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
7#[cfg(not(feature = "ferrocene_certified"))]
8use crate::borrow::{Borrow, BorrowMut};
9#[cfg(not(feature = "ferrocene_certified"))]
10use crate::cmp::Ordering;
11#[cfg(not(feature = "ferrocene_certified"))]
12use crate::convert::Infallible;
13#[cfg(not(feature = "ferrocene_certified"))]
14use crate::error::Error;
15#[cfg(not(feature = "ferrocene_certified"))]
16use crate::fmt;
17#[cfg(not(feature = "ferrocene_certified"))]
18use crate::hash::{self, Hash};
19#[cfg(not(feature = "ferrocene_certified"))]
20use crate::intrinsics::transmute_unchecked;
21#[cfg(not(feature = "ferrocene_certified"))]
22use crate::iter::{UncheckedIterator, repeat_n};
23#[cfg(not(feature = "ferrocene_certified"))]
24use crate::mem::{self, MaybeUninit};
25#[cfg(not(feature = "ferrocene_certified"))]
26use crate::ops::{
27    ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
28};
29#[cfg(not(feature = "ferrocene_certified"))]
30use crate::ptr::{null, null_mut};
31#[cfg(not(feature = "ferrocene_certified"))]
32use crate::slice::{Iter, IterMut};
33
34#[cfg(not(feature = "ferrocene_certified"))]
35mod ascii;
36#[cfg(not(feature = "ferrocene_certified"))]
37mod drain;
38#[cfg(not(feature = "ferrocene_certified"))]
39mod equality;
40#[cfg(not(feature = "ferrocene_certified"))]
41mod iter;
42
43#[cfg(not(feature = "ferrocene_certified"))]
44pub(crate) use drain::drain_array_with;
45#[stable(feature = "array_value_iter", since = "1.51.0")]
46// blocked by IntoIter
47#[cfg(not(feature = "ferrocene_certified"))]
48pub use iter::IntoIter;
49
50/// Creates an array of type `[T; N]` by repeatedly cloning a value.
51///
52/// This is the same as `[val; N]`, but it also works for types that do not
53/// implement [`Copy`].
54///
55/// The provided value will be used as an element of the resulting array and
56/// will be cloned N - 1 times to fill up the rest. If N is zero, the value
57/// will be dropped.
58///
59/// # Example
60///
61/// Creating multiple copies of a `String`:
62/// ```rust
63/// #![feature(array_repeat)]
64///
65/// use std::array;
66///
67/// let string = "Hello there!".to_string();
68/// let strings = array::repeat(string);
69/// assert_eq!(strings, ["Hello there!", "Hello there!"]);
70/// ```
71#[inline]
72#[unstable(feature = "array_repeat", issue = "126695")]
73#[cfg(not(feature = "ferrocene_certified"))]
74pub fn repeat<T: Clone, const N: usize>(val: T) -> [T; N] {
75    from_trusted_iterator(repeat_n(val, N))
76}
77
78/// Creates an array where each element is produced by calling `f` with
79/// that element's index while walking forward through the array.
80///
81/// This is essentially the same as writing
82/// ```text
83/// [f(0), f(1), f(2), …, f(N - 2), f(N - 1)]
84/// ```
85/// and is similar to `(0..i).map(f)`, just for arrays not iterators.
86///
87/// If `N == 0`, this produces an empty array without ever calling `f`.
88///
89/// # Example
90///
91/// ```rust
92/// // type inference is helping us here, the way `from_fn` knows how many
93/// // elements to produce is the length of array down there: only arrays of
94/// // equal lengths can be compared, so the const generic parameter `N` is
95/// // inferred to be 5, thus creating array of 5 elements.
96///
97/// let array = core::array::from_fn(|i| i);
98/// // indexes are:    0  1  2  3  4
99/// assert_eq!(array, [0, 1, 2, 3, 4]);
100///
101/// let array2: [usize; 8] = core::array::from_fn(|i| i * 2);
102/// // indexes are:     0  1  2  3  4  5   6   7
103/// assert_eq!(array2, [0, 2, 4, 6, 8, 10, 12, 14]);
104///
105/// let bool_arr = core::array::from_fn::<_, 5, _>(|i| i % 2 == 0);
106/// // indexes are:       0     1      2     3      4
107/// assert_eq!(bool_arr, [true, false, true, false, true]);
108/// ```
109///
110/// You can also capture things, for example to create an array full of clones
111/// where you can't just use `[item; N]` because it's not `Copy`:
112/// ```
113/// # // TBH `array::repeat` would be better for this, but it's not stable yet.
