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")]
201#[rustc_const_unstable(feature = "const_try", issue = "74935")]
202impl const From<Infallible> for TryFromSliceError {
203 fn from(x: Infallible) -> TryFromSliceError {
204 match x {}
205 }
206}
207
208#[stable(feature = "rust1", since = "1.0.0")]
209impl<T, const N: usize> AsRef<[T]> for [T; N] {
210 #[inline]
211 fn as_ref(&self) -> &[T] {
212 &self[..]
213 }
214}
215
216#[stable(feature = "rust1", since = "1.0.0")]
217impl<T, const N: usize> AsMut<[T]> for [T; N] {
218 #[inline]
219 fn as_mut(&mut self) -> &mut [T] {
220 &mut self[..]
221 }
222}
223
224#[stable(feature = "array_borrow", since = "1.4.0")]
225impl<T, const N: usize> Borrow<[T]> for [T; N] {
226 fn borrow(&self) -> &[T] {
227 self
228 }
229}
230
231#[stable(feature = "array_borrow", since = "1.4.0")]
232impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
233 fn borrow_mut(&mut self) -> &mut [T] {
234 self
235 }
236}
237
238/// Tries to create an array `[T; N]` by copying from a slice `&[T]`.
239/// Succeeds if `slice.len() == N`.
240///
241/// ```
242/// let bytes: [u8; 3] = [1, 0, 2];
243///
244/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
245/// assert_eq!(1, u16::from_le_bytes(bytes_head));
246///
247/// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
248/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
249/// ```
250#[stable(feature = "try_from", since = "1.34.0")]
251impl<T, const N: usize> TryFrom<&[T]> for [T; N]
252where
253 T: Copy,
254{
255 type Error = TryFromSliceError;
256
257 #[inline]
258 fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
259 <&Self>::try_from(slice).copied()
260 }
261}
262
263/// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
264/// Succeeds if `slice.len() == N`.
265///
266/// ```
267/// let mut bytes: [u8; 3] = [1, 0, 2];
268///
269/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
270/// assert_eq!(1, u16::from_le_bytes(bytes_head));
271///
272/// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
273/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
274/// ```
275#[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
276impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
277where
278 T: Copy,
279{
280 type Error = TryFromSliceError;
281
282 #[inline]
283 fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
284 <Self>::try_from(&*slice)
285 }
286}
287
288/// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
289/// `slice.len() == N`.
290///
291/// ```
292/// let bytes: [u8; 3] = [1, 0, 2];
293///
294/// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
295/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
296///
297/// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
298/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
299/// ```
300#[stable(feature = "try_from", since = "1.34.0")]
301impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
302 type Error = TryFromSliceError;
303
304 #[inline]
305 fn try_from(slice: &'a [T]) -> Result<&'a [T; N], TryFromSliceError> {
306 slice.as_array().ok_or(TryFromSliceError(()))
307 }
308}
309
310/// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
311/// `&mut [T]`. Succeeds if `slice.len() == N`.
312///
313/// ```
314/// let mut bytes: [u8; 3] = [1, 0, 2];
315///
316/// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
317/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
318///
319/// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
320/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
321/// ```
322#[stable(feature = "try_from", since = "1.34.0")]
323impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
324 type Error = TryFromSliceError;
325
326 #[inline]
327 fn try_from(slice: &'a mut [T]) -> Result<&'a mut [T; N], TryFromSliceError> {
328 slice.as_mut_array().ok_or(TryFromSliceError(()))
329 }
330}
331
332/// The hash of an array is the same as that of the corresponding slice,
333/// as required by the `Borrow` implementation.
