library/core/src/array/mod.rs

   1 //! Helper functions and types for fixed-length arrays.
   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::fmt;
  11 use crate::hash::{self, Hash};
  12 use crate::iter::TrustedLen;
  13 use crate::mem::{self, MaybeUninit};
  14 use crate::ops::{
  15     ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
  16 };
  17 use crate::slice::{Iter, IterMut};
  18
  19 mod equality;
  20 mod iter;
  21
  22 #[stable(feature = "array_value_iter", since = "1.51.0")]
  23 pub use iter::IntoIter;
  24
  25 /// Creates an array `[T; N]` where each array element `T` is returned by the `cb` call.
  26 ///
  27 /// # Arguments
  28 ///
  29 /// * `cb`: Callback where the passed argument is the current array index.
  30 ///
  31 /// # Example
  32 ///
  33 /// ```rust
  34 /// let array = core::array::from_fn(|i| i);
  35 /// assert_eq!(array, [0, 1, 2, 3, 4]);
  36 /// ```
  37 #[inline]
  38 #[stable(feature = "array_from_fn", since = "1.63.0")]
  39 pub fn from_fn<T, const N: usize, F>(mut cb: F) -> [T; N]
  40 where
  41     F: FnMut(usize) -> T,
  42 {
  43     let mut idx = 0;
  44     [(); N].map(|_| {
  45         let res = cb(idx);
  46         idx += 1;
  47         res
  48     })
  49 }
  50
  51 /// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
  52 /// Unlike [`from_fn`], where the element creation can't fail, this version will return an error
  53 /// if any element creation was unsuccessful.
  54 ///
  55 /// The return type of this function depends on the return type of the closure.
  56 /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N]; E>`.
  57 /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
  58 ///
  59 /// # Arguments
  60 ///
  61 /// * `cb`: Callback where the passed argument is the current array index.
  62 ///
  63 /// # Example
  64 ///
  65 /// ```rust
  66 /// #![feature(array_try_from_fn)]
  67 ///
  68 /// let array: Result<[u8; 5], _> = std::array::try_from_fn(|i| i.try_into());
  69 /// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
  70 ///
  71 /// let array: Result<[i8; 200], _> = std::array::try_from_fn(|i| i.try_into());
  72 /// assert!(array.is_err());
  73 ///
  74 /// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_add(100));
  75 /// assert_eq!(array, Some([100, 101, 102, 103]));
  76 ///
  77 /// let array: Option<[_; 4]> = std::array::try_from_fn(|i| i.checked_sub(100));
  78 /// assert_eq!(array, None);
  79 /// ```
  80 #[inline]
  81 #[unstable(feature = "array_try_from_fn", issue = "89379")]
  82 pub fn try_from_fn<R, const N: usize, F>(cb: F) -> ChangeOutputType<R, [R::Output; N]>
  83 where
  84     F: FnMut(usize) -> R,
  85     R: Try,
  86     R::Residual: Residual<[R::Output; N]>,
  87 {
  88     // SAFETY: we know for certain that this iterator will yield exactly `N`
  89     // items.
  90     unsafe { try_collect_into_array_unchecked(&mut (0..N).map(cb)) }
  91 }
  92
  93 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
  94 #[stable(feature = "array_from_ref", since = "1.53.0")]
  95 #[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
  96 pub const fn from_ref<T>(s: &T) -> &[T; 1] {
  97     // SAFETY: Converting `&T` to `&[T; 1]` is sound.
  98     unsafe { &*(s as *const T).cast::<[T; 1]>() }
  99 }
 100
 101 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
 102 #[stable(feature = "array_from_ref", since = "1.53.0")]
 103 #[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
 104 pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
 105     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
 106     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
 107 }
 108
 109 /// The error type returned when a conversion from a slice to an array fails.