114/// let my_string = String::from("Hello");
115/// let clones: [String; 42] = std::array::from_fn(|_| my_string.clone());
116/// assert!(clones.iter().all(|x| *x == my_string));
117/// ```
118///
119/// The array is generated in ascending index order, starting from the front
120/// and going towards the back, so you can use closures with mutable state:
121/// ```
122/// let mut state = 1;
123/// let a = std::array::from_fn(|_| { let x = state; state *= 2; x });
124/// assert_eq!(a, [1, 2, 4, 8, 16, 32]);
125/// ```
126#[inline]
127#[stable(feature = "array_from_fn", since = "1.63.0")]
128#[cfg(not(feature = "ferrocene_certified"))]
129pub fn from_fn<T, const N: usize, F>(f: F) -> [T; N]
130where
131    F: FnMut(usize) -> T,
132{
133    try_from_fn(NeverShortCircuit::wrap_mut_1(f)).0
134}
135
136/// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
137/// Unlike [`from_fn`], where the element creation can't fail, this version will return an error
138/// if any element creation was unsuccessful.
139///
140/// The return type of this function depends on the return type of the closure.
141/// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
142/// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
143///
144/// # Arguments
145///
146/// * `cb`: Callback where the passed argument is the current array index.
147///
148/// # Example
149///
150/// ```rust
151/// #![feature(array_try_from_fn)]
152///
153/// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
154/// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
155///
156/// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
157/// assert!(array.is_err());
158///
159/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
160/// assert_eq!(array, Some([100, 101, 102, 103]));
161///
162/// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
163/// assert_eq!(array, None);
164/// ```
165#[inline]
166#[unstable(feature = "array_try_from_fn", issue = "89379")]
167#[cfg(not(feature = "ferrocene_certified"))]
168pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
169where
170    F: FnMut(usize) -> R,
171    R: Try,
172    R::Residual: Residual<[R::Output; N]>,
173{
174    let mut array = [const { MaybeUninit::uninit() }; N];
175    match try_from_fn_erased(&mut array, cb) {
176        ControlFlow::Break(r) => FromResidual::from_residual(r),
177        ControlFlow::Continue(()) => {
178            // SAFETY: All elements of the array were populated.
179            try { unsafe { MaybeUninit::array_assume_init(array) } }
180        }
181    }
182}
183
184/// Converts a reference to `T` into a reference to an array of length 1 (without copying).
185#[stable(feature = "array_from_ref", since = "1.53.0")]
186#[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
187#[cfg(not(feature = "ferrocene_certified"))]
188pub const fn from_ref<T>(s: &T) -> &[T; 1] {
189    // SAFETY: Converting `&T` to `&[T; 1]` is sound.
190    unsafe { &*(s as *const T).cast::<[T; 1]>() }
191}
192
193/// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
194#[stable(feature = "array_from_ref", since = "1.53.0")]
195#[rustc_const_stable(feature = "const_array_from_ref", since = "1.83.0")]
196#[cfg(not(feature = "ferrocene_certified"))]
197pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
198    // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
199    unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
200}
201
202/// The error type returned when a conversion from a slice to an array fails.
203#[stable(feature = "try_from", since = "1.34.0")]
204#[derive(Debug, Copy, Clone)]
205#[cfg(not(feature = "ferrocene_certified"))]
206pub struct TryFromSliceError(());
207
208#[stable(feature = "core_array", since = "1.35.0")]
209#[cfg(not(feature = "ferrocene_certified"))]
210impl fmt::Display for TryFromSliceError {
211    #[inline]
212    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
213        #[allow(deprecated)]
214        self.description().fmt(f)
215    }
216}
217
218#[stable(feature = "try_from", since = "1.34.0")]
219#[cfg(not(feature = "ferrocene_certified"))]
220impl Error for TryFromSliceError {
221    #[allow(deprecated)]
222    fn description(&self) -> &str {
223        "could not convert slice to array"
224    }
225}
226
227#[stable(feature = "try_from_slice_error", since = "1.36.0")]
228#[rustc_const_unstable(feature = "const_try", issue = "74935")]
229#[cfg(not(feature = "ferrocene_certified"))]
230impl const From<Infallible> for TryFromSliceError {
231    fn from(x: Infallible) -> TryFromSliceError {
232        match x {}
233    }
234}
235
236#[stable(feature = "rust1", since = "1.0.0")]
237#[cfg(not(feature = "ferrocene_certified"))]
238impl<T, const N: usize> AsRef<[T]> for [T; N] {
239    #[inline]
240    fn as_ref(&self) -> &[T] {
241        &self[..]
242    }
243}
244
245#[stable(feature = "rust1", since = "1.0.0")]
246#[cfg(not(feature = "ferrocene_certified"))]
247impl<T, const N: usize> AsMut<[T]> for [T; N] {
248    #[inline]
249    fn as_mut(&mut self) -> &mut [T] {
250        &mut self[..]