334///
335/// ```
336/// use std::hash::BuildHasher;
337///
338/// let b = std::hash::RandomState::new();
339/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
340/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
341/// assert_eq!(b.hash_one(a), b.hash_one(s));
342/// ```
343#[stable(feature = "rust1", since = "1.0.0")]
344impl<T: Hash, const N: usize> Hash for [T; N] {
345 fn hash<H: hash::Hasher>(&self, state: &mut H) {
346 Hash::hash(&self[..], state)
347 }
348}
349
350#[stable(feature = "rust1", since = "1.0.0")]
351impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
352 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
353 fmt::Debug::fmt(&&self[..], f)
354 }
355}
356
357#[stable(feature = "rust1", since = "1.0.0")]
358impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
359 type Item = &'a T;
360 type IntoIter = Iter<'a, T>;
361
362 fn into_iter(self) -> Iter<'a, T> {
363 self.iter()
364 }
365}
366
367#[stable(feature = "rust1", since = "1.0.0")]
368impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
369 type Item = &'a mut T;
370 type IntoIter = IterMut<'a, T>;
371
372 fn into_iter(self) -> IterMut<'a, T> {
373 self.iter_mut()
374 }
375}
376
377#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
378#[rustc_const_unstable(feature = "const_index", issue = "143775")]
379impl<T, I, const N: usize> const Index<I> for [T; N]
380where
381 [T]: [const] Index<I>,
382{
383 type Output = <[T] as Index<I>>::Output;
384
385 #[inline]
386 fn index(&self, index: I) -> &Self::Output {
387 Index::index(self as &[T], index)
388 }
389}
390
391#[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
392#[rustc_const_unstable(feature = "const_index", issue = "143775")]
393impl<T, I, const N: usize> const IndexMut<I> for [T; N]
394where
395 [T]: [const] IndexMut<I>,
396{
397 #[inline]
398 fn index_mut(&mut self, index: I) -> &mut Self::Output {
399 IndexMut::index_mut(self as &mut [T], index)
400 }
401}
402
403/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
404#[stable(feature = "rust1", since = "1.0.0")]
405impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
406 #[inline]
407 fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
408 PartialOrd::partial_cmp(&&self[..], &&other[..])
409 }
410 #[inline]
411 fn lt(&self, other: &[T; N]) -> bool {
412 PartialOrd::lt(&&self[..], &&other[..])
413 }
414 #[inline]
415 fn le(&self, other: &[T; N]) -> bool {
416 PartialOrd::le(&&self[..], &&other[..])
417 }
418 #[inline]
419 fn ge(&self, other: &[T; N]) -> bool {
420 PartialOrd::ge(&&self[..], &&other[..])
421 }
422 #[inline]
423 fn gt(&self, other: &[T; N]) -> bool {
424 PartialOrd::gt(&&self[..], &&other[..])
425 }
426}
427
428/// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
429#[stable(feature = "rust1", since = "1.0.0")]
430impl<T: Ord, const N: usize> Ord for [T; N] {
431 #[inline]
432 fn cmp(&self, other: &[T; N]) -> Ordering {
433 Ord::cmp(&&self[..], &&other[..])
434 }
435}
436
437#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
438impl<T: Copy, const N: usize> Copy for [T; N] {}
439
440#[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
441impl<T: Clone, const N: usize> Clone for [T; N] {
442 #[inline]
443 fn clone(&self) -> Self {
444 SpecArrayClone::clone(self)
445 }
446
447 #[inline]
448 fn clone_from(&mut self, other: &Self) {
449 self.clone_from_slice(other);
450 }
451}
452
453trait SpecArrayClone: Clone {
454 fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
455}
456
457impl<T: Clone> SpecArrayClone for T {
458 #[inline]
459 default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
460 from_trusted_iterator(array.iter().cloned())
461 }
462}
463
464impl<T: Copy> SpecArrayClone for T {
465 #[inline]
466 fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
467 *array
468 }
469}
470
471// The Default impls cannot be done with const generics because `[T; 0]` doesn't
472// require Default to be implemented, and having different impl blocks for
473// different numbers isn't supported yet.
474
475macro_rules! array_impl_default {
476 {$n:expr, $t:ident $($ts:ident)*} => {
477 #[stable(since = "1.4.0", feature = "array_default")]
478 impl<T> Default for [T; $n] where T: Default {
479 fn default() -> [T; $n] {
480 [$t::default(), $($ts::default()),*]
481 }
482 }
483 array_impl_default!{($n - 1), $($ts)*}
484 };
485 {$n:expr,} => {
486 #[stable(since = "1.4.0", feature = "array_default")]
487 impl<T> Default for [T; $n] {
488 fn default() -> [T; $n] { [] }
489 }
490 };
491}
492
493array_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}
494
495impl<T, const N: usize> [T; N] {
496 /// Returns an array of the same size as `self`, with function `f` applied to each element
497 /// in order.
498 ///
499 /// If you don't necessarily need a new fixed-size array, consider using
500 /// [`Iterator::map`] instead.