 110 #[stable(feature = "try_from", since = "1.34.0")]
 111 #[derive(Debug, Copy, Clone)]
 112 pub struct TryFromSliceError(());
 113
 114 #[stable(feature = "core_array", since = "1.36.0")]
 115 impl fmt::Display for TryFromSliceError {
 116     #[inline]
 117     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 118         fmt::Display::fmt(self.__description(), f)
 119     }
 120 }
 121
 122 impl TryFromSliceError {
 123     #[unstable(
 124         feature = "array_error_internals",
 125         reason = "available through Error trait and this method should not \
 126                      be exposed publicly",
 127         issue = "none"
 128     )]
 129     #[inline]
 130     #[doc(hidden)]
 131     pub fn __description(&self) -> &str {
 132         "could not convert slice to array"
 133     }
 134 }
 135
 136 #[stable(feature = "try_from_slice_error", since = "1.36.0")]
 137 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 138 impl const From<Infallible> for TryFromSliceError {
 139     fn from(x: Infallible) -> TryFromSliceError {
 140         match x {}
 141     }
 142 }
 143
 144 #[stable(feature = "rust1", since = "1.0.0")]
 145 impl<T, const N: usize> AsRef<[T]> for [T; N] {
 146     #[inline]
 147     fn as_ref(&self) -> &[T] {
 148         &self[..]
 149     }
 150 }
 151
 152 #[stable(feature = "rust1", since = "1.0.0")]
 153 impl<T, const N: usize> AsMut<[T]> for [T; N] {
 154     #[inline]
 155     fn as_mut(&mut self) -> &mut [T] {
 156         &mut self[..]
 157     }
 158 }
 159
 160 #[stable(feature = "array_borrow", since = "1.4.0")]
 161 #[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
 162 impl<T, const N: usize> const Borrow<[T]> for [T; N] {
 163     fn borrow(&self) -> &[T] {
 164         self
 165     }
 166 }
 167
 168 #[stable(feature = "array_borrow", since = "1.4.0")]
 169 #[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
 170 impl<T, const N: usize> const BorrowMut<[T]> for [T; N] {
 171     fn borrow_mut(&mut self) -> &mut [T] {
 172         self
 173     }
 174 }
 175
 176 #[stable(feature = "try_from", since = "1.34.0")]
 177 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
 178 where
 179     T: Copy,
 180 {
 181     type Error = TryFromSliceError;
 182
 183     fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
 184         <&Self>::try_from(slice).map(|r| *r)
 185     }
 186 }
 187
 188 #[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
 189 impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
 190 where
 191     T: Copy,
 192 {
 193     type Error = TryFromSliceError;
 194
 195     fn try_from(slice: &mut [T]) -> Result<[T; N], TryFromSliceError> {
 196         <Self>::try_from(&*slice)
 197     }
 198 }
 199
 200 #[stable(feature = "try_from", since = "1.34.0")]
 201 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
 202     type Error = TryFromSliceError;
 203
 204     fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
 205         if slice.len() == N {
 206             let ptr = slice.as_ptr() as *const [T; N];
 207             // SAFETY: ok because we just checked that the length fits
 208             unsafe { Ok(&*ptr) }
 209         } else {
 210             Err(TryFromSliceError(()))
 211         }
 212     }
 213 }
 214
 215 #[stable(feature = "try_from", since = "1.34.0")]
 216 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
 217     type Error = TryFromSliceError;
 218
 219     fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
 220         if slice.len() == N {
 221             let ptr = slice.as_mut_ptr() as *mut [T; N];
 222             // SAFETY: ok because we just checked that the length fits
 223             unsafe { Ok(&mut *ptr) }
 224         } else {
 225             Err(TryFromSliceError(()))
 226         }
 227     }
 228 }
 229
 230 /// The hash of an array is the same as that of the corresponding slice,
 231 /// as required by the `Borrow` implementation.