251    }
252}
253
254#[stable(feature = "array_borrow", since = "1.4.0")]
255#[cfg(not(feature = "ferrocene_certified"))]
256impl<T, const N: usize> Borrow<[T]> for [T; N] {
257    fn borrow(&self) -> &[T] {
258        self
259    }
260}
261
262#[stable(feature = "array_borrow", since = "1.4.0")]
263#[cfg(not(feature = "ferrocene_certified"))]
264impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
265    fn borrow_mut(&mut self) -> &mut [T] {
266        self
267    }
268}
269
270/// Tries to create an array `[T; N]` by copying from a slice `&[T]`.
271/// Succeeds if `slice.len() == N`.
272///
273/// ```
274/// let bytes: [u8; 3] = [1, 0, 2];
275///
276/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
277/// assert_eq!(1, u16::from_le_bytes(bytes_head));
278///
279/// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
280/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
281/// ```
282#[stable(feature = "try_from", since = "1.34.0")]
283#[cfg(not(feature = "ferrocene_certified"))]
284impl<T, const N: usize> TryFrom<&[T]> for [T; N]
285where
286    T: Copy,
287{
288    type Error = TryFromSliceError;
289
290    #[inline]
291    fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
292        <&Self>::try_from(slice).copied()
293    }
294}
295
296/// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
297/// Succeeds if `slice.len() == N`.
298///
299/// ```
300/// let mut bytes: [u8; 3] = [1, 0, 2];
301///
302/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
303/// assert_eq!(1, u16::from_le_bytes(bytes_head));
304///
305/// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
306/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
307/// ```
308#[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
309#[cfg(not(feature = "ferrocene_certified"))]
310impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
311where
312    T: Copy,
313{
314    type Error = TryFromSliceError;
315
316    #[inline]
317    fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
318        <Self>::try_from(&*slice)
319    }
320}
321
322/// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
323/// `slice.len() == N`.
324///
325/// ```
326/// let bytes: [u8; 3] = [1, 0, 2];
327///
328/// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
329/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
330///
331/// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
332/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
333/// ```
334#[stable(feature = "try_from", since = "1.34.0")]
335#[cfg(not(feature = "ferrocene_certified"))]
336impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
337    type Error = TryFromSliceError;
338
339    #[inline]
340    fn try_from(slice: &'a [T]) -> Result<&'a [T; N], TryFromSliceError> {
341        slice.as_array().ok_or(TryFromSliceError(()))
342    }
343}
344
345/// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
346/// `&mut [T]`. Succeeds if `slice.len() == N`.
347///
348/// ```
349/// let mut bytes: [u8; 3] = [1, 0, 2];
350///
351/// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
352/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
353///
354/// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
355/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
356/// ```
357#[stable(feature = "try_from", since = "1.34.0")]
358#[cfg(not(feature = "ferrocene_certified"))]
359impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
360    type Error = TryFromSliceError;
361
362    #[inline]
363    fn try_from(slice: &'a mut [T]) -> Result<&'a mut [T; N], TryFromSliceError> {
364        slice.as_mut_array().ok_or(TryFromSliceError(()))
365    }
366}
367
368/// The hash of an array is the same as that of the corresponding slice,
369/// as required by the `Borrow` implementation.
370///
371/// ```
372/// use std::hash::BuildHasher;
373///
374/// let b = std::hash::RandomState::new();
375/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
376/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
377/// assert_eq!(b.hash_one(a), b.hash_one(s));
378/// ```
379#[stable(feature = "rust1", since = "1.0.0")]
380#[cfg(not(feature = "ferrocene_certified"))]
381impl<T: Hash, const N: usize> Hash for [T; N] {
382    fn hash<H: hash::Hasher>(&self, state: &mut H) {
383        Hash::hash(&self[..], state)
384    }
385}
386
387#[stable(feature = "rust1", since = "1.0.0")]
388#[cfg(not(feature = "ferrocene_certified"))]
389impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
390    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
391        fmt::Debug::fmt(&&self[..], f)
392    }
393}
394
395#[stable(feature = "rust1", since = "1.0.0")]
396#[cfg(not(feature = "ferrocene_certified"))]
397impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
398    type Item = &'a T;
399    type IntoIter = Iter<'a, T>;
400
401    fn into_iter(self) -> Iter<'a, T> {
402        self.iter()
403    }
404}
405
406#[stable(feature = "rust1", since = "1.0.0")]
407#[cfg(not(feature = "ferrocene_certified"))]
408impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
409    type Item = &'a mut T;
410    type IntoIter = IterMut<'a, T>;
411
412    fn into_iter(self) -> IterMut<'a, T> {
413        self.iter_mut()
414    }
415}
416
417#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
418#[rustc_const_unstable(feature = "const_index", issue = "143775")]
419#[cfg(not(feature = "ferrocene_certified"))]
420impl<T, I, const N: usize> const Index<I> for [T; N]
421where
422    [T]: [const] Index<I>,
423{
424    type Output = <[T] as Index<I>>::Output;
425
426    #[inline]
427    fn index(&self, index: I) -> &Self::Output {
428        Index::index(self as &[T], index)
429    }
430}
431
432#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
433#[rustc_const_unstable(feature = "const_index", issue = "143775")]
434#[cfg(not(feature = "ferrocene_certified"))]
435impl<T, I, const N: usize> const IndexMut<I> for [T; N]
436where
437    [T]: [const] IndexMut<I>,
438{
439    #[inline]
440    fn index_mut(&mut self, index: I) -> &mut Self::Output {
441        IndexMut::index_mut(self as &mut [T], index)
442    }
443}
444
445/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
446#[stable(feature = "rust1", since = "1.0.0")]
447// blocked by PartialOrd
448#[cfg(not(feature = "ferrocene_certified"))]
449impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
450    #[inline]
