1 //! Helper functions and types for fixed-length arrays.
3 //! *[See also the array primitive type](array).*
5 #![stable(feature = "core_array", since = "1.36.0")]
7 use crate::borrow::{Borrow, BorrowMut};
8 use crate::cmp::Ordering;
9 use crate::convert::{Infallible, TryFrom};
11 use crate::hash::{self, Hash};
12 use crate::iter::TrustedLen;
13 use crate::mem::{self, MaybeUninit};
14 use crate::ops::{Index, IndexMut};
15 use crate::slice::{Iter, IterMut};
19 #[stable(feature = "array_value_iter", since = "1.51.0")]
20 pub use iter::IntoIter;
22 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
23 #[stable(feature = "array_from_ref", since = "1.53.0")]
24 pub fn from_ref<T>(s: &T) -> &[T; 1] {
25 // SAFETY: Converting `&T` to `&[T; 1]` is sound.
26 unsafe { &*(s as *const T).cast::<[T; 1]>() }
29 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
30 #[stable(feature = "array_from_ref", since = "1.53.0")]
31 pub fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
32 // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
33 unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
36 /// The error type returned when a conversion from a slice to an array fails.
37 #[stable(feature = "try_from", since = "1.34.0")]
38 #[derive(Debug, Copy, Clone)]
39 pub struct TryFromSliceError(());
41 #[stable(feature = "core_array", since = "1.36.0")]
42 impl fmt::Display for TryFromSliceError {
44 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
45 fmt::Display::fmt(self.__description(), f)
49 impl TryFromSliceError {
51 feature = "array_error_internals",
52 reason = "available through Error trait and this method should not \
58 pub fn __description(&self) -> &str {
59 "could not convert slice to array"
63 #[stable(feature = "try_from_slice_error", since = "1.36.0")]
64 impl From<Infallible> for TryFromSliceError {
65 fn from(x: Infallible) -> TryFromSliceError {
70 #[stable(feature = "rust1", since = "1.0.0")]
71 impl<T, const N: usize> AsRef<[T]> for [T; N] {
73 fn as_ref(&self) -> &[T] {
78 #[stable(feature = "rust1", since = "1.0.0")]
79 impl<T, const N: usize> AsMut<[T]> for [T; N] {
81 fn as_mut(&mut self) -> &mut [T] {
86 #[stable(feature = "array_borrow", since = "1.4.0")]
87 impl<T, const N: usize> Borrow<[T]> for [T; N] {
88 fn borrow(&self) -> &[T] {
93 #[stable(feature = "array_borrow", since = "1.4.0")]
94 impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
95 fn borrow_mut(&mut self) -> &mut [T] {
100 #[stable(feature = "try_from", since = "1.34.0")]
101 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
105 type Error = TryFromSliceError;
107 fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
108 <&Self>::try_from(slice).map(|r| *r)
112 #[stable(feature = "try_from", since = "1.34.0")]
113 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
114 type Error = TryFromSliceError;
116 fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
117 if slice.len() == N {
118 let ptr = slice.as_ptr() as *const [T; N];
119 // SAFETY: ok because we just checked that the length fits
122 Err(TryFromSliceError(()))
127 #[stable(feature = "try_from", since = "1.34.0")]
128 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
129 type Error = TryFromSliceError;
131 fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
132 if slice.len() == N {
133 let ptr = slice.as_mut_ptr() as *mut [T; N];
134 // SAFETY: ok because we just checked that the length fits
135 unsafe { Ok(&mut *ptr) }
137 Err(TryFromSliceError(()))
142 /// The hash of an array is the same as that of the corresponding slice,
143 /// as required by the `Borrow` implementation.