 232 ///
 233 /// ```
 234 /// #![feature(build_hasher_simple_hash_one)]
 235 /// use std::hash::BuildHasher;
 236 ///
 237 /// let b = std::collections::hash_map::RandomState::new();
 238 /// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
 239 /// let s: &[u8] = &[0xa8, 0x3c, 0x09];
 240 /// assert_eq!(b.hash_one(a), b.hash_one(s));
 241 /// ```
 242 #[stable(feature = "rust1", since = "1.0.0")]
 243 impl<T: Hash, const N: usize> Hash for [T; N] {
 244     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 245         Hash::hash(&self[..], state)
 246     }
 247 }
 248
 249 #[stable(feature = "rust1", since = "1.0.0")]
 250 impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
 251     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 252         fmt::Debug::fmt(&&self[..], f)
 253     }
 254 }
 255
 256 #[stable(feature = "rust1", since = "1.0.0")]
 257 impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
 258     type Item = &'a T;
 259     type IntoIter = Iter<'a, T>;
 260
 261     fn into_iter(self) -> Iter<'a, T> {
 262         self.iter()
 263     }
 264 }
 265
 266 #[stable(feature = "rust1", since = "1.0.0")]
 267 impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
 268     type Item = &'a mut T;
 269     type IntoIter = IterMut<'a, T>;
 270
 271     fn into_iter(self) -> IterMut<'a, T> {
 272         self.iter_mut()
 273     }
 274 }
 275
 276 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 277 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 278 impl<T, I, const N: usize> const Index<I> for [T; N]
 279 where
 280     [T]: ~const Index<I>,
 281 {
 282     type Output = <[T] as Index<I>>::Output;
 283
 284     #[inline]
 285     fn index(&self, index: I) -> &Self::Output {
 286         Index::index(self as &[T], index)
 287     }
 288 }
 289
 290 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 291 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 292 impl<T, I, const N: usize> const IndexMut<I> for [T; N]
 293 where
 294     [T]: ~const IndexMut<I>,
 295 {
 296     #[inline]
 297     fn index_mut(&mut self, index: I) -> &mut Self::Output {
 298         IndexMut::index_mut(self as &mut [T], index)
 299     }
 300 }
 301
 302 #[stable(feature = "rust1", since = "1.0.0")]
 303 impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
 304     #[inline]
 305     fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
 306         PartialOrd::partial_cmp(&&self[..], &&other[..])
 307     }
 308     #[inline]
 309     fn lt(&self, other: &[T; N]) -> bool {
 310         PartialOrd::lt(&&self[..], &&other[..])
 311     }
 312     #[inline]
 313     fn le(&self, other: &[T; N]) -> bool {
 314         PartialOrd::le(&&self[..], &&other[..])
 315     }
 316     #[inline]
 317     fn ge(&self, other: &[T; N]) -> bool {
 318         PartialOrd::ge(&&self[..], &&other[..])
 319     }
 320     #[inline]
 321     fn gt(&self, other: &[T; N]) -> bool {
 322         PartialOrd::gt(&&self[..], &&other[..])
 323     }
 324 }
 325
 326 /// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
 327 #[stable(feature = "rust1", since = "1.0.0")]
 328 impl<T: Ord, const N: usize> Ord for [T; N] {
 329     #[inline]
 330     fn cmp(&self, other: &[T; N]) -> Ordering {
 331         Ord::cmp(&&self[..], &&other[..])
 332     }
 333 }
 334
 335 #[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
 336 impl<T: Copy, const N: usize> Copy for [T; N] {}
 337
 338 #[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
 339 impl<T: Clone, const N: usize> Clone for [T; N] {
 340     #[inline]
 341     fn clone(&self) -> Self {
 342         SpecArrayClone::clone(self)
 343     }
 344
 345     #[inline]
 346     fn clone_from(&mut self, other: &Self) {
 347         self.clone_from_slice(other);
 348     }
 349 }
 350
 351 trait SpecArrayClone: Clone {
 352     fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
 353 }
 354
 355 impl<T: Clone> SpecArrayClone for T {
 356     #[inline]
 357     default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 358         // SAFETY: we know for certain that this iterator will yield exactly `N`
 359         // items.