451    fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
452        PartialOrd::partial_cmp(&&self[..], &&other[..])
453    }
454    #[inline]
455    fn lt(&self, other: &[T; N]) -> bool {
456        PartialOrd::lt(&&self[..], &&other[..])
457    }
458    #[inline]
459    fn le(&self, other: &[T; N]) -> bool {
460        PartialOrd::le(&&self[..], &&other[..])
461    }
462    #[inline]
463    fn ge(&self, other: &[T; N]) -> bool {
464        PartialOrd::ge(&&self[..], &&other[..])
465    }
466    #[inline]
467    fn gt(&self, other: &[T; N]) -> bool {
468        PartialOrd::gt(&&self[..], &&other[..])
469    }
470}
471
472/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
473#[stable(feature = "rust1", since = "1.0.0")]
474#[cfg(not(feature = "ferrocene_certified"))]
475impl<T: Ord, const N: usize> Ord for [T; N] {
476    #[inline]
477    fn cmp(&self, other: &[T; N]) -> Ordering {
478        Ord::cmp(&&self[..], &&other[..])
479    }
480}
481
482#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
483#[cfg(not(feature = "ferrocene_certified"))]
484impl<T: Copy, const N: usize> Copy for [T; N] {}
485
486#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
487#[cfg(not(feature = "ferrocene_certified"))]
488impl<T: Clone, const N: usize> Clone for [T; N] {
489    #[inline]
490    fn clone(&self) -> Self {
491        SpecArrayClone::clone(self)
492    }
493
494    #[inline]
495    fn clone_from(&mut self, other: &Self) {
496        self.clone_from_slice(other);
497    }
498}
499
500#[cfg(not(feature = "ferrocene_certified"))]
501trait SpecArrayClone: Clone {
502    fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
503}
504
505#[cfg(not(feature = "ferrocene_certified"))]
506impl<T: Clone> SpecArrayClone for T {
507    #[inline]
508    default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
509        from_trusted_iterator(array.iter().cloned())
510    }
511}
512
513#[cfg(not(feature = "ferrocene_certified"))]
514impl<T: Copy> SpecArrayClone for T {
515    #[inline]
516    fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
517        *array
518    }
519}
520
521// The Default impls cannot be done with const generics because `[T; 0]` doesn't
522// require Default to be implemented, and having different impl blocks for
523// different numbers isn't supported yet.
524
525#[cfg(not(feature = "ferrocene_certified"))]
526macro_rules! array_impl_default {
527    {$n:expr, $t:ident $($ts:ident)*} => {
528        #[stable(since = "1.4.0", feature = "array_default")]
529        impl<T> Default for [T; $n] where T: Default {
530            fn default() -> [T; $n] {
531                [$t::default(), $($ts::default()),*]
532            }
533        }
534        array_impl_default!{($n - 1), $($ts)*}
535    };
536    {$n:expr,} => {
537        #[stable(since = "1.4.0", feature = "array_default")]
538        impl<T> Default for [T; $n] {
539            fn default() -> [T; $n] { [] }
540        }
541    };
542}
543
544#[cfg(not(feature = "ferrocene_certified"))]
545array_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}
546
547impl<T, const N: usize> [T; N] {
548    /// Returns an array of the same size as `self`, with function `f` applied to each element
549    /// in order.
550    ///
551    /// If you don't necessarily need a new fixed-size array, consider using
552    /// [`Iterator::map`] instead.
