library/core/src/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 = "1.36.0")]
   6
   7 use crate::borrow::{Borrow, BorrowMut};
   8 use crate::cmp::Ordering;
   9 use crate::convert::{Infallible, TryFrom};
  10 use crate::error::Error;
  11 use crate::fmt;
  12 use crate::hash::{self, Hash};
  13 use crate::iter::TrustedLen;
  14 use crate::mem::{self, MaybeUninit};
  15 use crate::ops::{
  16     ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
  17 };
  18 use crate::slice::{Iter, IterMut};
  19
  20 mod equality;
  21 mod iter;
  22
  23 #[stable(feature = "array_value_iter", since = "1.51.0")]
  24 pub use iter::IntoIter;
  25
  26 /// Creates an array of type [T; N], where each element `T` is the returned value from `cb`
  27 /// using that element's index.
  28 ///
  29 /// # Arguments
  30 ///
  31 /// * `cb`: Callback where the passed argument is the current array index.
  32 ///
  33 /// # Example
  34 ///
  35 /// ```rust
  36 /// // type inference is helping us here, the way `from_fn` knows how many
  37 /// // elements to produce is the length of array down there: only arrays of
  38 /// // equal lengths can be compared, so the const generic parameter `N` is
  39 /// // inferred to be 5, thus creating array of 5 elements.
  40 ///
  41 /// let array = core::array::from_fn(|i| i);
  42 /// // indexes are:    0  1  2  3  4
  43 /// assert_eq!(array, [0, 1, 2, 3, 4]);
  44 ///
  45 /// let array2: [usize; 8] = core::array::from_fn(|i| i * 2);
  46 /// // indexes are:     0  1  2  3  4  5   6   7
  47 /// assert_eq!(array2, [0, 2, 4, 6, 8, 10, 12, 14]);
  48 ///
  49 /// let bool_arr = core::array::from_fn::<_, 5, _>(|i| i % 2 == 0);
  50 /// // indexes are:       0     1      2     3      4
  51 /// assert_eq!(bool_arr, [true, false, true, false, true]);
  52 /// ```
  53 #[inline]
  54 #[stable(feature = "array_from_fn", since = "1.63.0")]
  55 pub fn from_fn<T, const N: usize, F>(mut cb: F) -> [T; N]
  56 where
  57     F: FnMut(usize) -> T,
  58 {
  59     let mut idx = 0;
  60     [(); N].map(|_| {
  61         let res = cb(idx);
  62         idx += 1;
  63         res
  64     })
  65 }
  66
  67 /// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
  68 /// Unlike [`from_fn`], where the element creation can't fail, this version will return an error
  69 /// if any element creation was unsuccessful.
  70 ///
  71 /// The return type of this function depends on the return type of the closure.
  72 /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N]; E>`.
  73 /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
  74 ///
  75 /// # Arguments
  76 ///
  77 /// * `cb`: Callback where the passed argument is the current array index.
  78 ///
  79 /// # Example
  80 ///
  81 /// ```rust
  82 /// #![feature(array_try_from_fn)]
  83 ///
  84 /// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
  85 /// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
  86 ///
  87 /// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
  88 /// assert!(array.is_err());
  89 ///
  90 /// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
  91 /// assert_eq!(array, Some([100, 101, 102, 103]));
  92 ///
  93 /// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
  94 /// assert_eq!(array, None);
  95 /// ```
  96 #[inline]
  97 #[unstable(feature = "array_try_from_fn", issue = "89379")]
  98 pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
  99 where
 100     F: FnMut(usize) -> R,
 101     R: Try,
 102     R::Residual: Residual<[R::Output; N]>,
 103 {
 104     // SAFETY: we know for certain that this iterator will yield exactly `N`
 105     // items.
 106     unsafe { try_collect_into_array_unchecked(&mut (0..N).map(cb)) }
 107 }
 108
 109 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
 110 #[stable(feature = "array_from_ref", since = "1.53.0")]
 111 #[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
 112 pub const fn from_ref<T>(s: &T) -> &[T; 1] {
 113     // SAFETY: Converting `&T` to `&[T; 1]` is sound.
