Hello there!".to_string();
49/// let strings = array::repeat(string);
50/// assert_eq!(strings, ["Hello there!", "Hello there!"]);
51/// ```
52#[inline]
53#[unstable(feature = "array_repeat", issue = "126695")]
54pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
55    from_trusted_iterator(repeat_n(val, N))
56}
57
58/// Creates an array where each element is produced by calling `f` with
59/// that element's index while walking forward through the array.
60///
61/// This is essentially the same as writing
62/// ```text
63/// [f(0), f(1), f(2), …, f(N - 2), f(N - 1)]
64/// ```
65/// and is similar to `(0..i).map(f)`, just for arrays not iterators.
66///
67/// If `N == 0`, this produces an empty array without ever calling `f`.
68///
69/// # Example
70///
71/// ```rust
72/// // type inference is helping us here, the way `from_fn` knows how many
73/// // elements to produce is the length of array down there: only arrays of
74/// // equal lengths can be compared, so the const generic parameter `N` is
75/// // inferred to be 5, thus creating array of 5 elements.
76///
77/// let array = core::array::from_fn(|i| i);
78/// // indexes are:    0  1  2  3  4
79/// assert_eq!(array, [0, 1, 2, 3, 4]);
80///
81/// let array2: [usize; 8] = core::array::from_fn(|i| i * 2);
82/// // indexes are:     0  1  2  3  4  5   6   7
83/// assert_eq!(array2, [0, 2, 4, 6, 8, 10, 12, 14]);
84///
85/// let bool_arr = core::array::from_fn::<_, 5, _>(|i| i % 2 == 0);
86/// // indexes are:       0     1      2     3      4
87/// assert_eq!(bool_arr, [true, false, true, false, true]);
88/// ```
89///
90/// You can also capture things, for example to create an array full of clones
91/// where you can't just use `[item; N]` because it's not `Copy`:
92/// ```
93/// # // TBH `array::repeat` would be better for this, but it's not stable yet.
94/// let my_string = String::from("Hello");
95/// let clones: [String; 42] = std::array::from_fn(|_| my_string.clone());
96/// assert!(clones.iter().all(|x| *x == my_string));
97/// ```
98///
99/// The array is generated in ascending index order, starting from the front
100/// and going towards the back, so you can use closures with mutable state:
101/// ```
102/// let mut state = 1;
103/// let a = std::array::from_fn(|_| { let x = state; state *= 2; x });
104/// assert_eq!(a, [1, 2, 4, 8, 16, 32]);
105/// ```
106#[inline]
107#[stable(feature = "array_from_fn", since = "1.63.0")]
108pub fn from_fn<T, const N: usize, F>(f: F) -> [T; N]
109where
110    F: FnMut(usize) -> T,
111{
112    try_from_fn(NeverShortCircuit::wrap_mut_1(f)).0
113}
114
115/// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
116/// Unlike [`from_fn`], where the element creation can't fail, this version will return an error
117/// if any element creation was unsuccessful.
118///
119/// The return type of this function depends on the return type of the closure.
120/// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
121/// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
122///
123/// # Arguments
124///
125/// * `cb`: Callback where the passed argument is the current array index.
126///
127/// # Example
128///
129/// ```rust
130/// #![feature(array_try_from_fn)]
131///
132/// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
133/// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
134///
135/// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
136/// assert!(array.is_err());
137///
138/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
139/// assert_eq!(array, Some([100, 101, 102, 103]));
140///
141/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
142/// assert_eq!(array, None);
143/// ```
144#[inline]
145#[unstable(feature = "array_try_from_fn", issue = "89379")]
146pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
147where
148    F: FnMut(usize) -> R,
149    R: Try,
150    R::Residual: Residual<[R::Output; N]>,
151{
152    let mut array = [const { MaybeUninit::uninit() }; N];
153    match try_from_fn_erased(&mut array, cb) {
154        ControlFlow::Break(r) => FromResidual::from_residual(r),
155        ControlFlow::Continue(()) => {
156            // SAFETY: All elements of the array were populated.