553    ///
554    ///
555    /// # Note on performance and stack usage
556    ///
557    /// Unfortunately, usages of this method are currently not always optimized
558    /// as well as they could be. This mainly concerns large arrays, as mapping
559    /// over small arrays seem to be optimized just fine. Also note that in
560    /// debug mode (i.e. without any optimizations), this method can use a lot
561    /// of stack space (a few times the size of the array or more).
562    ///
563    /// Therefore, in performance-critical code, try to avoid using this method
564    /// on large arrays or check the emitted code. Also try to avoid chained
565    /// maps (e.g. `arr.map(...).map(...)`).
566    ///
567    /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
568    /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
569    /// really need a new array of the same size as the result. Rust's lazy
570    /// iterators tend to get optimized very well.
571    ///
572    ///
573    /// # Examples
574    ///
575    /// ```
576    /// let x = [1, 2, 3];
577    /// let y = x.map(|v| v + 1);
578    /// assert_eq!(y, [2, 3, 4]);
579    ///
580    /// let x = [1, 2, 3];
581    /// let mut temp = 0;
582    /// let y = x.map(|v| { temp += 1; v * temp });
583    /// assert_eq!(y, [1, 4, 9]);
584    ///
585    /// let x = ["Ferris", "Bueller's", "Day", "Off"];
586    /// let y = x.map(|v| v.len());
587    /// assert_eq!(y, [6, 9, 3, 3]);
588    /// ```
589    #[must_use]
590    #[stable(feature = "array_map", since = "1.55.0")]
591    #[cfg(not(feature = "ferrocene_certified"))]
592    pub fn map<F, U>(self, f: F) -> [U; N]
593    where
594        F: FnMut(T) -> U,
595    {
596        self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
597    }
598
599    /// A fallible function `f` applied to each element on array `self` in order to
600    /// return an array the same size as `self` or the first error encountered.
601    ///
602    /// The return type of this function depends on the return type of the closure.
603    /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
604    /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
605    ///
606    /// # Examples
607    ///
608    /// ```
609    /// #![feature(array_try_map)]
610    ///
611    /// let a = ["1", "2", "3"];
612    /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
613    /// assert_eq!(b, [2, 3, 4]);
614    ///
615    /// let a = ["1", "2a", "3"];
616    /// let b = a.try_map(|v| v.parse::<u32>());
617    /// assert!(b.is_err());
618    ///
619    /// use std::num::NonZero;
620    ///
621    /// let z = [1, 2, 0, 3, 4];
622    /// assert_eq!(z.try_map(NonZero::new), None);
623    ///
624    /// let a = [1, 2, 3];
625    /// let b = a.try_map(NonZero::new);
626    /// let c = b.map(|x| x.map(NonZero::get));
627    /// assert_eq!(c, Some(a));
628    /// ```
629    #[unstable(feature = "array_try_map", issue = "79711")]
630    #[cfg(not(feature = "ferrocene_certified"))]
631    pub fn try_map<R>(self, f: impl FnMut(T) -> R) -> ChangeOutputType<R, [R::Output; N]>
632    where
633        R: Try<Residual: Residual<[R::Output; N]>>,
634    {
635        drain_array_with(self, |iter| try_from_trusted_iterator(iter.map(f)))
636    }
637
638    /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
639    #[stable(feature = "array_as_slice", since = "1.57.0")]
640    #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
641    pub const fn as_slice(&self) -> &[T] {
642        self
643    }
644
645    /// Returns a mutable slice containing the entire array. Equivalent to
646    /// `&mut s[..]`.
647    #[stable(feature = "array_as_slice", since = "1.57.0")]
648    #[rustc_const_stable(feature = "const_array_as_mut_slice", since = "1.89.0")]
649    pub const fn as_mut_slice(&mut self) -> &mut [T] {
650        self
651    }
652
653    /// Borrows each element and returns an array of references with the same
654    /// size as `self`.
655    ///
656    ///
657    /// # Example
658    ///
659    /// ```
660    /// let floats = [3.1, 2.7, -1.0];
661    /// let float_refs: [&f64; 3] = floats.each_ref();
662    /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
663    /// ```
664    ///
665    /// This method is particularly useful if combined with other methods, like
666    /// [`map`](#method.map). This way, you can avoid moving the original
667    /// array if its elements are not [`Copy`].
668    ///
669    /// ```
670    /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
671    /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
672    /// assert_eq!(is_ascii, [true, false, true]);
673    ///
674    /// // We can still access the original array: it has not been moved.
675    /// assert_eq!(strings.len(), 3);
676    /// ```
677    #[stable(feature = "array_methods", since = "1.77.0")]
678    #[rustc_const_stable(feature = "const_array_each_ref", since = "CURRENT_RUSTC_VERSION")]
679    #[cfg(not(feature = "ferrocene_certified"))]
680    pub const fn each_ref(&self) -> [&T; N] {
681        let mut buf = [null::<T>(); N];
682
683        // FIXME(const_trait_impl): We would like to simply use iterators for this (as in the original implementation), but this is not allowed in constant expressions.