501 ///
502 ///
503 /// # Note on performance and stack usage
504 ///
505 /// Unfortunately, usages of this method are currently not always optimized
506 /// as well as they could be. This mainly concerns large arrays, as mapping
507 /// over small arrays seem to be optimized just fine. Also note that in
508 /// debug mode (i.e. without any optimizations), this method can use a lot
509 /// of stack space (a few times the size of the array or more).
510 ///
511 /// Therefore, in performance-critical code, try to avoid using this method
512 /// on large arrays or check the emitted code. Also try to avoid chained
513 /// maps (e.g. `arr.map(...).map(...)`).
514 ///
515 /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
516 /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
517 /// really need a new array of the same size as the result. Rust's lazy
518 /// iterators tend to get optimized very well.
519 ///
520 ///
521 /// # Examples
522 ///
523 /// ```
524 /// let x = [1, 2, 3];
525 /// let y = x.map(|v| v + 1);
526 /// assert_eq!(y, [2, 3, 4]);
527 ///
528 /// let x = [1, 2, 3];
529 /// let mut temp = 0;
530 /// let y = x.map(|v| { temp += 1; v * temp });
531 /// assert_eq!(y, [1, 4, 9]);
532 ///
533 /// let x = ["Ferris", "Bueller's", "Day", "Off"];
534 /// let y = x.map(|v| v.len());
535 /// assert_eq!(y, [6, 9, 3, 3]);
536 /// ```
537 #[must_use]
538 #[stable(feature = "array_map", since = "1.55.0")]
539 pub fn map<F, U>(self, f: F) -> [U; N]
540 where
541 F: FnMut(T) -> U,
542 {
543 self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
544 }
545
546 /// A fallible function `f` applied to each element on array `self` in order to
547 /// return an array the same size as `self` or the first error encountered.
548 ///
549 /// The return type of this function depends on the return type of the closure.
550 /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N], E>`.
551 /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
552 ///
553 /// # Examples
554 ///
555 /// ```
556 /// #![feature(array_try_map)]
557 ///
558 /// let a = ["1", "2", "3"];
559 /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
560 /// assert_eq!(b, [2, 3, 4]);
561 ///
562 /// let a = ["1", "2a", "3"];
563 /// let b = a.try_map(|v| v.parse::<u32>());
564 /// assert!(b.is_err());
565 ///
566 /// use std::num::NonZero;
567 ///
568 /// let z = [1, 2, 0, 3, 4];
569 /// assert_eq!(z.try_map(NonZero::new), None);
570 ///
571 /// let a = [1, 2, 3];
572 /// let b = a.try_map(NonZero::new);
573 /// let c = b.map(|x| x.map(NonZero::get));
574 /// assert_eq!(c, Some(a));
575 /// ```
576 #[unstable(feature = "array_try_map", issue = "79711")]
577 pub fn try_map<R>(self, f: impl FnMut(T) -> R) -> ChangeOutputType<R, [R::Output; N]>
578 where
579 R: Try<Residual: Residual<[R::Output; N]>>,
580 {
581 drain_array_with(self, |iter| try_from_trusted_iterator(iter.map(f)))
582 }
583
584 /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
585 #[stable(feature = "array_as_slice", since = "1.57.0")]
586 #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
587 pub const fn as_slice(&self) -> &[T] {
588 self
589 }
590
591 /// Returns a mutable slice containing the entire array. Equivalent to
592 /// `&mut s[..]`.
593 #[stable(feature = "array_as_slice", since = "1.57.0")]
594 #[rustc_const_stable(feature = "const_array_as_mut_slice", since = "1.89.0")]
595 pub const fn as_mut_slice(&mut self) -> &mut [T] {
596 self
597 }
598
599 /// Borrows each element and returns an array of references with the same
600 /// size as `self`.
601 ///
602 ///
603 /// # Example
604 ///
605 /// ```
606 /// let floats = [3.1, 2.7, -1.0];
607 /// let float_refs: [&f64; 3] = floats.each_ref();
608 /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
609 /// ```
610 ///
611 /// This method is particularly useful if combined with other methods, like
612 /// [`map`](#method.map). This way, you can avoid moving the original
613 /// array if its elements are not [`Copy`].
614 ///
615 /// ```
616 /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
617 /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
618 /// assert_eq!(is_ascii, [true, false, true]);
619 ///
620 /// // We can still access the original array: it has not been moved.