157            try { unsafe { MaybeUninit::array_assume_init(array) } }
158        }
159    }
160}
161
162/// Converts a reference to `T` into a reference to an array of length 1 (without copying).
163#[stable(feature = "array_from_ref", since = "1.53.0")]
164#[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
165pub const fn from_ref<T>(s: &T) -> &[T; 1] {
166    // SAFETY: Converting `&T` to `&[T; 1]` is sound.
167    unsafe { &*(s as *const T).cast::<[T; 1]>() }
168}
169
170/// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
171#[stable(feature = "array_from_ref", since = "1.53.0")]
172#[rustc_const_stable(feature = "const_array_from_ref", since = "1.83.0")]
173pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
174    // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
175    unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
176}
177
178/// The error type returned when a conversion from a slice to an array fails.
179#[stable(feature = "try_from", since = "1.34.0")]
180#[derive(Debug, Copy, Clone)]
181pub struct TryFromSliceError(());
182
183#[stable(feature = "core_array", since = "1.35.0")]
184impl fmt::Display for TryFromSliceError {
185    #[inline]
186    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
187        #[allow(deprecated)]
188        self.description().fmt(f)
189    }
190}
191
192#[stable(feature = "try_from", since = "1.34.0")]
193impl Error for TryFromSliceError {
194    #[allow(deprecated)]
195    fn description(&self) -> &str {
196        "could not convert slice to array"
197    }
198}
199
200#[stable(feature = "try_from_slice_error", since = "1.36.0")]
201impl From<Infallible> for TryFromSliceError {
202    fn from(x: Infallible) -> TryFromSliceError {
203        match x {}
204    }
205}
206
207#[stable(feature = "rust1", since = "1.0.0")]
208impl<T, const N: usize> AsRef<[T]> for [T; N] {
209    #[inline]
210    fn as_ref(&self) -> &[T] {
211        &self[..]
212    }
213}
214
215#[stable(feature = "rust1", since = "1.0.0")]
216impl<T, const N: usize> AsMut<[T]> for [T; N] {
217    #[inline]
218    fn as_mut(&mut self) -> &mut [T] {
219        &mut self[..]
220    }
221}
222
223#[stable(feature = "array_borrow", since = "1.4.0")]
224impl<T, const N: usize> Borrow<[T]> for [T; N] {
225    fn borrow(&self) -> &[T] {
226        self
227    }
228}
229
230#[stable(feature = "array_borrow", since = "1.4.0")]
231impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
232    fn borrow_mut(&mut self) -> &mut [T] {
233        self
234    }
235}
236
237/// Tries to create an array `[T; N]` by copying from a slice `&[T]`.
238/// Succeeds if `slice.len() == N`.
239///
240/// ```
241/// let bytes: [u8; 3] = [1, 0, 2];
242///
243/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
244/// assert_eq!(1, u16::from_le_bytes(bytes_head));
245///
246/// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
247/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
248/// ```
249#[stable(feature = "try_from", since = "1.34.0")]
250impl<T, const N: usize> TryFrom<&[T]> for [T; N]
251where
252    T: Copy,
253{
254    type Error = TryFromSliceError;
255
256    #[inline]
257    fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
258        <&Self>::try_from(slice).copied()
259    }
260}
261
262/// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
263/// Succeeds if `slice.len() == N`.
264///
265/// ```
266/// let mut bytes: [u8; 3] = [1, 0, 2];
267///
268/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
269/// assert_eq!(1, u16::from_le_bytes(bytes_head));
270///
271/// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
272/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
273/// ```
274#[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
275impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
276where
277    T: Copy,
278{
279    type Error = TryFromSliceError;
280
281    #[inline]
282    fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
283        <Self>::try_from(&*slice)
284    }
285}
286
287/// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
288/// `slice.len() == N`.