 114     unsafe { &*(s as *const T).cast::<[T; 1]>() }
 115 }
 116
 117 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
 118 #[stable(feature = "array_from_ref", since = "1.53.0")]
 119 #[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
 120 pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
 121     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
 122     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
 123 }
 124
 125 /// The error type returned when a conversion from a slice to an array fails.
 126 #[stable(feature = "try_from", since = "1.34.0")]
 127 #[derive(Debug, Copy, Clone)]
 128 pub struct TryFromSliceError(());
 129
 130 #[stable(feature = "core_array", since = "1.36.0")]
 131 impl fmt::Display for TryFromSliceError {
 132     #[inline]
 133     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 134         fmt::Display::fmt(self.__description(), f)
 135     }
 136 }
 137
 138 #[stable(feature = "try_from", since = "1.34.0")]
 139 impl Error for TryFromSliceError {
 140     #[allow(deprecated)]
 141     fn description(&self) -> &str {
 142         self.__description()
 143     }
 144 }
 145
 146 impl TryFromSliceError {
 147     #[unstable(
 148         feature = "array_error_internals",
 149         reason = "available through Error trait and this method should not \
 150                      be exposed publicly",
 151         issue = "none"
 152     )]
 153     #[inline]
 154     #[doc(hidden)]
 155     pub fn __description(&self) -> &str {
 156         "could not convert slice to array"
 157     }
 158 }
 159
 160 #[stable(feature = "try_from_slice_error", since = "1.36.0")]
 161 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 162 impl const From<Infallible> for TryFromSliceError {
 163     fn from(x: Infallible) -> TryFromSliceError {
 164         match x {}
 165     }
 166 }
 167
 168 #[stable(feature = "rust1", since = "1.0.0")]
 169 impl<T, const N: usize> AsRef<[T]> for [T; N] {
 170     #[inline]
 171     fn as_ref(&self) -> &[T] {
 172         &self[..]
 173     }
 174 }
 175
 176 #[stable(feature = "rust1", since = "1.0.0")]
 177 impl<T, const N: usize> AsMut<[T]> for [T; N] {
 178     #[inline]
 179     fn as_mut(&mut self) -> &mut [T] {
 180         &mut self[..]
 181     }
 182 }
 183
 184 #[stable(feature = "array_borrow", since = "1.4.0")]
 185 #[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
 186 impl<T, const N: usize> const Borrow<[T]> for [T; N] {
 187     fn borrow(&self) -> &[T] {
 188         self
 189     }
 190 }
 191
 192 #[stable(feature = "array_borrow", since = "1.4.0")]
 193 #[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
 194 impl<T, const N: usize> const BorrowMut<[T]> for [T; N] {
 195     fn borrow_mut(&mut self) -> &mut [T] {
 196         self
 197     }
 198 }
 199
 200 /// Tries to create an array `[T; N]` by copying from a slice `&[T]`. Succeeds if
 201 /// `slice.len() == N`.
 202 ///
 203 /// ```
 204 /// let bytes: [u8; 3] = [1, 0, 2];
 205 ///
 206 /// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
 207 /// assert_eq!(1, u16::from_le_bytes(bytes_head));
 208 ///
 209 /// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
 210 /// assert_eq!(512, u16::from_le_bytes(bytes_tail));
 211 /// ```
 212 #[stable(feature = "try_from", since = "1.34.0")]
 213 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
 214 where
 215     T: Copy,
 216 {
 217     type Error = TryFromSliceError;
 218
 219     fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
 220         <&Self>::try_from(slice).map(|r| *r)
 221     }
 222 }
 223
 224 /// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
 225 /// Succeeds if `slice.len() == N`.
 226 ///
 227 /// ```
 228 /// let mut bytes: [u8; 3] = [1, 0, 2];
 229 ///
 230 /// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
 231 /// assert_eq!(1, u16::from_le_bytes(bytes_head));
 232 ///
 233 /// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
 234 /// assert_eq!(512, u16::from_le_bytes(bytes_tail));
 235 /// ```
 236 #[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
 237 impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
 238 where
 239     T: Copy,
 240 {
 241     type Error = TryFromSliceError;
 242
 243     fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
 244         <Self>::try_from(&*slice)
 245     }
 246 }
 247
 248 /// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
 249 /// `slice.len() == N`.