684        let mut i = 0;
685        while i < N {
686            buf[i] = &raw const self[i];
687
688            i += 1;
689        }
690
691        // SAFETY: `*const T` has the same layout as `&T`, and we've also initialised each pointer as a valid reference.
692        unsafe { transmute_unchecked(buf) }
693    }
694
695    /// Borrows each element mutably and returns an array of mutable references
696    /// with the same size as `self`.
697    ///
698    ///
699    /// # Example
700    ///
701    /// ```
702    ///
703    /// let mut floats = [3.1, 2.7, -1.0];
704    /// let float_refs: [&mut f64; 3] = floats.each_mut();
705    /// *float_refs[0] = 0.0;
706    /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
707    /// assert_eq!(floats, [0.0, 2.7, -1.0]);
708    /// ```
709    #[stable(feature = "array_methods", since = "1.77.0")]
710    #[rustc_const_stable(feature = "const_array_each_ref", since = "CURRENT_RUSTC_VERSION")]
711    #[cfg(not(feature = "ferrocene_certified"))]
712    pub const fn each_mut(&mut self) -> [&mut T; N] {
713        let mut buf = [null_mut::<T>(); N];
714
715        // FIXME(const_trait_impl): We would like to simply use iterators for this (as in the original implementation), but this is not allowed in constant expressions.
716        let mut i = 0;
717        while i < N {
718            buf[i] = &raw mut self[i];
719
720            i += 1;
721        }
722
723        // SAFETY: `*mut T` has the same layout as `&mut T`, and we've also initialised each pointer as a valid reference.
724        unsafe { transmute_unchecked(buf) }
725    }
726
727    /// Divides one array reference into two at an index.
728    ///
729    /// The first will contain all indices from `[0, M)` (excluding
730    /// the index `M` itself) and the second will contain all
731    /// indices from `[M, N)` (excluding the index `N` itself).
732    ///
733    /// # Panics
734    ///
735    /// Panics if `M > N`.
736    ///
737    /// # Examples
738    ///
739    /// ```
740    /// #![feature(split_array)]
741    ///
742    /// let v = [1, 2, 3, 4, 5, 6];
743    ///
744    /// {
745    ///    let (left, right) = v.split_array_ref::<0>();
746    ///    assert_eq!(left, &[]);
747    ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
748    /// }
749    ///
750    /// {
751    ///     let (left, right) = v.split_array_ref::<2>();
752    ///     assert_eq!(left, &[1, 2]);
753    ///     assert_eq!(right, &[3, 4, 5, 6]);
754    /// }
755    ///
756    /// {
757    ///     let (left, right) = v.split_array_ref::<6>();
758    ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
759    ///     assert_eq!(right, &[]);
760    /// }
761    /// ```
762    #[unstable(
763        feature = "split_array",
764        reason = "return type should have array as 2nd element",
765        issue = "90091"
766    )]
767    #[inline]
768    #[cfg(not(feature = "ferrocene_certified"))]
769    pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
770        self.split_first_chunk::<M>().unwrap()
771    }
772
773    /// Divides one mutable array reference into two at an index.
774    ///
775    /// The first will contain all indices from `[0, M)` (excluding
776    /// the index `M` itself) and the second will contain all
777    /// indices from `[M, N)` (excluding the index `N` itself).
778    ///
779    /// # Panics
780    ///
781    /// Panics if `M > N`.
782    ///
783    /// # Examples
784    ///
785    /// ```
786    /// #![feature(split_array)]
787    ///
788    /// let mut v = [1, 0, 3, 0, 5, 6];
789    /// let (left, right) = v.split_array_mut::<2>();
790    /// assert_eq!(left, &mut [1, 0][..]);
791    /// assert_eq!(right, &mut [3, 0, 5, 6]);
792    /// left[1] = 2;
793    /// right[1] = 4;
794    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
795    /// ```
796    #[unstable(
797        feature = "split_array",
798        reason = "return type should have array as 2nd element",
799        issue = "90091"
800    )]
801    #[inline]
802    #[cfg(not(feature = "ferrocene_certified"))]
803    pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
804        self.split_first_chunk_mut::<M>().unwrap()
805    }
806
807    /// Divides one array reference into two at an index from the end.
808    ///
809    /// The first will contain all indices from `[0, N - M)` (excluding
810    /// the index `N - M` itself) and the second will contain all
811    /// indices from `[N - M, N)` (excluding the index `N` itself).
812    ///
813    /// # Panics
814    ///
815    /// Panics if `M > N`.