289///
290/// ```
291/// let bytes: [u8; 3] = [1, 0, 2];
292///
293/// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
294/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
295///
296/// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
297/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
298/// ```
299#[stable(feature = "try_from", since = "1.34.0")]
300impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
301    type Error = TryFromSliceError;
302
303    #[inline]
304    fn try_from(slice: &'a [T]) -> Result<&'a [T; N], TryFromSliceError> {
305        slice.as_array().ok_or(TryFromSliceError(()))
306    }
307}
308
309/// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
310/// `&mut [T]`. Succeeds if `slice.len() == N`.
311///
312/// ```
313/// let mut bytes: [u8; 3] = [1, 0, 2];
314///
315/// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
316/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
317///
318/// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
319/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
320/// ```
321#[stable(feature = "try_from", since = "1.34.0")]
322impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
323    type Error = TryFromSliceError;
324
325    #[inline]
326    fn try_from(slice: &'a mut [T]) -> Result<&'a mut [T; N], TryFromSliceError> {
327        slice.as_mut_array().ok_or(TryFromSliceError(()))
328    }
329}
330
331/// The hash of an array is the same as that of the corresponding slice,
332/// as required by the `Borrow` implementation.
333///
334/// ```
335/// use std::hash::BuildHasher;
336///
337/// let b = std::hash::RandomState::new();
338/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
339/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
340/// assert_eq!(b.hash_one(a), b.hash_one(s));
341/// ```
342#[stable(feature = "rust1", since = "1.0.0")]
343impl<T: Hash, const N: usize> Hash for [T; N] {
344    fn hash<H: hash::Hasher>(&self, state: &mut H) {
345        Hash::hash(&self[..], state)
346    }
347}
348
349#[stable(feature = "rust1", since = "1.0.0")]
350impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
351    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
352        fmt::Debug::fmt(&&self[..], f)
353    }
354}
355
356#[stable(feature = "rust1", since = "1.0.0")]
357impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
358    type Item = &'a T;
359    type IntoIter = Iter<'a, T>;
360
361    fn into_iter(self) -> Iter<'a, T> {
362        self.iter()
363    }
364}
365
366#[stable(feature = "rust1", since = "1.0.0")]
367impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
368    type Item = &'a mut T;
369    type IntoIter = IterMut<'a, T>;
370
371    fn into_iter(self) -> IterMut<'a, T> {
372        self.iter_mut()
373    }
374}
375
376#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
377impl<T, I, const N: usize> Index<I> for [T; N]
378where
379    [T]: Index<I>,
380{
381    type Output = <[T] as Index<I>>::Output;
382
383    #[inline]
384    fn index(&self, index: I) -> &Self::Output {
385        Index::index(self as &[T], index)
386    }
387}
388
389#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
390impl<T, I, const N: usize> IndexMut<I> for [T; N]
391where
392    [T]: IndexMut<I>,
393{
394    #[inline]
395    fn index_mut(&mut self, index: I) -> &mut Self::Output {
396        IndexMut::index_mut(self as &mut [T], index)
397    }
398}
399
400/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
401#[stable(feature = "rust1", since = "1.0.0")]
402impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
403    #[inline]
404    fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
405        PartialOrd::partial_cmp(&&self[..], &&other[..])
406    }
407    #[inline]
408    fn lt(&self, other: &[T; N]) -> bool {
409        PartialOrd::lt(&&self[..], &&other[..])
410    }
411    #[inline]
412    fn le(&self, other: &[T; N]) -> bool {
413        PartialOrd::le(&&self[..], &&other[..])
414    }
415    #[inline]
416    fn ge(&self, other: &[T; N]) -> bool {
417        PartialOrd::ge(&&self[..], &&other[..])
418    }
419    #[inline]
420    fn gt(&self, other: &[T; N]) -> bool {
421        PartialOrd::gt(&&self[..], &&other[..])
422    }
423}
424
425/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
426#[stable(feature = "rust1", since = "1.0.0")]
427impl<T: Ord, const N: usize> Ord for [T; N] {
428    #[inline]