 250 ///
 251 /// ```
 252 /// let bytes: [u8; 3] = [1, 0, 2];
 253 ///
 254 /// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
 255 /// assert_eq!(1, u16::from_le_bytes(*bytes_head));
 256 ///
 257 /// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
 258 /// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
 259 /// ```
 260 #[stable(feature = "try_from", since = "1.34.0")]
 261 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
 262     type Error = TryFromSliceError;
 263
 264     fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
 265         if slice.len() == N {
 266             let ptr = slice.as_ptr() as *const [T; N];
 267             // SAFETY: ok because we just checked that the length fits
 268             unsafe { Ok(&*ptr) }
 269         } else {
 270             Err(TryFromSliceError(()))
 271         }
 272     }
 273 }
 274
 275 /// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
 276 /// `&mut [T]`. Succeeds if `slice.len() == N`.
 277 ///
 278 /// ```
 279 /// let mut bytes: [u8; 3] = [1, 0, 2];
 280 ///
 281 /// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
 282 /// assert_eq!(1, u16::from_le_bytes(*bytes_head));
 283 ///
 284 /// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
 285 /// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
 286 /// ```
 287 #[stable(feature = "try_from", since = "1.34.0")]
 288 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
 289     type Error = TryFromSliceError;
 290
 291     fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
 292         if slice.len() == N {
 293             let ptr = slice.as_mut_ptr() as *mut [T; N];
 294             // SAFETY: ok because we just checked that the length fits
 295             unsafe { Ok(&mut *ptr) }
 296         } else {
 297             Err(TryFromSliceError(()))
 298         }
 299     }
 300 }
 301
 302 /// The hash of an array is the same as that of the corresponding slice,
 303 /// as required by the `Borrow` implementation.
 304 ///
 305 /// ```
 306 /// #![feature(build_hasher_simple_hash_one)]
 307 /// use std::hash::BuildHasher;
 308 ///
 309 /// let b = std::collections::hash_map::RandomState::new();
 310 /// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
 311 /// let s: &[u8] = &[0xa8, 0x3c, 0x09];
 312 /// assert_eq!(b.hash_one(a), b.hash_one(s));
 313 /// ```
 314 #[stable(feature = "rust1", since = "1.0.0")]
 315 impl<T: Hash, const N: usize> Hash for [T; N] {
 316     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 317         Hash::hash(&self[..], state)
 318     }
 319 }
 320
 321 #[stable(feature = "rust1", since = "1.0.0")]
 322 impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
 323     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 324         fmt::Debug::fmt(&&self[..], f)
 325     }
 326 }
 327
 328 #[stable(feature = "rust1", since = "1.0.0")]
 329 impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
 330     type Item = &'a T;
 331     type IntoIter = Iter<'a, T>;
 332
 333     fn into_iter(self) -> Iter<'a, T> {
 334         self.iter()
 335     }
 336 }
 337
 338 #[stable(feature = "rust1", since = "1.0.0")]
 339 impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
 340     type Item = &'a mut T;
 341     type IntoIter = IterMut<'a, T>;
 342
 343     fn into_iter(self) -> IterMut<'a, T> {
 344         self.iter_mut()
 345     }
 346 }
 347
 348 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 349 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 350 impl<T, I, const N: usize> const Index<I> for [T; N]
 351 where
 352     [T]: ~const Index<I>,
 353 {
 354     type Output = <[T] as Index<I>>::Output;
 355
 356     #[inline]
 357     fn index(&self, index: I) -> &Self::Output {
 358         Index::index(self as &[T], index)
 359     }
 360 }
 361
 362 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 363 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 364 impl<T, I, const N: usize> const IndexMut<I> for [T; N]
 365 where
 366     [T]: ~const IndexMut<I>,
 367 {
 368     #[inline]
 369     fn index_mut(&mut self, index: I) -> &mut Self::Output {
 370         IndexMut::index_mut(self as &mut [T], index)
 371     }
 372 }
 373
 374 #[stable(feature = "rust1", since = "1.0.0")]
 375 impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
 376     #[inline]
 377     fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
 378         PartialOrd::partial_cmp(&&self[..], &&other[..])
 379     }
 380     #[inline]