146 /// #![feature(build_hasher_simple_hash_one)]
147 /// use std::hash::BuildHasher;
149 /// let b = std::collections::hash_map::RandomState::new();
150 /// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
151 /// let s: &[u8] = &[0xa8, 0x3c, 0x09];
152 /// assert_eq!(b.hash_one(a), b.hash_one(s));
154 #[stable(feature = "rust1", since = "1.0.0")]
155 impl<T: Hash, const N: usize> Hash for [T; N] {
156 fn hash<H: hash::Hasher>(&self, state: &mut H) {
157 Hash::hash(&self[..], state)
161 #[stable(feature = "rust1", since = "1.0.0")]
162 impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
163 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
164 fmt::Debug::fmt(&&self[..], f)
168 // Note: the `#[rustc_skip_array_during_method_dispatch]` on `trait IntoIterator`
169 // hides this implementation from explicit `.into_iter()` calls on editions < 2021,
170 // so those calls will still resolve to the slice implementation, by reference.
171 #[stable(feature = "array_into_iter_impl", since = "1.53.0")]
172 impl<T, const N: usize> IntoIterator for [T; N] {
174 type IntoIter = IntoIter<T, N>;
176 /// Creates a consuming iterator, that is, one that moves each value out of
177 /// the array (from start to end). The array cannot be used after calling
178 /// this unless `T` implements `Copy`, so the whole array is copied.
180 /// Arrays have special behavior when calling `.into_iter()` prior to the
181 /// 2021 edition -- see the [array] Editions section for more information.
183 /// [array]: prim@array
184 fn into_iter(self) -> Self::IntoIter {
189 #[stable(feature = "rust1", since = "1.0.0")]
190 impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
192 type IntoIter = Iter<'a, T>;
194 fn into_iter(self) -> Iter<'a, T> {
199 #[stable(feature = "rust1", since = "1.0.0")]
200 impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
201 type Item = &'a mut T;
202 type IntoIter = IterMut<'a, T>;
204 fn into_iter(self) -> IterMut<'a, T> {
209 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
210 impl<T, I, const N: usize> Index<I> for [T; N]
214 type Output = <[T] as Index<I>>::Output;
217 fn index(&self, index: I) -> &Self::Output {
218 Index::index(self as &[T], index)
222 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
223 impl<T, I, const N: usize> IndexMut<I> for [T; N]
228 fn index_mut(&mut self, index: I) -> &mut Self::Output {
229 IndexMut::index_mut(self as &mut [T], index)
233 #[stable(feature = "rust1", since = "1.0.0")]
234 impl<A, B, const N: usize> PartialEq<[B; N]> for [A; N]
239 fn eq(&self, other: &[B; N]) -> bool {
240 self[..] == other[..]
243 fn ne(&self, other: &[B; N]) -> bool {
244 self[..] != other[..]
248 #[stable(feature = "rust1", since = "1.0.0")]
249 impl<A, B, const N: usize> PartialEq<[B]> for [A; N]
254 fn eq(&self, other: &[B]) -> bool {
255 self[..] == other[..]
258 fn ne(&self, other: &[B]) -> bool {
259 self[..] != other[..]
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<A, B, const N: usize> PartialEq<[A; N]> for [B]
269 fn eq(&self, other: &[A; N]) -> bool {
270 self[..] == other[..]
273 fn ne(&self, other: &[A; N]) -> bool {
274 self[..] != other[..]
278 #[stable(feature = "rust1", since = "1.0.0")]
279 impl<A, B, const N: usize> PartialEq<&[B]> for [A; N]
284 fn eq(&self, other: &&[B]) -> bool {
285 self[..] == other[..]
288 fn ne(&self, other: &&[B]) -> bool {
289 self[..] != other[..]
293 #[stable(feature = "rust1", since = "1.0.0")]
294 impl<A, B, const N: usize> PartialEq<[A; N]> for &[B]
299 fn eq(&self, other: &[A; N]) -> bool {
300 self[..] == other[..]
303 fn ne(&self, other: &[A; N]) -> bool {
304 self[..] != other[..]
308 #[stable(feature = "rust1", since = "1.0.0")]
309 impl<A, B, const N: usize> PartialEq<&mut [B]> for [A; N]
314 fn eq(&self, other: &&mut [B]) -> bool {
315 self[..] == other[..]
318 fn ne(&self, other: &&mut [B]) -> bool {
319 self[..] != other[..]
323 #[stable(feature = "rust1", since = "1.0.0")]
324 impl<A, B, const N: usize> PartialEq<[A; N]> for &mut [B]
329 fn eq(&self, other: &[A; N]) -> bool {
330 self[..] == other[..]