429    fn cmp(&self, other: &[T; N]) -> Ordering {
430        Ord::cmp(&&self[..], &&other[..])
431    }
432}
433
434#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
435impl<T: Copy, const N: usize> Copy for [T; N] {}
436
437#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
438impl<T: Clone, const N: usize> Clone for [T; N] {
439    #[inline]
440    fn clone(&self) -> Self {
441        SpecArrayClone::clone(self)
442    }
443
444    #[inline]
445    fn clone_from(&mut self, other: &Self) {
446        self.clone_from_slice(other);
447    }
448}
449
450trait SpecArrayClone: Clone {
451    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
452}
453
454impl<T: Clone> SpecArrayClone for T {
455    #[inline]
456    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
457        from_trusted_iterator(array.iter().cloned())
458    }
459}
460
461impl<T: Copy> SpecArrayClone for T {
462    #[inline]
463    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
464        *array
465    }
466}
467
468// The Default impls cannot be done with const generics because `[T; 0]` doesn't
469// require Default to be implemented, and having different impl blocks for
470// different numbers isn't supported yet.
471
472macro_rules! array_impl_default {
473    {$n:expr, $t:ident $($ts:ident)*} => {
474        #[stable(since = "1.4.0", feature = "array_default")]
475        impl<T> Default for [T; $n] where T: Default {
476            fn default() -> [T; $n] {
477                [$t::default(), $($ts::default()),*]
478            }
479        }
480        array_impl_default!{($n - 1), $($ts)*}
481    };
482    {$n:expr,} => {
483        #[stable(since = "1.4.0", feature = "array_default")]
484        impl<T> Default for [T; $n] {
485            fn default() -> [T; $n] { [] }
486        }
487    };
488}
489
490array_impl_default! {32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T}
491
492impl<T, const N: usize> [T; N] {
493    /// Returns an array of the same size as `self`, with function `f` applied to each element
494    /// in order.
495    ///
496    /// If you don't necessarily need a new fixed-size array, consider using
497    /// [`Iterator::map`] instead.
498    ///
499    ///
500    /// # Note on performance and stack usage
501    ///
502    /// Unfortunately, usages of this method are currently not always optimized
503    /// as well as they could be. This mainly concerns large arrays, as mapping
504    /// over small arrays seem to be optimized just fine. Also note that in
505    /// debug mode (i.e. without any optimizations), this method can use a lot
506    /// of stack space (a few times the size of the array or more).
507    ///
508    /// Therefore, in performance-critical code, try to avoid using this method
509    /// on large arrays or check the emitted code. Also try to avoid chained
510    /// maps (e.g. `arr.map(...).map(...)`).
511    ///
512    /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
513    /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
514    /// really need a new array of the same size as the result. Rust's lazy
515    /// iterators tend to get optimized very well.
516    ///
517    ///
518    /// # Examples
519    ///
520    /// ```
521    /// let x = [1, 2, 3];
522    /// let y = x.map(|v| v + 1);
523    /// assert_eq!(y, [2, 3, 4]);
524    ///
525    /// let x = [1, 2, 3];
526    /// let mut temp = 0;
527    /// let y = x.map(|v| { temp += 1; v * temp });
528    /// assert_eq!(y, [1, 4, 9]);
529    ///
530    /// let x = ["Ferris", "Bueller's", "Day", "Off"];
531    /// let y = x.map(|v| v.len());
532    /// assert_eq!(y, [6, 9, 3, 3]);
533    /// ```
534    #[must_use]
535    #[stable(feature = "array_map", since = "1.55.0")]
536    pub fn map<F, U>(self, f: F) -> [U; N]
537    where
538        F: FnMut(T) -> U,
539    {
540        self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
541    }
542
543    /// A fallible function `f` applied to each element on array `self` in order to
544    /// return an array the same size as `self` or the first error encountered.
545    ///
546    /// The return type of this function depends on the return type of the closure.