 381     fn lt(&self, other: &[T; N]) -> bool {
 382         PartialOrd::lt(&&self[..], &&other[..])
 383     }
 384     #[inline]
 385     fn le(&self, other: &[T; N]) -> bool {
 386         PartialOrd::le(&&self[..], &&other[..])
 387     }
 388     #[inline]
 389     fn ge(&self, other: &[T; N]) -> bool {
 390         PartialOrd::ge(&&self[..], &&other[..])
 391     }
 392     #[inline]
 393     fn gt(&self, other: &[T; N]) -> bool {
 394         PartialOrd::gt(&&self[..], &&other[..])
 395     }
 396 }
 397
 398 /// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
 399 #[stable(feature = "rust1", since = "1.0.0")]
 400 impl<T: Ord, const N: usize> Ord for [T; N] {
 401     #[inline]
 402     fn cmp(&self, other: &[T; N]) -> Ordering {
 403         Ord::cmp(&&self[..], &&other[..])
 404     }
 405 }
 406
 407 #[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
 408 impl<T: Copy, const N: usize> Copy for [T; N] {}
 409
 410 #[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
 411 impl<T: Clone, const N: usize> Clone for [T; N] {
 412     #[inline]
 413     fn clone(&self) -> Self {
 414         SpecArrayClone::clone(self)
 415     }
 416
 417     #[inline]
 418     fn clone_from(&mut self, other: &Self) {
 419         self.clone_from_slice(other);
 420     }
 421 }
 422
 423 trait SpecArrayClone: Clone {
 424     fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
 425 }
 426
 427 impl<T: Clone> SpecArrayClone for T {
 428     #[inline]
 429     default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 430         // SAFETY: we know for certain that this iterator will yield exactly `N`
 431         // items.
 432         unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
 433     }
 434 }
 435
 436 impl<T: Copy> SpecArrayClone for T {
 437     #[inline]
 438     fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 439         *array
 440     }
 441 }
 442
 443 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 444 // require Default to be implemented, and having different impl blocks for
 445 // different numbers isn't supported yet.
 446
 447 macro_rules! array_impl_default {
 448     {$n:expr, $t:ident $($ts:ident)*} => {
 449         #[stable(since = "1.4.0", feature = "array_default")]
 450         #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 451         impl<T> const Default for [T; $n] where T: ~const Default {
 452             fn default() -> [T; $n] {
 453                 [$t::default(), $($ts::default()),*]
 454             }
 455         }
 456         array_impl_default!{($n - 1), $($ts)*}
 457     };
 458     {$n:expr,} => {
 459         #[stable(since = "1.4.0", feature = "array_default")]
 460         #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 461         impl<T> const Default for [T; $n] {
 462             fn default() -> [T; $n] { [] }
 463         }
 464     };
 465 }
 466
 467 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}
 468
 469 impl<T, const N: usize> [T; N] {
 470     /// Returns an array of the same size as `self`, with function `f` applied to each element
 471     /// in order.
 472     ///
 473     /// If you don't necessarily need a new fixed-size array, consider using
 474     /// [`Iterator::map`] instead.
 475     ///
 476     ///
 477     /// # Note on performance and stack usage
 478     ///
 479     /// Unfortunately, usages of this method are currently not always optimized
 480     /// as well as they could be. This mainly concerns large arrays, as mapping
 481     /// over small arrays seem to be optimized just fine. Also note that in
 482     /// debug mode (i.e. without any optimizations), this method can use a lot
 483     /// of stack space (a few times the size of the array or more).
 484     ///
 485     /// Therefore, in performance-critical code, try to avoid using this method
 486     /// on large arrays or check the emitted code. Also try to avoid chained
 487     /// maps (e.g. `arr.map(...).map(...)`).
 488     ///
 489     /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
 490     /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
 491     /// really need a new array of the same size as the result. Rust's lazy
 492     /// iterators tend to get optimized very well.
 493     ///
 494     ///
 495     /// # Examples
 496     ///
 497     /// ```
 498     /// let x = [1, 2, 3];
 499     /// let y = x.map(|v| v + 1);
 500     /// assert_eq!(y, [2, 3, 4]);
 501     ///
 502     /// let x = [1, 2, 3];
 503     /// let mut temp = 0;
 504     /// let y = x.map(|v| { temp += 1; v * temp });
 505     /// assert_eq!(y, [1, 4, 9]);
 506     ///
 507     /// let x = ["Ferris", "Bueller's", "Day", "Off"];
 508     /// let y = x.map(|v| v.len());
 509     /// assert_eq!(y, [6, 9, 3, 3]);
 510     /// ```
 511     #[stable(feature = "array_map", since = "1.55.0")]
 512     pub fn map<F, U>(self, f: F) -> [U; N]
 513     where
 514         F: FnMut(T) -> U,
 515     {
 516         // SAFETY: we know for certain that this iterator will yield exactly `N`
 517         // items.