333 fn ne(&self, other: &[A; N]) -> bool {
334 self[..] != other[..]
338 // NOTE: some less important impls are omitted to reduce code bloat
339 // __impl_slice_eq2! { [A; $N], &'b [B; $N] }
340 // __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }
342 #[stable(feature = "rust1", since = "1.0.0")]
343 impl<T: Eq, const N: usize> Eq for [T; N] {}
345 #[stable(feature = "rust1", since = "1.0.0")]
346 impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
348 fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
349 PartialOrd::partial_cmp(&&self[..], &&other[..])
352 fn lt(&self, other: &[T; N]) -> bool {
353 PartialOrd::lt(&&self[..], &&other[..])
356 fn le(&self, other: &[T; N]) -> bool {
357 PartialOrd::le(&&self[..], &&other[..])
360 fn ge(&self, other: &[T; N]) -> bool {
361 PartialOrd::ge(&&self[..], &&other[..])
364 fn gt(&self, other: &[T; N]) -> bool {
365 PartialOrd::gt(&&self[..], &&other[..])
369 /// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
370 #[stable(feature = "rust1", since = "1.0.0")]
371 impl<T: Ord, const N: usize> Ord for [T; N] {
373 fn cmp(&self, other: &[T; N]) -> Ordering {
374 Ord::cmp(&&self[..], &&other[..])
378 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
379 // require Default to be implemented, and having different impl blocks for
380 // different numbers isn't supported yet.
382 macro_rules! array_impl_default {
383 {$n:expr, $t:ident $($ts:ident)*} => {
384 #[stable(since = "1.4.0", feature = "array_default")]
385 impl<T> Default for [T; $n] where T: Default {
386 fn default() -> [T; $n] {
387 [$t::default(), $($ts::default()),*]
390 array_impl_default!{($n - 1), $($ts)*}
393 #[stable(since = "1.4.0", feature = "array_default")]
394 impl<T> Default for [T; $n] {
395 fn default() -> [T; $n] { [] }
400 array_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}
403 impl<T, const N: usize> [T; N] {
404 /// Returns an array of the same size as `self`, with function `f` applied to each element
410 /// #![feature(array_map)]
411 /// let x = [1, 2, 3];
412 /// let y = x.map(|v| v + 1);
413 /// assert_eq!(y, [2, 3, 4]);
415 /// let x = [1, 2, 3];
416 /// let mut temp = 0;
417 /// let y = x.map(|v| { temp += 1; v * temp });
418 /// assert_eq!(y, [1, 4, 9]);
420 /// let x = ["Ferris", "Bueller's", "Day", "Off"];
421 /// let y = x.map(|v| v.len());
422 /// assert_eq!(y, [6, 9, 3, 3]);
424 #[unstable(feature = "array_map", issue = "75243")]
425 pub fn map<F, U>(self, f: F) -> [U; N]
429 // SAFETY: we know for certain that this iterator will yield exactly `N`
431 unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
434 /// 'Zips up' two arrays into a single array of pairs.
436 /// `zip()` returns a new array where every element is a tuple where the
437 /// first element comes from the first array, and the second element comes
438 /// from the second array. In other words, it zips two arrays together,
439 /// into a single one.
444 /// #![feature(array_zip)]
445 /// let x = [1, 2, 3];
446 /// let y = [4, 5, 6];
447 /// let z = x.zip(y);
448 /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
450 #[unstable(feature = "array_zip", issue = "80094")]
451 pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
452 let mut iter = IntoIterator::into_iter(self).zip(rhs);
454 // SAFETY: we know for certain that this iterator will yield exactly `N`
456 unsafe { collect_into_array_unchecked(&mut iter) }
459 /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
460 #[unstable(feature = "array_methods", issue = "76118")]
461 pub fn as_slice(&self) -> &[T] {
465 /// Returns a mutable slice containing the entire array. Equivalent to
467 #[unstable(feature = "array_methods", issue = "76118")]
468 pub fn as_mut_slice(&mut self) -> &mut [T] {
472 /// Borrows each element and returns an array of references with the same
479 /// #![feature(array_methods)]
481 /// let floats = [3.1, 2.7, -1.0];
482 /// let float_refs: [&f64; 3] = floats.each_ref();
483 /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
486 /// This method is particularly useful if combined with other methods, like
487 /// [`map`](#method.map). This way, you can avoid moving the original
488 /// array if its elements are not `Copy`.