547    /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
548    /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
549    ///
550    /// # Examples
551    ///
552    /// ```
553    /// #![feature(array_try_map)]
554    ///
555    /// let a = ["1", "2", "3"];
556    /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
557    /// assert_eq!(b, [2, 3, 4]);
558    ///
559    /// let a = ["1", "2a", "3"];
560    /// let b = a.try_map(|v| v.parse::<u32>());
561    /// assert!(b.is_err());
562    ///
563    /// use std::num::NonZero;
564    ///
565    /// let z = [1, 2, 0, 3, 4];
566    /// assert_eq!(z.try_map(NonZero::new), None);
567    ///
568    /// let a = [1, 2, 3];
569    /// let b = a.try_map(NonZero::new);
570    /// let c = b.map(|x| x.map(NonZero::get));
571    /// assert_eq!(c, Some(a));
572    /// ```
573    #[unstable(feature = "array_try_map", issue = "79711")]
574    pub fn try_map<R>(self, f: impl FnMut(T) -> R) -> ChangeOutputType<R, [R::Output; N]>
575    where
576        R: Try<Residual: Residual<[R::Output; N]>>,
577    {
578        drain_array_with(self, |iter| try_from_trusted_iterator(iter.map(f)))
579    }
580
581    /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
582    #[stable(feature = "array_as_slice", since = "1.57.0")]
583    #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
584    pub const fn as_slice(&self) -> &[T] {
585        self
586    }
587
588    /// Returns a mutable slice containing the entire array. Equivalent to
589    /// `&mut s[..]`.
590    #[stable(feature = "array_as_slice", since = "1.57.0")]
591    #[rustc_const_stable(feature = "const_array_as_mut_slice", since = "CURRENT_RUSTC_VERSION")]
592    pub const fn as_mut_slice(&mut self) -> &mut [T] {
593        self
594    }
595
596    /// Borrows each element and returns an array of references with the same
597    /// size as `self`.
598    ///
599    ///
600    /// # Example
601    ///
602    /// ```
603    /// let floats = [3.1, 2.7, -1.0];
604    /// let float_refs: [&f64; 3] = floats.each_ref();
605    /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
606    /// ```
607    ///
608    /// This method is particularly useful if combined with other methods, like
609    /// [`map`](#method.map). This way, you can avoid moving the original
610    /// array if its elements are not [`Copy`].
611    ///
612    /// ```
613    /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
614    /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
615    /// assert_eq!(is_ascii, [true, false, true]);
616    ///
617    /// // We can still access the original array: it has not been moved.
618    /// assert_eq!(strings.len(), 3);
619    /// ```
620    #[stable(feature = "array_methods", since = "1.77.0")]
621    #[rustc_const_unstable(feature = "const_array_each_ref", issue = "133289")]
622    pub const fn each_ref(&self) -> [&T; N] {
623        let mut buf = [null::<T>(); N];
624
625        // FIXME(const-hack): We would like to simply use iterators for this (as in the original implementation), but this is not allowed in constant expressions.
626        let mut i = 0;
627        while i < N {
628            buf[i] = &raw const self[i];
629
630            i += 1;
631        }
632
633        // SAFETY: `*const T` has the same layout as `&T`, and we've also initialised each pointer as a valid reference.
634        unsafe { transmute_unchecked(buf) }
635    }
636
637    /// Borrows each element mutably and returns an array of mutable references
638    /// with the same size as `self`.
639    ///
640    ///
641    /// # Example
642    ///
643    /// ```
644    ///
645    /// let mut floats = [3.1, 2.7, -1.0];
646    /// let float_refs: [&mut f64; 3] = floats.each_mut();
647    /// *float_refs[0] = 0.0;
648    /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
649    /// assert_eq!(floats, [0.0, 2.7, -1.0]);
650    /// ```
651    #[stable(feature = "array_methods", since = "1.77.0")]
652    #[rustc_const_unstable(feature = "const_array_each_ref", issue = "133289")]
653    pub const fn each_mut(&mut self) -> [&mut T; N] {
654        let mut buf = [null_mut::<T>(); N];
655
656        // FIXME(const-hack): We would like to simply use iterators for this (as in the original implementation), but this is not allowed in constant expressions.