 518         unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 519     }
 520
 521     /// A fallible function `f` applied to each element on array `self` in order to
 522     /// return an array the same size as `self` or the first error encountered.
 523     ///
 524     /// The return type of this function depends on the return type of the closure.
 525     /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N]; E>`.
 526     /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
 527     ///
 528     /// # Examples
 529     ///
 530     /// ```
 531     /// #![feature(array_try_map)]
 532     /// let a = ["1", "2", "3"];
 533     /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
 534     /// assert_eq!(b, [2, 3, 4]);
 535     ///
 536     /// let a = ["1", "2a", "3"];
 537     /// let b = a.try_map(|v| v.parse::<u32>());
 538     /// assert!(b.is_err());
 539     ///
 540     /// use std::num::NonZeroU32;
 541     /// let z = [1, 2, 0, 3, 4];
 542     /// assert_eq!(z.try_map(NonZeroU32::new), None);
 543     /// let a = [1, 2, 3];
 544     /// let b = a.try_map(NonZeroU32::new);
 545     /// let c = b.map(|x| x.map(NonZeroU32::get));
 546     /// assert_eq!(c, Some(a));
 547     /// ```
 548     #[unstable(feature = "array_try_map", issue = "79711")]
 549     pub fn try_map<F, R>(self, f: F) -> ChangeOutputType<R, [R::Output; N]>
 550     where
 551         F: FnMut(T) -> R,
 552         R: Try,
 553         R::Residual: Residual<[R::Output; N]>,
 554     {
 555         // SAFETY: we know for certain that this iterator will yield exactly `N`
 556         // items.
 557         unsafe { try_collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 558     }
 559
 560     /// 'Zips up' two arrays into a single array of pairs.
 561     ///
 562     /// `zip()` returns a new array where every element is a tuple where the
 563     /// first element comes from the first array, and the second element comes
 564     /// from the second array. In other words, it zips two arrays together,
 565     /// into a single one.
 566     ///
 567     /// # Examples
 568     ///
 569     /// ```
 570     /// #![feature(array_zip)]
 571     /// let x = [1, 2, 3];
 572     /// let y = [4, 5, 6];
 573     /// let z = x.zip(y);
 574     /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
 575     /// ```
 576     #[unstable(feature = "array_zip", issue = "80094")]
 577     pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
 578         let mut iter = IntoIterator::into_iter(self).zip(rhs);
 579
 580         // SAFETY: we know for certain that this iterator will yield exactly `N`
 581         // items.
 582         unsafe { collect_into_array_unchecked(&mut iter) }
 583     }
 584
 585     /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
 586     #[stable(feature = "array_as_slice", since = "1.57.0")]
 587     #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
 588     pub const fn as_slice(&self) -> &[T] {
 589         self
 590     }
 591
 592     /// Returns a mutable slice containing the entire array. Equivalent to
 593     /// `&mut s[..]`.
 594     #[stable(feature = "array_as_slice", since = "1.57.0")]
 595     pub 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     /// #![feature(array_methods)]
 607     ///
 608     /// let floats = [3.1, 2.7, -1.0];
 609     /// let float_refs: [&f64; 3] = floats.each_ref();
 610     /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
 611     /// ```
 612     ///
 613     /// This method is particularly useful if combined with other methods, like
 614     /// [`map`](#method.map). This way, you can avoid moving the original
 615     /// array if its elements are not [`Copy`].
 616     ///
 617     /// ```
 618     /// #![feature(array_methods)]
 619     ///
 620     /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
 621     /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
 622     /// assert_eq!(is_ascii, [true, false, true]);
 623     ///
 624     /// // We can still access the original array: it has not been moved.