816    ///
817    /// # Examples
818    ///
819    /// ```
820    /// #![feature(split_array)]
821    ///
822    /// let v = [1, 2, 3, 4, 5, 6];
823    ///
824    /// {
825    ///    let (left, right) = v.rsplit_array_ref::<0>();
826    ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
827    ///    assert_eq!(right, &[]);
828    /// }
829    ///
830    /// {
831    ///     let (left, right) = v.rsplit_array_ref::<2>();
832    ///     assert_eq!(left, &[1, 2, 3, 4]);
833    ///     assert_eq!(right, &[5, 6]);
834    /// }
835    ///
836    /// {
837    ///     let (left, right) = v.rsplit_array_ref::<6>();
838    ///     assert_eq!(left, &[]);
839    ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
840    /// }
841    /// ```
842    #[unstable(
843        feature = "split_array",
844        reason = "return type should have array as 2nd element",
845        issue = "90091"
846    )]
847    #[inline]
848    #[cfg(not(feature = "ferrocene_certified"))]
849    pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
850        self.split_last_chunk::<M>().unwrap()
851    }
852
853    /// Divides one mutable array reference into two at an index from the end.
854    ///
855    /// The first will contain all indices from `[0, N - M)` (excluding
856    /// the index `N - M` itself) and the second will contain all
857    /// indices from `[N - M, N)` (excluding the index `N` itself).
858    ///
859    /// # Panics
860    ///
861    /// Panics if `M > N`.
862    ///
863    /// # Examples
864    ///
865    /// ```
866    /// #![feature(split_array)]
867    ///
868    /// let mut v = [1, 0, 3, 0, 5, 6];
869    /// let (left, right) = v.rsplit_array_mut::<4>();
870    /// assert_eq!(left, &mut [1, 0]);
871    /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
872    /// left[1] = 2;
873    /// right[1] = 4;
874    /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
875    /// ```
876    #[unstable(
877        feature = "split_array",
878        reason = "return type should have array as 2nd element",
879        issue = "90091"
880    )]
881    #[inline]
882    #[cfg(not(feature = "ferrocene_certified"))]
883    pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
884        self.split_last_chunk_mut::<M>().unwrap()
885    }
886}
887
888/// Populate an array from the first `N` elements of `iter`
889///
890/// # Panics
891///
892/// If the iterator doesn't actually have enough items.
893///
894/// By depending on `TrustedLen`, however, we can do that check up-front (where
895/// it easily optimizes away) so it doesn't impact the loop that fills the array.
896#[inline]
897#[cfg(not(feature = "ferrocene_certified"))]
898fn from_trusted_iterator<T, const N: usize>(iter: impl UncheckedIterator<Item = T>) -> [T; N] {
899    try_from_trusted_iterator(iter.map(NeverShortCircuit)).0
900}
901
902#[inline]
903#[cfg(not(feature = "ferrocene_certified"))]
904fn try_from_trusted_iterator<T, R, const N: usize>(
905    iter: impl UncheckedIterator<Item = R>,
906) -> ChangeOutputType<R, [T; N]>
907where
908    R: Try<Output = T>,
909    R::Residual: Residual<[T; N]>,
910{
911    assert!(iter.size_hint().0 >= N);
912    fn next<T>(mut iter: impl UncheckedIterator<Item = T>) -> impl FnMut(usize) -> T {
913        move |_| {
914            // SAFETY: We know that `from_fn` will call this at most N times,
915            // and we checked to ensure that we have at least that many items.
916            unsafe { iter.next_unchecked() }
917        }
918    }
919
920    try_from_fn(next(iter))
921}
922
923/// Version of [`try_from_fn`] using a passed-in slice in order to avoid
924/// needing to monomorphize for every array length.
925///
926/// This takes a generator rather than an iterator so that *at the type level*
927/// it never needs to worry about running out of items.  When combined with
928/// an infallible `Try` type, that means the loop canonicalizes easily, allowing
929/// it to optimize well.
930///
931/// It would be *possible* to unify this and [`iter_next_chunk_erased`] into one
932/// function that does the union of both things, but last time it was that way
933/// it resulted in poor codegen from the "are there enough source items?" checks
934/// not optimizing away.  So if you give it a shot, make sure to watch what
935/// happens in the codegen tests.
936#[inline]
937#[cfg(not(feature = "ferrocene_certified"))]
938fn try_from_fn_erased<T, R>(
939    buffer: &mut [MaybeUninit<T>],
940    mut generator: impl FnMut(usize) -> R,
941) -> ControlFlow<R::Residual>
942where
943    R: Try<Output = T>,
944{
945    let mut guard = Guard { array_mut: buffer, initialized: 0 };
946
947    while guard.initialized < guard.array_mut.len() {
948        let item = generator(guard.initialized).branch()?;
949
950        // SAFETY: The loop condition ensures we have space to push the item
951        unsafe { guard.push_unchecked(item) };
952    }
953
954    mem::forget(guard);
955    ControlFlow::Continue(())
956}
957
958/// Panic guard for incremental initialization of arrays.