657        let mut i = 0;
658        while i < N {
659            buf[i] = &raw mut self[i];
660
661            i += 1;
662        }
663
664        // SAFETY: `*mut T` has the same layout as `&mut T`, and we've also initialised each pointer as a valid reference.
665        unsafe { transmute_unchecked(buf) }
666    }
667
668    /// Divides one array reference into two at an index.
669    ///
670    /// The first will contain all indices from `[0, M)` (excluding
671    /// the index `M` itself) and the second will contain all
672    /// indices from `[M, N)` (excluding the index `N` itself).
673    ///
674    /// # Panics
675    ///
676    /// Panics if `M > N`.
677    ///
678    /// # Examples
679    ///
680    /// ```
681    /// #![feature(split_array)]
682    ///
683    /// let v = [1, 2, 3, 4, 5, 6];
684    ///
685    /// {
686    ///    let (left, right) = v.split_array_ref::<0>();
687    ///    assert_eq!(left, &[]);
688    ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
689    /// }
690    ///
691    /// {
692    ///     let (left, right) = v.split_array_ref::<2>();
693    ///     assert_eq!(left, &[1, 2]);
694    ///     assert_eq!(right, &[3, 4, 5, 6]);
695    /// }
696    ///
697    /// {
698    ///     let (left, right) = v.split_array_ref::<6>();
699    ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
700    ///     assert_eq!(right, &[]);
701    /// }
702    /// ```
703    #[unstable(
704        feature = "split_array",
705        reason = "return type should have array as 2nd element",
706        issue = "90091"
707    )]
708    #[inline]
709    pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
710        (&self[..]).split_first_chunk::<M>().unwrap()
711    }
712
713    /// Divides one mutable array reference into two at an index.
714    ///
715    /// The first will contain all indices from `[0, M)` (excluding
716    /// the index `M` itself) and the second will contain all
717    /// indices from `[M, N)` (excluding the index `N` itself).
718    ///
719    /// # Panics
720    ///
721    /// Panics if `M > N`.
722    ///
723    /// # Examples
724    ///
725    /// ```
726    /// #![feature(split_array)]
727    ///
728    /// let mut v = [1, 0, 3, 0, 5, 6];
729    /// let (left, right) = v.split_array_mut::<2>();
730    /// assert_eq!(left, &mut [1, 0][..]);
731    /// assert_eq!(right, &mut [3, 0, 5, 6]);
732    /// left[1] = 2;
733    /// right[1] = 4;
734    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
735    /// ```
736    #[unstable(
737        feature = "split_array",
738        reason = "return type should have array as 2nd element",
739        issue = "90091"
740    )]
741    #[inline]
742    pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
743        (&mut self[..]).split_first_chunk_mut::<M>().unwrap()
744    }
745
746    /// Divides one array reference into two at an index from the end.
747    ///
748    /// The first will contain all indices from `[0, N - M)` (excluding
749    /// the index `N - M` itself) and the second will contain all
750    /// indices from `[N - M, N)` (excluding the index `N` itself).
751    ///
752    /// # Panics
753    ///
754    /// Panics if `M > N`.
755    ///
756    /// # Examples
757    ///
758    /// ```
759    /// #![feature(split_array)]
760    ///
761    /// let v = [1, 2, 3, 4, 5, 6];
762    ///
763    /// {
764    ///    let (left, right) = v.rsplit_array_ref::<0>();
765    ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
766    ///    assert_eq!(right, &[]);
767    /// }
768    ///
769    /// {
770    ///     let (left, right) = v.rsplit_array_ref::<2>();
771    ///     assert_eq!(left, &[1, 2, 3, 4]);
772    ///     assert_eq!(right, &[5, 6]);
773    /// }
774    ///
775    /// {
776    ///     let (left, right) = v.rsplit_array_ref::<6>();
777    ///     assert_eq!(left, &[]);
778    ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
779    /// }
780    /// ```
781    #[unstable(
782        feature = "split_array",
783        reason = "return type should have array as 2nd element",
784        issue = "90091"
785    )]
786    #[inline]
787    pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
788        (&self[..]).split_last_chunk::<M>().unwrap()
789    }
790
791    /// Divides one mutable array reference into two at an index from the end.