 625     /// assert_eq!(strings.len(), 3);
 626     /// ```
 627     #[unstable(feature = "array_methods", issue = "76118")]
 628     pub fn each_ref(&self) -> [&T; N] {
 629         // SAFETY: we know for certain that this iterator will yield exactly `N`
 630         // items.
 631         unsafe { collect_into_array_unchecked(&mut self.iter()) }
 632     }
 633
 634     /// Borrows each element mutably and returns an array of mutable references
 635     /// with the same size as `self`.
 636     ///
 637     ///
 638     /// # Example
 639     ///
 640     /// ```
 641     /// #![feature(array_methods)]
 642     ///
 643     /// let mut floats = [3.1, 2.7, -1.0];
 644     /// let float_refs: [&mut f64; 3] = floats.each_mut();
 645     /// *float_refs[0] = 0.0;
 646     /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
 647     /// assert_eq!(floats, [0.0, 2.7, -1.0]);
 648     /// ```
 649     #[unstable(feature = "array_methods", issue = "76118")]
 650     pub fn each_mut(&mut self) -> [&mut T; N] {
 651         // SAFETY: we know for certain that this iterator will yield exactly `N`
 652         // items.
 653         unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
 654     }
 655
 656     /// Divides one array reference into two at an index.
 657     ///
 658     /// The first will contain all indices from `[0, M)` (excluding
 659     /// the index `M` itself) and the second will contain all
 660     /// indices from `[M, N)` (excluding the index `N` itself).
 661     ///
 662     /// # Panics
 663     ///
 664     /// Panics if `M > N`.
 665     ///
 666     /// # Examples
 667     ///
 668     /// ```
 669     /// #![feature(split_array)]
 670     ///
 671     /// let v = [1, 2, 3, 4, 5, 6];
 672     ///
 673     /// {
 674     ///    let (left, right) = v.split_array_ref::<0>();
 675     ///    assert_eq!(left, &[]);
 676     ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
 677     /// }
 678     ///
 679     /// {
 680     ///     let (left, right) = v.split_array_ref::<2>();
 681     ///     assert_eq!(left, &[1, 2]);
 682     ///     assert_eq!(right, &[3, 4, 5, 6]);
 683     /// }
 684     ///
 685     /// {
 686     ///     let (left, right) = v.split_array_ref::<6>();
 687     ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
 688     ///     assert_eq!(right, &[]);
 689     /// }
 690     /// ```
 691     #[unstable(
 692         feature = "split_array",
 693         reason = "return type should have array as 2nd element",
 694         issue = "90091"
 695     )]
 696     #[inline]
 697     pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
 698         (&self[..]).split_array_ref::<M>()
 699     }
 700
 701     /// Divides one mutable array reference into two at an index.
 702     ///
 703     /// The first will contain all indices from `[0, M)` (excluding
 704     /// the index `M` itself) and the second will contain all
 705     /// indices from `[M, N)` (excluding the index `N` itself).
 706     ///
 707     /// # Panics
 708     ///
 709     /// Panics if `M > N`.
 710     ///
 711     /// # Examples
 712     ///
 713     /// ```
 714     /// #![feature(split_array)]
 715     ///
 716     /// let mut v = [1, 0, 3, 0, 5, 6];
 717     /// let (left, right) = v.split_array_mut::<2>();
 718     /// assert_eq!(left, &mut [1, 0][..]);
 719     /// assert_eq!(right, &mut [3, 0, 5, 6]);
 720     /// left[1] = 2;
 721     /// right[1] = 4;
 722     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
 723     /// ```
 724     #[unstable(
 725         feature = "split_array",
 726         reason = "return type should have array as 2nd element",
 727         issue = "90091"
 728     )]
 729     #[inline]
 730     pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
 731         (&mut self[..]).split_array_mut::<M>()
 732     }
 733
 734     /// Divides one array reference into two at an index from the end.
 735     ///
 736     /// The first will contain all indices from `[0, N - M)` (excluding
 737     /// the index `N - M` itself) and the second will contain all
 738     /// indices from `[N - M, N)` (excluding the index `N` itself).
 739     ///
 740     /// # Panics
 741     ///
 742     /// Panics if `M > N`.