621 /// assert_eq!(strings.len(), 3);
622 /// ```
623 #[stable(feature = "array_methods", since = "1.77.0")]
624 #[rustc_const_unstable(feature = "const_array_each_ref", issue = "133289")]
625 pub const fn each_ref(&self) -> [&T; N] {
626 let mut buf = [null::<T>(); N];
627
628 // 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.
629 let mut i = 0;
630 while i < N {
631 buf[i] = &raw const self[i];
632
633 i += 1;
634 }
635
636 // SAFETY: `*const T` has the same layout as `&T`, and we've also initialised each pointer as a valid reference.
637 unsafe { transmute_unchecked(buf) }
638 }
639
640 /// Borrows each element mutably and returns an array of mutable references
641 /// with the same size as `self`.
642 ///
643 ///
644 /// # Example
645 ///
646 /// ```
647 ///
648 /// let mut floats = [3.1, 2.7, -1.0];
649 /// let float_refs: [&mut f64; 3] = floats.each_mut();
650 /// *float_refs[0] = 0.0;
651 /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
652 /// assert_eq!(floats, [0.0, 2.7, -1.0]);
653 /// ```
654 #[stable(feature = "array_methods", since = "1.77.0")]
655 #[rustc_const_unstable(feature = "const_array_each_ref", issue = "133289")]
656 pub const fn each_mut(&mut self) -> [&mut T; N] {
657 let mut buf = [null_mut::<T>(); N];
658
659 // 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.
660 let mut i = 0;
661 while i < N {
662 buf[i] = &raw mut self[i];
663
664 i += 1;
665 }
666
667 // SAFETY: `*mut T` has the same layout as `&mut T`, and we've also initialised each pointer as a valid reference.
668 unsafe { transmute_unchecked(buf) }
669 }
670
671 /// Divides one array reference into two at an index.
672 ///
673 /// The first will contain all indices from `[0, M)` (excluding
674 /// the index `M` itself) and the second will contain all
675 /// indices from `[M, N)` (excluding the index `N` itself).
676 ///
677 /// # Panics
678 ///
679 /// Panics if `M > N`.
680 ///
681 /// # Examples
682 ///
683 /// ```
684 /// #![feature(split_array)]
685 ///
686 /// let v = [1, 2, 3, 4, 5, 6];
687 ///
688 /// {
689 /// let (left, right) = v.split_array_ref::<0>();
690 /// assert_eq!(left, &[]);
691 /// assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
692 /// }
693 ///
694 /// {
695 /// let (left, right) = v.split_array_ref::<2>();
696 /// assert_eq!(left, &[1, 2]);
697 /// assert_eq!(right, &[3, 4, 5, 6]);
698 /// }
699 ///
700 /// {
701 /// let (left, right) = v.split_array_ref::<6>();
702 /// assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
703 /// assert_eq!(right, &[]);
704 /// }
705 /// ```
706 #[unstable(
707 feature = "split_array",
708 reason = "return type should have array as 2nd element",
709 issue = "90091"
710 )]
711 #[inline]
712 pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
713 self.split_first_chunk::<M>().unwrap()
714 }
715
716 /// Divides one mutable array reference into two at an index.
717 ///
718 /// The first will contain all indices from `[0, M)` (excluding
719 /// the index `M` itself) and the second will contain all
720 /// indices from `[M, N)` (excluding the index `N` itself).
721 ///
722 /// # Panics
723 ///
724 /// Panics if `M > N`.
725 ///
726 /// # Examples
727 ///
728 /// ```
729 /// #![feature(split_array)]
730 ///
731 /// let mut v = [1, 0, 3, 0, 5, 6];
732 /// let (left, right) = v.split_array_mut::<2>();
733 /// assert_eq!(left, &mut [1, 0][..]);
734 /// assert_eq!(right, &mut [3, 0, 5, 6]);
735 /// left[1] = 2;
736 /// right[1] = 4;
737 /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
738 /// ```
739 #[unstable(
740 feature = "split_array",
741 reason = "return type should have array as 2nd element",
742 issue = "90091"
743 )]
744 #[inline]
745 pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
746 self.split_first_chunk_mut::<M>().unwrap()
747 }
748
749 /// Divides one array reference into two at an index from the end.
750 ///
751 /// The first will contain all indices from `[0, N - M)` (excluding
752 /// the index `N - M` itself) and the second will contain all
753 /// indices from `[N - M, N)` (excluding the index `N` itself).