792    ///
793    /// The first will contain all indices from `[0, N - M)` (excluding
794    /// the index `N - M` itself) and the second will contain all
795    /// indices from `[N - M, N)` (excluding the index `N` itself).
796    ///
797    /// # Panics
798    ///
799    /// Panics if `M > N`.
800    ///
801    /// # Examples
802    ///
803    /// ```
804    /// #![feature(split_array)]
805    ///
806    /// let mut v = [1, 0, 3, 0, 5, 6];
807    /// let (left, right) = v.rsplit_array_mut::<4>();
808    /// assert_eq!(left, &mut [1, 0]);
809    /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
810    /// left[1] = 2;
811    /// right[1] = 4;
812    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
813    /// ```
814    #[unstable(
815        feature = "split_array",
816        reason = "return type should have array as 2nd element",
817        issue = "90091"
818    )]
819    #[inline]
820    pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
821        (&mut self[..]).split_last_chunk_mut::<M>().unwrap()
822    }
823}
824
825/// Populate an array from the first `N` elements of `iter`
826///
827/// # Panics
828///
829/// If the iterator doesn't actually have enough items.
830///
831/// By depending on `TrustedLen`, however, we can do that check up-front (where
832/// it easily optimizes away) so it doesn't impact the loop that fills the array.
833#[inline]
834fn from_trusted_iterator<T, const N: usize>(iter: impl UncheckedIterator<Item = T>) -> [T; N] {
835    try_from_trusted_iterator(iter.map(NeverShortCircuit)).0
836}
837
838#[inline]
839fn try_from_trusted_iterator<T, R, const N: usize>(
840    iter: impl UncheckedIterator<Item = R>,
841) -> ChangeOutputType<R, [T; N]>
842where
843    R: Try<Output = T>,
844    R::Residual: Residual<[T; N]>,
845{
846    assert!(iter.size_hint().0 >= N);
847    fn next<T>(mut iter: impl UncheckedIterator<Item = T>) -> impl FnMut(usize) -> T {
848        move |_| {
849            // SAFETY: We know that `from_fn` will call this at most N times,
850            // and we checked to ensure that we have at least that many items.
851            unsafe { iter.next_unchecked() }
852        }
853    }
854
855    try_from_fn(next(iter))
856}
857
858/// Version of [`try_from_fn`] using a passed-in slice in order to avoid
859/// needing to monomorphize for every array length.
860///
861/// This takes a generator rather than an iterator so that *at the type level*
862/// it never needs to worry about running out of items.  When combined with
863/// an infallible `Try` type, that means the loop canonicalizes easily, allowing
864/// it to optimize well.
865///
866/// It would be *possible* to unify this and [`iter_next_chunk_erased`] into one
867/// function that does the union of both things, but last time it was that way
868/// it resulted in poor codegen from the "are there enough source items?" checks
869/// not optimizing away.  So if you give it a shot, make sure to watch what
870/// happens in the codegen tests.
871#[inline]
872fn try_from_fn_erased<T, R>(
873    buffer: &mut [MaybeUninit<T>],
874    mut generator: impl FnMut(usize) -> R,
875) -> ControlFlow<R::Residual>
876where
877    R: Try<Output = T>,
878{
879    let mut guard = Guard { array_mut: buffer, initialized: 0 };
880
881    while guard.initialized < guard.array_mut.len() {
882        let item = generator(guard.initialized).branch()?;
883
884        // SAFETY: The loop condition ensures we have space to push the item
885        unsafe { guard.push_unchecked(item) };
886    }
887
888    mem::forget(guard);
889    ControlFlow::Continue(())
890}
891
892/// Panic guard for incremental initialization of arrays.
