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::{Index, IndexMut};
  15 use crate::slice::{Iter, IterMut};
  16
  17 mod equality;
  18 mod iter;
  19
  20 #[stable(feature = "array_value_iter", since = "1.51.0")]
  21 pub use iter::IntoIter;
  22
  23 /// Creates an array `[T; N]` where each array element `T` is returned by the `cb` call.
  24 ///
  25 /// # Arguments
  26 ///
  27 /// * `cb`: Callback where the passed argument is the current array index.
  28 ///
  29 /// # Example
  30 ///
  31 /// ```rust
  32 /// #![feature(array_from_fn)]
  33 ///
  34 /// let array = core::array::from_fn(|i| i);
  35 /// assert_eq!(array, [0, 1, 2, 3, 4]);
  36 /// ```
  37 #[inline]
  38 #[unstable(feature = "array_from_fn", issue = "89379")]
  39 pub fn from_fn<F, T, const N: usize>(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 `core::array::from_fn`, where the element creation can't fail, this version will return an error
  53 /// if any element creation was unsuccessful.
  54 ///
  55 /// # Arguments
  56 ///
  57 /// * `cb`: Callback where the passed argument is the current array index.
  58 ///
  59 /// # Example
  60 ///
  61 /// ```rust
  62 /// #![feature(array_from_fn)]
  63 ///
  64 /// #[derive(Debug, PartialEq)]
  65 /// enum SomeError {
  66 ///     Foo,
  67 /// }
  68 ///
  69 /// let array = core::array::try_from_fn(|i| Ok::<_, SomeError>(i));
  70 /// assert_eq!(array, Ok([0, 1, 2, 3, 4]));
  71 ///
  72 /// let another_array = core::array::try_from_fn::<SomeError, _, (), 2>(|_| Err(SomeError::Foo));
  73 /// assert_eq!(another_array, Err(SomeError::Foo));
  74 /// ```
  75 #[inline]
  76 #[unstable(feature = "array_from_fn", issue = "89379")]
  77 pub fn try_from_fn<E, F, T, const N: usize>(cb: F) -> Result<[T; N], E>
  78 where
  79     F: FnMut(usize) -> Result<T, E>,
  80 {
  81     // SAFETY: we know for certain that this iterator will yield exactly `N`
  82     // items.
  83     unsafe { collect_into_array_rslt_unchecked(&mut (0..N).map(cb)) }
  84 }
  85
  86 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
  87 #[stable(feature = "array_from_ref", since = "1.53.0")]
  88 #[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
  89 pub const fn from_ref<T>(s: &T) -> &[T; 1] {
  90     // SAFETY: Converting `&T` to `&[T; 1]` is sound.
  91     unsafe { &*(s as *const T).cast::<[T; 1]>() }
  92 }
  93
  94 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
  95 #[stable(feature = "array_from_ref", since = "1.53.0")]
  96 #[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
  97 pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
  98     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
  99     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
 100 }
 101
 102 /// The error type returned when a conversion from a slice to an array fails.
 103 #[stable(feature = "try_from", since = "1.34.0")]
 104 #[derive(Debug, Copy, Clone)]
 105 pub struct TryFromSliceError(());
 106
 107 #[stable(feature = "core_array", since = "1.36.0")]
 108 impl fmt::Display for TryFromSliceError {
 109     #[inline]
 110     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 111         fmt::Display::fmt(self.__description(), f)
 112     }
 113 }
 114
 115 impl TryFromSliceError {
 116     #[unstable(
 117         feature = "array_error_internals",
 118         reason = "available through Error trait and this method should not \
 119                      be exposed publicly",
 120         issue = "none"
 121     )]
 122     #[inline]
 123     #[doc(hidden)]
 124     pub fn __description(&self) -> &str {
 125         "could not convert slice to array"
 126     }
 127 }
 128
 129 #[stable(feature = "try_from_slice_error", since = "1.36.0")]
 130 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 131 impl const From<Infallible> for TryFromSliceError {
 132     fn from(x: Infallible) -> TryFromSliceError {
 133         match x {}
 134     }
 135 }
 136
 137 #[stable(feature = "rust1", since = "1.0.0")]
 138 impl<T, const N: usize> AsRef<[T]> for [T; N] {
 139     #[inline]
 140     fn as_ref(&self) -> &[T] {
 141         &self[..]