754 ///
755 /// # Panics
756 ///
757 /// Panics if `M > N`.
758 ///
759 /// # Examples
760 ///
761 /// ```
762 /// #![feature(split_array)]
763 ///
764 /// let v = [1, 2, 3, 4, 5, 6];
765 ///
766 /// {
767 /// let (left, right) = v.rsplit_array_ref::<0>();
768 /// assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
769 /// assert_eq!(right, &[]);
770 /// }
771 ///
772 /// {
773 /// let (left, right) = v.rsplit_array_ref::<2>();
774 /// assert_eq!(left, &[1, 2, 3, 4]);
775 /// assert_eq!(right, &[5, 6]);
776 /// }
777 ///
778 /// {
779 /// let (left, right) = v.rsplit_array_ref::<6>();
780 /// assert_eq!(left, &[]);
781 /// assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
782 /// }
783 /// ```
784 #[unstable(
785 feature = "split_array",
786 reason = "return type should have array as 2nd element",
787 issue = "90091"
788 )]
789 #[inline]
790 pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
791 self.split_last_chunk::<M>().unwrap()
792 }
793
794 /// Divides one mutable array reference into two at an index from the end.
795 ///
796 /// The first will contain all indices from `[0, N - M)` (excluding
797 /// the index `N - M` itself) and the second will contain all
798 /// indices from `[N - M, N)` (excluding the index `N` itself).
799 ///
800 /// # Panics
801 ///
802 /// Panics if `M > N`.
803 ///
804 /// # Examples
805 ///
806 /// ```
807 /// #![feature(split_array)]
808 ///
809 /// let mut v = [1, 0, 3, 0, 5, 6];
810 /// let (left, right) = v.rsplit_array_mut::<4>();
811 /// assert_eq!(left, &mut [1, 0]);
812 /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
813 /// left[1] = 2;
814 /// right[1] = 4;
815 /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
816 /// ```
817 #[unstable(
818 feature = "split_array",
819 reason = "return type should have array as 2nd element",
820 issue = "90091"
821 )]
822 #[inline]
823 pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
824 self.split_last_chunk_mut::<M>().unwrap()
825 }
826}
827
828/// Populate an array from the first `N` elements of `iter`
829///
830/// # Panics
831///
832/// If the iterator doesn't actually have enough items.
833///
834/// By depending on `TrustedLen`, however, we can do that check up-front (where
835/// it easily optimizes away) so it doesn't impact the loop that fills the array.
836#[inline]
837fn from_trusted_iterator<T, const N: usize>(iter: impl UncheckedIterator<Item = T>) -> [T; N] {
838 try_from_trusted_iterator(iter.map(NeverShortCircuit)).0
839}
840
841#[inline]
842fn try_from_trusted_iterator<T, R, const N: usize>(
843 iter: impl UncheckedIterator<Item = R>,
844) -> ChangeOutputType<R, [T; N]>
845where
846 R: Try<Output = T>,
847 R::Residual: Residual<[T; N]>,
848{
849 assert!(iter.size_hint().0 >= N);
850 fn next<T>(mut iter: impl UncheckedIterator<Item = T>) -> impl FnMut(usize) -> T {
851 move |_| {
852 // SAFETY: We know that `from_fn` will call this at most N times,
853 // and we checked to ensure that we have at least that many items.
854 unsafe { iter.next_unchecked() }
855 }
856 }
857
858 try_from_fn(next(iter))
859}
860
861/// Version of [`try_from_fn`] using a passed-in slice in order to avoid
862/// needing to monomorphize for every array length.
863///
864/// This takes a generator rather than an iterator so that *at the type level*
865/// it never needs to worry about running out of items. When combined with
866/// an infallible `Try` type, that means the loop canonicalizes easily, allowing
867/// it to optimize well.
868///
869/// It would be *possible* to unify this and [`iter_next_chunk_erased`] into one
870/// function that does the union of both things, but last time it was that way
871/// it resulted in poor codegen from the "are there enough source items?" checks
872/// not optimizing away. So if you give it a shot, make sure to watch what
873/// happens in the codegen tests.