 360         unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
 361     }
 362 }
 363
 364 impl<T: Copy> SpecArrayClone for T {
 365     #[inline]
 366     fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 367         *array
 368     }
 369 }
 370
 371 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 372 // require Default to be implemented, and having different impl blocks for
 373 // different numbers isn't supported yet.
 374
 375 macro_rules! array_impl_default {
 376     {$n:expr, $t:ident $($ts:ident)*} => {
 377         #[stable(since = "1.4.0", feature = "array_default")]
 378         impl<T> Default for [T; $n] where T: Default {
 379             fn default() -> [T; $n] {
 380                 [$t::default(), $($ts::default()),*]
 381             }
 382         }
 383         array_impl_default!{($n - 1), $($ts)*}
 384     };
 385     {$n:expr,} => {
 386         #[stable(since = "1.4.0", feature = "array_default")]
 387         #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 388         impl<T> const Default for [T; $n] {
 389             fn default() -> [T; $n] { [] }
 390         }
 391     };
 392 }
 393
 394 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}
 395
 396 impl<T, const N: usize> [T; N] {
 397     /// Returns an array of the same size as `self`, with function `f` applied to each element
 398     /// in order.
 399     ///
 400     /// If you don't necessarily need a new fixed-size array, consider using
 401     /// [`Iterator::map`] instead.
 402     ///
 403     ///
 404     /// # Note on performance and stack usage
 405     ///
 406     /// Unfortunately, usages of this method are currently not always optimized
 407     /// as well as they could be. This mainly concerns large arrays, as mapping
 408     /// over small arrays seem to be optimized just fine. Also note that in
 409     /// debug mode (i.e. without any optimizations), this method can use a lot
 410     /// of stack space (a few times the size of the array or more).
 411     ///
 412     /// Therefore, in performance-critical code, try to avoid using this method
 413     /// on large arrays or check the emitted code. Also try to avoid chained
 414     /// maps (e.g. `arr.map(...).map(...)`).
 415     ///
 416     /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
 417     /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
 418     /// really need a new array of the same size as the result. Rust's lazy
 419     /// iterators tend to get optimized very well.
 420     ///
 421     ///
 422     /// # Examples
 423     ///
 424     /// ```
 425     /// let x = [1, 2, 3];
 426     /// let y = x.map(|v| v + 1);
 427     /// assert_eq!(y, [2, 3, 4]);
 428     ///
 429     /// let x = [1, 2, 3];
 430     /// let mut temp = 0;
 431     /// let y = x.map(|v| { temp += 1; v * temp });
 432     /// assert_eq!(y, [1, 4, 9]);
 433     ///
 434     /// let x = ["Ferris", "Bueller's", "Day", "Off"];
 435     /// let y = x.map(|v| v.len());
 436     /// assert_eq!(y, [6, 9, 3, 3]);
 437     /// ```
 438     #[stable(feature = "array_map", since = "1.55.0")]
 439     pub fn map<F, U>(self, f: F) -> [U; N]
 440     where
 441         F: FnMut(T) -> U,
 442     {
 443         // SAFETY: we know for certain that this iterator will yield exactly `N`
 444         // items.
 445         unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 446     }
 447
 448     /// A fallible function `f` applied to each element on array `self` in order to
 449     /// return an array the same size as `self` or the first error encountered.
 450     ///
 451     /// The return type of this function depends on the return type of the closure.
 452     /// If you return `Result<T, E>` from the closure, you'll get a `Result<[T; N]; E>`.
 453     /// If you return `Option<T>` from the closure, you'll get an `Option<[T; N]>`.