893///
894/// Disarm the guard with `mem::forget` once the array has been initialized.
895///
896/// # Safety
897///
898/// All write accesses to this structure are unsafe and must maintain a correct
899/// count of `initialized` elements.
900///
901/// To minimize indirection fields are still pub but callers should at least use
902/// `push_unchecked` to signal that something unsafe is going on.
903struct Guard<'a, T> {
904    /// The array to be initialized.
905    pub array_mut: &'a mut [MaybeUninit<T>],
906    /// The number of items that have been initialized so far.
907    pub initialized: usize,
908}
909
910impl<T> Guard<'_, T> {
911    /// Adds an item to the array and updates the initialized item counter.
912    ///
913    /// # Safety
914    ///
915    /// No more than N elements must be initialized.
916    #[inline]
917    pub(crate) unsafe fn push_unchecked(&mut self, item: T) {
918        // SAFETY: If `initialized` was correct before and the caller does not
919        // invoke this method more than N times then writes will be in-bounds
920        // and slots will not be initialized more than once.
921        unsafe {
922            self.array_mut.get_unchecked_mut(self.initialized).write(item);
923            self.initialized = self.initialized.unchecked_add(1);
924        }
925    }
926}
927
928impl<T> Drop for Guard<'_, T> {
929    #[inline]
930    fn drop(&mut self) {
931        debug_assert!(self.initialized <= self.array_mut.len());
932
933        // SAFETY: this slice will contain only initialized objects.
934        unsafe {
935            self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();
936        }
937    }
938}
939
940/// Pulls `N` items from `iter` and returns them as an array. If the iterator
941/// yields fewer than `N` items, `Err` is returned containing an iterator over
942/// the already yielded items.
943///
944/// Since the iterator is passed as a mutable reference and this function calls
945/// `next` at most `N` times, the iterator can still be used afterwards to
946/// retrieve the remaining items.
947///
948/// If `iter.next()` panicks, all items already yielded by the iterator are
949/// dropped.
950///
951/// Used for [`Iterator::next_chunk`].
952#[inline]
953pub(crate) fn iter_next_chunk<T, const N: usize>(
954    iter: &mut impl Iterator<Item = T>,
955) -> Result<[T; N], IntoIter<T, N>> {
956    let mut array = [const { MaybeUninit::uninit() }; N];
957    let r = iter_next_chunk_erased(&mut array, iter);
958    match r {
959        Ok(()) => {
960            // SAFETY: All elements of `array` were populated.
961            Ok(unsafe { MaybeUninit::array_assume_init(array) })
962        }
963        Err(initialized) => {
964            // SAFETY: Only the first `initialized` elements were populated
965            Err(unsafe { IntoIter::new_unchecked(array, 0..initialized) })
966        }
967    }
968}
969
970/// Version of [`iter_next_chunk`] using a passed-in slice in order to avoid
971/// needing to monomorphize for every array length.
972///
973/// Unfortunately this loop has two exit conditions, the buffer filling up
974/// or the iterator running out of items, making it tend to optimize poorly.
975#[inline]
976fn iter_next_chunk_erased<T>(
977    buffer: &mut [MaybeUninit<T>],
978    iter: &mut impl Iterator<Item = T>,
979) -> Result<(), usize> {
980    let mut guard = Guard { array_mut: buffer, initialized: 0 };
981    while guard.initialized < guard.array_mut.len() {
982        let Some(item) = iter.next() else {
983            // Unlike `try_from_fn_erased`, we want to keep the partial results,
984            // so we need to defuse the guard instead of using `?`.
985            let initialized = guard.initialized;
986            mem::forget(guard);
987            return Err(initialized);
988        };
989
990        // SAFETY: The loop condition ensures we have space to push the item
991        unsafe { guard.push_unchecked(item) };
992    }
993
994    mem::forget(guard);
995    Ok(())
996}