 743     ///
 744     /// # Examples
 745     ///
 746     /// ```
 747     /// #![feature(split_array)]
 748     ///
 749     /// let v = [1, 2, 3, 4, 5, 6];
 750     ///
 751     /// {
 752     ///    let (left, right) = v.rsplit_array_ref::<0>();
 753     ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
 754     ///    assert_eq!(right, &[]);
 755     /// }
 756     ///
 757     /// {
 758     ///     let (left, right) = v.rsplit_array_ref::<2>();
 759     ///     assert_eq!(left, &[1, 2, 3, 4]);
 760     ///     assert_eq!(right, &[5, 6]);
 761     /// }
 762     ///
 763     /// {
 764     ///     let (left, right) = v.rsplit_array_ref::<6>();
 765     ///     assert_eq!(left, &[]);
 766     ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
 767     /// }
 768     /// ```
 769     #[unstable(
 770         feature = "split_array",
 771         reason = "return type should have array as 2nd element",
 772         issue = "90091"
 773     )]
 774     #[inline]
 775     pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
 776         (&self[..]).rsplit_array_ref::<M>()
 777     }
 778
 779     /// Divides one mutable array reference into two at an index from the end.
 780     ///
 781     /// The first will contain all indices from `[0, N - M)` (excluding
 782     /// the index `N - M` itself) and the second will contain all
 783     /// indices from `[N - M, N)` (excluding the index `N` itself).
 784     ///
 785     /// # Panics
 786     ///
 787     /// Panics if `M > N`.
 788     ///
 789     /// # Examples
 790     ///
 791     /// ```
 792     /// #![feature(split_array)]
 793     ///
 794     /// let mut v = [1, 0, 3, 0, 5, 6];
 795     /// let (left, right) = v.rsplit_array_mut::<4>();
 796     /// assert_eq!(left, &mut [1, 0]);
 797     /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
 798     /// left[1] = 2;
 799     /// right[1] = 4;
 800     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
 801     /// ```
 802     #[unstable(
 803         feature = "split_array",
 804         reason = "return type should have array as 2nd element",
 805         issue = "90091"
 806     )]
 807     #[inline]
 808     pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
 809         (&mut self[..]).rsplit_array_mut::<M>()
 810     }
 811 }
 812
 813 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 814 /// yields fewer than `N` items, this function exhibits undefined behavior.
 815 ///
 816 /// See [`try_collect_into_array`] for more information.
 817 ///
 818 ///
 819 /// # Safety
 820 ///
 821 /// It is up to the caller to guarantee that `iter` yields at least `N` items.
 822 /// Violating this condition causes undefined behavior.
 823 unsafe fn try_collect_into_array_unchecked<I, T, R, const N: usize>(iter: &mut I) -> R::TryType
 824 where
 825     // Note: `TrustedLen` here is somewhat of an experiment. This is just an
 826     // internal function, so feel free to remove if this bound turns out to be a
 827     // bad idea. In that case, remember to also remove the lower bound
 828     // `debug_assert!` below!
 829     I: Iterator + TrustedLen,
 830     I::Item: Try<Output = T, Residual = R>,
 831     R: Residual<[T; N]>,
 832 {
 833     debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
 834     debug_assert!(N <= iter.size_hint().0);
 835
 836     // SAFETY: covered by the function contract.
 837     unsafe { try_collect_into_array(iter).unwrap_unchecked() }
 838 }
 839
 840 // Infallible version of `try_collect_into_array_unchecked`.
 841 unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
 842 where
 843     I: Iterator + TrustedLen,
 844 {
 845     let mut map = iter.map(NeverShortCircuit);
 846
 847     // SAFETY: The same safety considerations w.r.t. the iterator length
 848     // apply for `try_collect_into_array_unchecked` as for
 849     // `collect_into_array_unchecked`
 850     match unsafe { try_collect_into_array_unchecked(&mut map) } {
 851         NeverShortCircuit(array) => array,
 852     }
 853 }
 854
 855 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 856 /// yields fewer than `N` items, `Err` is returned containing an iterator over
 857 /// the already yielded items.
 858 ///
 859 /// Since the iterator is passed as a mutable reference and this function calls
 860 /// `next` at most `N` times, the iterator can still be used afterwards to
 861 /// retrieve the remaining items.
 862 ///
 863 /// If `iter.next()` panicks, all items already yielded by the iterator are
 864 /// dropped.