 454     ///
 455     /// # Examples
 456     ///
 457     /// ```
 458     /// #![feature(array_try_map)]
 459     /// let a = ["1", "2", "3"];
 460     /// let b = a.try_map(|v| v.parse::<u32>()).unwrap().map(|v| v + 1);
 461     /// assert_eq!(b, [2, 3, 4]);
 462     ///
 463     /// let a = ["1", "2a", "3"];
 464     /// let b = a.try_map(|v| v.parse::<u32>());
 465     /// assert!(b.is_err());
 466     ///
 467     /// use std::num::NonZeroU32;
 468     /// let z = [1, 2, 0, 3, 4];
 469     /// assert_eq!(z.try_map(NonZeroU32::new), None);
 470     /// let a = [1, 2, 3];
 471     /// let b = a.try_map(NonZeroU32::new);
 472     /// let c = b.map(|x| x.map(NonZeroU32::get));
 473     /// assert_eq!(c, Some(a));
 474     /// ```
 475     #[unstable(feature = "array_try_map", issue = "79711")]
 476     pub fn try_map<F, R>(self, f: F) -> ChangeOutputType<R, [R::Output; N]>
 477     where
 478         F: FnMut(T) -> R,
 479         R: Try,
 480         R::Residual: Residual<[R::Output; N]>,
 481     {
 482         // SAFETY: we know for certain that this iterator will yield exactly `N`
 483         // items.
 484         unsafe { try_collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 485     }
 486
 487     /// 'Zips up' two arrays into a single array of pairs.
 488     ///
 489     /// `zip()` returns a new array where every element is a tuple where the
 490     /// first element comes from the first array, and the second element comes
 491     /// from the second array. In other words, it zips two arrays together,
 492     /// into a single one.
 493     ///
 494     /// # Examples
 495     ///
 496     /// ```
 497     /// #![feature(array_zip)]
 498     /// let x = [1, 2, 3];
 499     /// let y = [4, 5, 6];
 500     /// let z = x.zip(y);
 501     /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
 502     /// ```
 503     #[unstable(feature = "array_zip", issue = "80094")]
 504     pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
 505         let mut iter = IntoIterator::into_iter(self).zip(rhs);
 506
 507         // SAFETY: we know for certain that this iterator will yield exactly `N`
 508         // items.
 509         unsafe { collect_into_array_unchecked(&mut iter) }
 510     }
 511
 512     /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
 513     #[stable(feature = "array_as_slice", since = "1.57.0")]
 514     #[rustc_const_stable(feature = "array_as_slice", since = "1.57.0")]
 515     pub const fn as_slice(&self) -> &[T] {
 516         self
 517     }
 518
 519     /// Returns a mutable slice containing the entire array. Equivalent to
 520     /// `&mut s[..]`.
 521     #[stable(feature = "array_as_slice", since = "1.57.0")]
 522     pub fn as_mut_slice(&mut self) -> &mut [T] {
 523         self
 524     }
 525
 526     /// Borrows each element and returns an array of references with the same
 527     /// size as `self`.
 528     ///
 529     ///
 530     /// # Example
 531     ///
 532     /// ```
 533     /// #![feature(array_methods)]
 534     ///
 535     /// let floats = [3.1, 2.7, -1.0];
 536     /// let float_refs: [&f64; 3] = floats.each_ref();
 537     /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
 538     /// ```
 539     ///
 540     /// This method is particularly useful if combined with other methods, like
 541     /// [`map`](#method.map). This way, you can avoid moving the original
 542     /// array if its elements are not [`Copy`].
 543     ///
 544     /// ```
 545     /// #![feature(array_methods)]
 546     ///
 547     /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
 548     /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
 549     /// assert_eq!(is_ascii, [true, false, true]);
 550     ///
 551     /// // We can still access the original array: it has not been moved.
 552     /// assert_eq!(strings.len(), 3);
 553     /// ```
 554     #[unstable(feature = "array_methods", issue = "76118")]
 555     pub fn each_ref(&self) -> [&T; N] {
 556         // SAFETY: we know for certain that this iterator will yield exactly `N`
 557         // items.