491 /// #![feature(array_methods, array_map)]
493 /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
494 /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
495 /// assert_eq!(is_ascii, [true, false, true]);
497 /// // We can still access the original array: it has not been moved.
498 /// assert_eq!(strings.len(), 3);
500 #[unstable(feature = "array_methods", issue = "76118")]
501 pub fn each_ref(&self) -> [&T; N] {
502 // SAFETY: we know for certain that this iterator will yield exactly `N`
504 unsafe { collect_into_array_unchecked(&mut self.iter()) }
507 /// Borrows each element mutably and returns an array of mutable references
508 /// with the same size as `self`.
514 /// #![feature(array_methods)]
516 /// let mut floats = [3.1, 2.7, -1.0];
517 /// let float_refs: [&mut f64; 3] = floats.each_mut();
518 /// *float_refs[0] = 0.0;
519 /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
520 /// assert_eq!(floats, [0.0, 2.7, -1.0]);
522 #[unstable(feature = "array_methods", issue = "76118")]
523 pub fn each_mut(&mut self) -> [&mut T; N] {
524 // SAFETY: we know for certain that this iterator will yield exactly `N`
526 unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
530 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
531 /// yields fewer than `N` items, this function exhibits undefined behavior.
533 /// See [`collect_into_array`] for more information.
538 /// It is up to the caller to guarantee that `iter` yields at least `N` items.
539 /// Violating this condition causes undefined behavior.
540 unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
542 // Note: `TrustedLen` here is somewhat of an experiment. This is just an
543 // internal function, so feel free to remove if this bound turns out to be a
544 // bad idea. In that case, remember to also remove the lower bound
545 // `debug_assert!` below!
546 I: Iterator + TrustedLen,
548 debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
549 debug_assert!(N <= iter.size_hint().0);
551 match collect_into_array(iter) {
552 Some(array) => array,
553 // SAFETY: covered by the function contract.
554 None => unsafe { crate::hint::unreachable_unchecked() },
558 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
559 /// yields fewer than `N` items, `None` is returned and all already yielded
560 /// items are dropped.
562 /// Since the iterator is passed as a mutable reference and this function calls
563 /// `next` at most `N` times, the iterator can still be used afterwards to
564 /// retrieve the remaining items.
566 /// If `iter.next()` panicks, all items already yielded by the iterator are
568 fn collect_into_array<I, const N: usize>(iter: &mut I) -> Option<[I::Item; N]>
573 // SAFETY: An empty array is always inhabited and has no validity invariants.
574 return unsafe { Some(mem::zeroed()) };
577 struct Guard<T, const N: usize> {
582 impl<T, const N: usize> Drop for Guard<T, N> {
584 debug_assert!(self.initialized <= N);
586 let initialized_part = crate::ptr::slice_from_raw_parts_mut(self.ptr, self.initialized);
588 // SAFETY: this raw slice will contain only initialized objects.
590 crate::ptr::drop_in_place(initialized_part);
595 let mut array = MaybeUninit::uninit_array::<N>();
596 let mut guard: Guard<_, N> =
597 Guard { ptr: MaybeUninit::slice_as_mut_ptr(&mut array), initialized: 0 };
599 while let Some(item) = iter.next() {
600 // SAFETY: `guard.initialized` starts at 0, is increased by one in the
601 // loop and the loop is aborted once it reaches N (which is
604 array.get_unchecked_mut(guard.initialized).write(item);
606 guard.initialized += 1;
608 // Check if the whole array was initialized.
609 if guard.initialized == N {
612 // SAFETY: the condition above asserts that all elements are
614 let out = unsafe { MaybeUninit::array_assume_init(array) };
619 // This is only reached if the iterator is exhausted before
620 // `guard.initialized` reaches `N`. Also note that `guard` is dropped here,
621 // dropping all already initialized elements.