 865 #[inline]
 866 fn try_collect_into_array<I, T, R, const N: usize>(
 867     iter: &mut I,
 868 ) -> Result<R::TryType, IntoIter<T, N>>
 869 where
 870     I: Iterator,
 871     I::Item: Try<Output = T, Residual = R>,
 872     R: Residual<[T; N]>,
 873 {
 874     if N == 0 {
 875         // SAFETY: An empty array is always inhabited and has no validity invariants.
 876         return Ok(Try::from_output(unsafe { mem::zeroed() }));
 877     }
 878
 879     let mut array = MaybeUninit::uninit_array::<N>();
 880     let mut guard = Guard { array_mut: &mut array, initialized: 0 };
 881
 882     for _ in 0..N {
 883         match iter.next() {
 884             Some(item_rslt) => {
 885                 let item = match item_rslt.branch() {
 886                     ControlFlow::Break(r) => {
 887                         return Ok(FromResidual::from_residual(r));
 888                     }
 889                     ControlFlow::Continue(elem) => elem,
 890                 };
 891
 892                 // SAFETY: `guard.initialized` starts at 0, which means push can be called
 893                 // at most N times, which this loop does.
 894                 unsafe {
 895                     guard.push_unchecked(item);
 896                 }
 897             }
 898             None => {
 899                 let alive = 0..guard.initialized;
 900                 mem::forget(guard);
 901                 // SAFETY: `array` was initialized with exactly `initialized`
 902                 // number of elements.
 903                 return Err(unsafe { IntoIter::new_unchecked(array, alive) });
 904             }
 905         }
 906     }
 907
 908     mem::forget(guard);
 909     // SAFETY: All elements of the array were populated in the loop above.
 910     let output = unsafe { array.transpose().assume_init() };
 911     Ok(Try::from_output(output))
 912 }
 913
 914 /// Panic guard for incremental initialization of arrays.
 915 ///
 916 /// Disarm the guard with `mem::forget` once the array has been initialized.
 917 ///
 918 /// # Safety
 919 ///
 920 /// All write accesses to this structure are unsafe and must maintain a correct
 921 /// count of `initialized` elements.
 922 ///
 923 /// To minimize indirection fields are still pub but callers should at least use
 924 /// `push_unchecked` to signal that something unsafe is going on.
 925 pub(crate) struct Guard<'a, T, const N: usize> {
 926     /// The array to be initialized.
 927     pub array_mut: &'a mut [MaybeUninit<T>; N],
 928     /// The number of items that have been initialized so far.
 929     pub initialized: usize,
 930 }
 931
 932 impl<T, const N: usize> Guard<'_, T, N> {
 933     /// Adds an item to the array and updates the initialized item counter.
 934     ///
 935     /// # Safety
 936     ///
 937     /// No more than N elements must be initialized.
 938     #[inline]
 939     pub unsafe fn push_unchecked(&mut self, item: T) {
 940         // SAFETY: If `initialized` was correct before and the caller does not
 941         // invoke this method more than N times then writes will be in-bounds
 942         // and slots will not be initialized more than once.
 943         unsafe {
 944             self.array_mut.get_unchecked_mut(self.initialized).write(item);
 945             self.initialized = self.initialized.unchecked_add(1);
 946         }
 947     }
 948 }
 949
 950 impl<T, const N: usize> Drop for Guard<'_, T, N> {
 951     fn drop(&mut self) {
 952         debug_assert!(self.initialized <= N);
 953
 954         // SAFETY: this slice will contain only initialized objects.
 955         unsafe {
 956             crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
 957                 &mut self.array_mut.get_unchecked_mut(..self.initialized),
 958             ));
 959         }
 960     }
 961 }
 962
 963 /// Returns the next chunk of `N` items from the iterator or errors with an
 964 /// iterator over the remainder. Used for `Iterator::next_chunk`.
 965 #[inline]
 966 pub(crate) fn iter_next_chunk<I, const N: usize>(
 967     iter: &mut I,
 968 ) -> Result<[I::Item; N], IntoIter<I::Item, N>>
 969 where
 970     I: Iterator,
 971 {
 972     let mut map = iter.map(NeverShortCircuit);
 973     try_collect_into_array(&mut map).map(|NeverShortCircuit(arr)| arr)
 974 }