 558         unsafe { collect_into_array_unchecked(&mut self.iter()) }
 559     }
 560
 561     /// Borrows each element mutably and returns an array of mutable references
 562     /// with the same size as `self`.
 563     ///
 564     ///
 565     /// # Example
 566     ///
 567     /// ```
 568     /// #![feature(array_methods)]
 569     ///
 570     /// let mut floats = [3.1, 2.7, -1.0];
 571     /// let float_refs: [&mut f64; 3] = floats.each_mut();
 572     /// *float_refs[0] = 0.0;
 573     /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
 574     /// assert_eq!(floats, [0.0, 2.7, -1.0]);
 575     /// ```
 576     #[unstable(feature = "array_methods", issue = "76118")]
 577     pub fn each_mut(&mut self) -> [&mut T; N] {
 578         // SAFETY: we know for certain that this iterator will yield exactly `N`
 579         // items.
 580         unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
 581     }
 582
 583     /// Divides one array reference into two at an index.
 584     ///
 585     /// The first will contain all indices from `[0, M)` (excluding
 586     /// the index `M` itself) and the second will contain all
 587     /// indices from `[M, N)` (excluding the index `N` itself).
 588     ///
 589     /// # Panics
 590     ///
 591     /// Panics if `M > N`.
 592     ///
 593     /// # Examples
 594     ///
 595     /// ```
 596     /// #![feature(split_array)]
 597     ///
 598     /// let v = [1, 2, 3, 4, 5, 6];
 599     ///
 600     /// {
 601     ///    let (left, right) = v.split_array_ref::<0>();
 602     ///    assert_eq!(left, &[]);
 603     ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
 604     /// }
 605     ///
 606     /// {
 607     ///     let (left, right) = v.split_array_ref::<2>();
 608     ///     assert_eq!(left, &[1, 2]);
 609     ///     assert_eq!(right, &[3, 4, 5, 6]);
 610     /// }
 611     ///
 612     /// {
 613     ///     let (left, right) = v.split_array_ref::<6>();
 614     ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
 615     ///     assert_eq!(right, &[]);
 616     /// }
 617     /// ```
 618     #[unstable(
 619         feature = "split_array",
 620         reason = "return type should have array as 2nd element",
 621         issue = "90091"
 622     )]
 623     #[inline]
 624     pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
 625         (&self[..]).split_array_ref::<M>()
 626     }
 627
 628     /// Divides one mutable array reference into two at an index.
 629     ///
 630     /// The first will contain all indices from `[0, M)` (excluding
 631     /// the index `M` itself) and the second will contain all
 632     /// indices from `[M, N)` (excluding the index `N` itself).
 633     ///
 634     /// # Panics
 635     ///
 636     /// Panics if `M > N`.
 637     ///
 638     /// # Examples
 639     ///
 640     /// ```
 641     /// #![feature(split_array)]
 642     ///
 643     /// let mut v = [1, 0, 3, 0, 5, 6];
 644     /// let (left, right) = v.split_array_mut::<2>();
 645     /// assert_eq!(left, &mut [1, 0][..]);
 646     /// assert_eq!(right, &mut [3, 0, 5, 6]);
 647     /// left[1] = 2;
 648     /// right[1] = 4;
 649     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
 650     /// ```
 651     #[unstable(
 652         feature = "split_array",
 653         reason = "return type should have array as 2nd element",
 654         issue = "90091"
 655     )]
 656     #[inline]
 657     pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
 658         (&mut self[..]).split_array_mut::<M>()
 659     }
 660
 661     /// Divides one array reference into two at an index from the end.
 662     ///
 663     /// The first will contain all indices from `[0, N - M)` (excluding
 664     /// the index `N - M` itself) and the second will contain all
 665     /// indices from `[N - M, N)` (excluding the index `N` itself).
 666     ///
 667     /// # Panics
 668     ///
 669     /// Panics if `M > N`.