874#[inline]
875fn try_from_fn_erased<T, R>(
876 buffer: &mut [MaybeUninit<T>],
877 mut generator: impl FnMut(usize) -> R,
878) -> ControlFlow<R::Residual>
879where
880 R: Try<Output = T>,
881{
882 let mut guard = Guard { array_mut: buffer, initialized: 0 };
883
884 while guard.initialized < guard.array_mut.len() {
885 let item = generator(guard.initialized).branch()?;
886
887 // SAFETY: The loop condition ensures we have space to push the item
888 unsafe { guard.push_unchecked(item) };
889 }
890
891 mem::forget(guard);
892 ControlFlow::Continue(())
893}
894
895/// Panic guard for incremental initialization of arrays.
896///
897/// Disarm the guard with `mem::forget` once the array has been initialized.
898///
899/// # Safety
900///
901/// All write accesses to this structure are unsafe and must maintain a correct
902/// count of `initialized` elements.
903///
904/// To minimize indirection fields are still pub but callers should at least use
905/// `push_unchecked` to signal that something unsafe is going on.
906struct Guard<'a, T> {
907 /// The array to be initialized.
908 pub array_mut: &'a mut [MaybeUninit<T>],
909 /// The number of items that have been initialized so far.
910 pub initialized: usize,
911}
912
913impl<T> Guard<'_, T> {
914 /// Adds an item to the array and updates the initialized item counter.
915 ///
916 /// # Safety
917 ///
918 /// No more than N elements must be initialized.
919 #[inline]
920 pub(crate) unsafe fn push_unchecked(&mut self, item: T) {
921 // SAFETY: If `initialized` was correct before and the caller does not
922 // invoke this method more than N times then writes will be in-bounds
923 // and slots will not be initialized more than once.
924 unsafe {
925 self.array_mut.get_unchecked_mut(self.initialized).write(item);
926 self.initialized = self.initialized.unchecked_add(1);
927 }
928 }
929}
930
931impl<T> Drop for Guard<'_, T> {
932 #[inline]
933 fn drop(&mut self) {
934 debug_assert!(self.initialized <= self.array_mut.len());
935
936 // SAFETY: this slice will contain only initialized objects.
937 unsafe {
938 self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();
939 }
940 }
941}
942
943/// Pulls `N` items from `iter` and returns them as an array. If the iterator
944/// yields fewer than `N` items, `Err` is returned containing an iterator over
945/// the already yielded items.
946///
947/// Since the iterator is passed as a mutable reference and this function calls
948/// `next` at most `N` times, the iterator can still be used afterwards to
949/// retrieve the remaining items.
950///
951/// If `iter.next()` panicks, all items already yielded by the iterator are
952/// dropped.
953///
954/// Used for [`Iterator::next_chunk`].
955#[inline]
956pub(crate) fn iter_next_chunk<T, const N: usize>(
957 iter: &mut impl Iterator<Item = T>,
958) -> Result<[T; N], IntoIter<T, N>> {
959 let mut array = [const { MaybeUninit::uninit() }; N];
960 let r = iter_next_chunk_erased(&mut array, iter);
961 match r {
962 Ok(()) => {
963 // SAFETY: All elements of `array` were populated.
964 Ok(unsafe { MaybeUninit::array_assume_init(array) })
965 }
966 Err(initialized) => {
967 // SAFETY: Only the first `initialized` elements were populated
968 Err(unsafe { IntoIter::new_unchecked(array, 0..initialized) })
969 }
970 }
971}
972
973/// Version of [`iter_next_chunk`] using a passed-in slice in order to avoid
974/// needing to monomorphize for every array length.
975///
976/// Unfortunately this loop has two exit conditions, the buffer filling up
977/// or the iterator running out of items, making it tend to optimize poorly.
978#[inline]
979fn iter_next_chunk_erased<T>(
980 buffer: &mut [MaybeUninit<T>],
981 iter: &mut impl Iterator<Item = T>,
982) -> Result<(), usize> {
983 let mut guard = Guard { array_mut: buffer, initialized: 0 };
984 while guard.initialized < guard.array_mut.len() {
985 let Some(item) = iter.next() else {
986 // Unlike `try_from_fn_erased`, we want to keep the partial results,
987 // so we need to defuse the guard instead of using `?`.
988 let initialized = guard.initialized;
989 mem::forget(guard);
990 return Err(initialized);
991 };
992
993 // SAFETY: The loop condition ensures we have space to push the item
994 unsafe { guard.push_unchecked(item) };
995 }
996
997 mem::forget(guard);
998 Ok(())
999}