959///
960/// Disarm the guard with `mem::forget` once the array has been initialized.
961///
962/// # Safety
963///
964/// All write accesses to this structure are unsafe and must maintain a correct
965/// count of `initialized` elements.
966///
967/// To minimize indirection fields are still pub but callers should at least use
968/// `push_unchecked` to signal that something unsafe is going on.
969#[cfg(not(feature = "ferrocene_certified"))]
970struct Guard<'a, T> {
971    /// The array to be initialized.
972    pub array_mut: &'a mut [MaybeUninit<T>],
973    /// The number of items that have been initialized so far.
974    pub initialized: usize,
975}
976
977#[cfg(not(feature = "ferrocene_certified"))]
978impl<T> Guard<'_, T> {
979    /// Adds an item to the array and updates the initialized item counter.
980    ///
981    /// # Safety
982    ///
983    /// No more than N elements must be initialized.
984    #[inline]
985    pub(crate) unsafe fn push_unchecked(&mut self, item: T) {
986        // SAFETY: If `initialized` was correct before and the caller does not
987        // invoke this method more than N times then writes will be in-bounds
988        // and slots will not be initialized more than once.
989        unsafe {
990            self.array_mut.get_unchecked_mut(self.initialized).write(item);
991            self.initialized = self.initialized.unchecked_add(1);
992        }
993    }
994}
995
996#[cfg(not(feature = "ferrocene_certified"))]
997impl<T> Drop for Guard<'_, T> {
998    #[inline]
999    fn drop(&mut self) {
1000        debug_assert!(self.initialized <= self.array_mut.len());
1001
1002        // SAFETY: this slice will contain only initialized objects.
1003        unsafe {
1004            self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();
1005        }
1006    }
1007}
1008
1009/// Pulls `N` items from `iter` and returns them as an array. If the iterator
1010/// yields fewer than `N` items, `Err` is returned containing an iterator over
1011/// the already yielded items.
1012///
1013/// Since the iterator is passed as a mutable reference and this function calls
1014/// `next` at most `N` times, the iterator can still be used afterwards to
1015/// retrieve the remaining items.
1016///
1017/// If `iter.next()` panicks, all items already yielded by the iterator are
1018/// dropped.
1019///
1020/// Used for [`Iterator::next_chunk`].
1021#[inline]
1022#[cfg(not(feature = "ferrocene_certified"))]
1023pub(crate) fn iter_next_chunk<T, const N: usize>(
1024    iter: &mut impl Iterator<Item = T>,
1025) -> Result<[T; N], IntoIter<T, N>> {
1026    let mut array = [const { MaybeUninit::uninit() }; N];
1027    let r = iter_next_chunk_erased(&mut array, iter);
1028    match r {
1029        Ok(()) => {
1030            // SAFETY: All elements of `array` were populated.
1031            Ok(unsafe { MaybeUninit::array_assume_init(array) })
1032        }
1033        Err(initialized) => {
1034            // SAFETY: Only the first `initialized` elements were populated
1035            Err(unsafe { IntoIter::new_unchecked(array, 0..initialized) })
1036        }
1037    }
1038}
1039
1040/// Version of [`iter_next_chunk`] using a passed-in slice in order to avoid
1041/// needing to monomorphize for every array length.
1042///
1043/// Unfortunately this loop has two exit conditions, the buffer filling up
1044/// or the iterator running out of items, making it tend to optimize poorly.
1045#[inline]
1046#[cfg(not(feature = "ferrocene_certified"))]
1047fn iter_next_chunk_erased<T>(
1048    buffer: &mut [MaybeUninit<T>],
1049    iter: &mut impl Iterator<Item = T>,
1050) -> Result<(), usize> {
1051    let mut guard = Guard { array_mut: buffer, initialized: 0 };
1052    while guard.initialized < guard.array_mut.len() {
1053        let Some(item) = iter.next() else {
1054            // Unlike `try_from_fn_erased`, we want to keep the partial results,
1055            // so we need to defuse the guard instead of using `?`.
1056            let initialized = guard.initialized;
1057            mem::forget(guard);
1058            return Err(initialized);
1059        };
1060
1061        // SAFETY: The loop condition ensures we have space to push the item
1062        unsafe { guard.push_unchecked(item) };
1063    }
1064
1065    mem::forget(guard);
1066    Ok(())
1067}