 670     ///
 671     /// # Examples
 672     ///
 673     /// ```
 674     /// #![feature(split_array)]
 675     ///
 676     /// let v = [1, 2, 3, 4, 5, 6];
 677     ///
 678     /// {
 679     ///    let (left, right) = v.rsplit_array_ref::<0>();
 680     ///    assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
 681     ///    assert_eq!(right, &[]);
 682     /// }
 683     ///
 684     /// {
 685     ///     let (left, right) = v.rsplit_array_ref::<2>();
 686     ///     assert_eq!(left, &[1, 2, 3, 4]);
 687     ///     assert_eq!(right, &[5, 6]);
 688     /// }
 689     ///
 690     /// {
 691     ///     let (left, right) = v.rsplit_array_ref::<6>();
 692     ///     assert_eq!(left, &[]);
 693     ///     assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
 694     /// }
 695     /// ```
 696     #[unstable(
 697         feature = "split_array",
 698         reason = "return type should have array as 2nd element",
 699         issue = "90091"
 700     )]
 701     #[inline]
 702     pub fn rsplit_array_ref<const M: usize>(&self) -> (&[T], &[T; M]) {
 703         (&self[..]).rsplit_array_ref::<M>()
 704     }
 705
 706     /// Divides one mutable array reference into two at an index from the end.
 707     ///
 708     /// The first will contain all indices from `[0, N - M)` (excluding
 709     /// the index `N - M` itself) and the second will contain all
 710     /// indices from `[N - M, N)` (excluding the index `N` itself).
 711     ///
 712     /// # Panics
 713     ///
 714     /// Panics if `M > N`.
 715     ///
 716     /// # Examples
 717     ///
 718     /// ```
 719     /// #![feature(split_array)]
 720     ///
 721     /// let mut v = [1, 0, 3, 0, 5, 6];
 722     /// let (left, right) = v.rsplit_array_mut::<4>();
 723     /// assert_eq!(left, &mut [1, 0]);
 724     /// assert_eq!(right, &mut [3, 0, 5, 6][..]);
 725     /// left[1] = 2;
 726     /// right[1] = 4;
 727     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
 728     /// ```
 729     #[unstable(
 730         feature = "split_array",
 731         reason = "return type should have array as 2nd element",
 732         issue = "90091"
 733     )]
 734     #[inline]
 735     pub fn rsplit_array_mut<const M: usize>(&mut self) -> (&mut [T], &mut [T; M]) {
 736         (&mut self[..]).rsplit_array_mut::<M>()
 737     }
 738 }
 739
 740 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 741 /// yields fewer than `N` items, this function exhibits undefined behavior.
 742 ///
 743 /// See [`try_collect_into_array`] for more information.
 744 ///
 745 ///
 746 /// # Safety
 747 ///
 748 /// It is up to the caller to guarantee that `iter` yields at least `N` items.
 749 /// Violating this condition causes undefined behavior.
 750 unsafe fn try_collect_into_array_unchecked<I, T, R, const N: usize>(iter: &mut I) -> R::TryType
 751 where
 752     // Note: `TrustedLen` here is somewhat of an experiment. This is just an
 753     // internal function, so feel free to remove if this bound turns out to be a
 754     // bad idea. In that case, remember to also remove the lower bound
 755     // `debug_assert!` below!
 756     I: Iterator + TrustedLen,
 757     I::Item: Try<Output = T, Residual = R>,
 758     R: Residual<[T; N]>,
 759 {
 760     debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
 761     debug_assert!(N <= iter.size_hint().0);
 762
 763     // SAFETY: covered by the function contract.
 764     unsafe { try_collect_into_array(iter).unwrap_unchecked() }
 765 }
 766
 767 // Infallible version of `try_collect_into_array_unchecked`.
 768 unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
 769 where
 770     I: Iterator + TrustedLen,
 771 {
 772     let mut map = iter.map(NeverShortCircuit);
 773
 774     // SAFETY: The same safety considerations w.r.t. the iterator length
 775     // apply for `try_collect_into_array_unchecked` as for
 776     // `collect_into_array_unchecked`
 777     match unsafe { try_collect_into_array_unchecked(&mut map) } {
 778         NeverShortCircuit(array) => array,
 779     }
 780 }
 781
 782 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 783 /// yields fewer than `N` items, `Err` is returned containing an iterator over
 784 /// the already yielded items.
 785 ///
 786 /// Since the iterator is passed as a mutable reference and this function calls
 787 /// `next` at most `N` times, the iterator can still be used afterwards to
 788 /// retrieve the remaining items.
 789 ///
 790 /// If `iter.next()` panicks, all items already yielded by the iterator are
 791 /// dropped.
 792 #[inline]
 793 fn try_collect_into_array<I, T, R, const N: usize>(
 794     iter: &mut I,
 795 ) -> Result<R::TryType, IntoIter<T, N>>
 796 where
 797     I: Iterator,
 798     I::Item: Try<Output = T, Residual = R>,
 799     R: Residual<[T; N]>,
 800 {
 801     if N == 0 {
 802         // SAFETY: An empty array is always inhabited and has no validity invariants.
 803         return Ok(Try::from_output(unsafe { mem::zeroed() }));
 804     }
 805
 806     struct Guard<'a, T, const N: usize> {
 807         array_mut: &'a mut [MaybeUninit<T>; N],
 808         initialized: usize,
 809     }
 810
 811     impl<T, const N: usize> Drop for Guard<'_, T, N> {
 812         fn drop(&mut self) {
 813             debug_assert!(self.initialized <= N);
 814
 815             // SAFETY: this slice will contain only initialized objects.
 816             unsafe {
 817                 crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
 818                     &mut self.array_mut.get_unchecked_mut(..self.initialized),
 819                 ));
 820             }
 821         }
 822     }
 823
 824     let mut array = MaybeUninit::uninit_array::<N>();
 825     let mut guard = Guard { array_mut: &mut array, initialized: 0 };
 826
 827     for _ in 0..N {
 828         match iter.next() {
 829             Some(item_rslt) => {
 830                 let item = match item_rslt.branch() {
 831                     ControlFlow::Break(r) => {
 832                         return Ok(FromResidual::from_residual(r));
 833                     }
 834                     ControlFlow::Continue(elem) => elem,
 835                 };
 836
 837                 // SAFETY: `guard.initialized` starts at 0, is increased by one in the
 838                 // loop and the loop is aborted once it reaches N (which is
 839                 // `array.len()`).
 840                 unsafe {
 841                     guard.array_mut.get_unchecked_mut(guard.initialized).write(item);
 842                 }
 843                 guard.initialized += 1;
 844             }
 845             None => {
 846                 let alive = 0..guard.initialized;
 847                 mem::forget(guard);
 848                 // SAFETY: `array` was initialized with exactly `initialized`
 849                 // number of elements.
 850                 return Err(unsafe { IntoIter::new_unchecked(array, alive) });
 851             }
 852         }
 853     }
 854
 855     mem::forget(guard);
 856     // SAFETY: All elements of the array were populated in the loop above.
 857     let output = unsafe { MaybeUninit::array_assume_init(array) };
 858     Ok(Try::from_output(output))
 859 }
 860
 861 /// Returns the next chunk of `N` items from the iterator or errors with an
 862 /// iterator over the remainder. Used for `Iterator::next_chunk`.
 863 #[inline]
 864 pub(crate) fn iter_next_chunk<I, const N: usize>(
 865     iter: &mut I,
 866 ) -> Result<[I::Item; N], IntoIter<I::Item, N>>
 867 where
 868     I: Iterator,
 869 {
 870     let mut map = iter.map(NeverShortCircuit);
 871     try_collect_into_array(&mut map).map(|NeverShortCircuit(arr)| arr)
 872 }