 142     }
 143 }
 144
 145 #[stable(feature = "rust1", since = "1.0.0")]
 146 impl<T, const N: usize> AsMut<[T]> for [T; N] {
 147     #[inline]
 148     fn as_mut(&mut self) -> &mut [T] {
 149         &mut self[..]
 150     }
 151 }
 152
 153 #[stable(feature = "array_borrow", since = "1.4.0")]
 154 impl<T, const N: usize> Borrow<[T]> for [T; N] {
 155     fn borrow(&self) -> &[T] {
 156         self
 157     }
 158 }
 159
 160 #[stable(feature = "array_borrow", since = "1.4.0")]
 161 impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
 162     fn borrow_mut(&mut self) -> &mut [T] {
 163         self
 164     }
 165 }
 166
 167 #[stable(feature = "try_from", since = "1.34.0")]
 168 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
 169 where
 170     T: Copy,
 171 {
 172     type Error = TryFromSliceError;
 173
 174     fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
 175         <&Self>::try_from(slice).map(|r| *r)
 176     }
 177 }
 178
 179 #[stable(feature = "try_from", since = "1.34.0")]
 180 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
 181     type Error = TryFromSliceError;
 182
 183     fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
 184         if slice.len() == N {
 185             let ptr = slice.as_ptr() as *const [T; N];
 186             // SAFETY: ok because we just checked that the length fits
 187             unsafe { Ok(&*ptr) }
 188         } else {
 189             Err(TryFromSliceError(()))
 190         }
 191     }
 192 }
 193
 194 #[stable(feature = "try_from", since = "1.34.0")]
 195 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
 196     type Error = TryFromSliceError;
 197
 198     fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
 199         if slice.len() == N {
 200             let ptr = slice.as_mut_ptr() as *mut [T; N];
 201             // SAFETY: ok because we just checked that the length fits
 202             unsafe { Ok(&mut *ptr) }
 203         } else {
 204             Err(TryFromSliceError(()))
 205         }
 206     }
 207 }
 208
 209 /// The hash of an array is the same as that of the corresponding slice,
 210 /// as required by the `Borrow` implementation.
 211 ///
 212 /// ```
 213 /// #![feature(build_hasher_simple_hash_one)]
 214 /// use std::hash::BuildHasher;
 215 ///
 216 /// let b = std::collections::hash_map::RandomState::new();
 217 /// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
 218 /// let s: &[u8] = &[0xa8, 0x3c, 0x09];
 219 /// assert_eq!(b.hash_one(a), b.hash_one(s));
 220 /// ```
 221 #[stable(feature = "rust1", since = "1.0.0")]
 222 impl<T: Hash, const N: usize> Hash for [T; N] {
 223     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 224         Hash::hash(&self[..], state)
 225     }
 226 }
 227
 228 #[stable(feature = "rust1", since = "1.0.0")]
 229 impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
 230     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 231         fmt::Debug::fmt(&&self[..], f)
 232     }
 233 }
 234
 235 // Note: the `#[rustc_skip_array_during_method_dispatch]` on `trait IntoIterator`
 236 // hides this implementation from explicit `.into_iter()` calls on editions < 2021,
 237 // so those calls will still resolve to the slice implementation, by reference.
 238 #[stable(feature = "array_into_iter_impl", since = "1.53.0")]
 239 impl<T, const N: usize> IntoIterator for [T; N] {
 240     type Item = T;
 241     type IntoIter = IntoIter<T, N>;
 242
 243     /// Creates a consuming iterator, that is, one that moves each value out of
 244     /// the array (from start to end). The array cannot be used after calling
 245     /// this unless `T` implements `Copy`, so the whole array is copied.
 246     ///
 247     /// Arrays have special behavior when calling `.into_iter()` prior to the
 248     /// 2021 edition -- see the [array] Editions section for more information.
 249     ///
 250     /// [array]: prim@array
 251     fn into_iter(self) -> Self::IntoIter {
 252         IntoIter::new(self)
 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 impl<T, I, const N: usize> Index<I> for [T; N]
 278 where
 279     [T]: Index<I>,
 280 {
 281     type Output = <[T] as Index<I>>::Output;
 282
 283     #[inline]
 284     fn index(&self, index: I) -> &Self::Output {
 285         Index::index(self as &[T], index)
 286     }
 287 }
 288
 289 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 290 impl<T, I, const N: usize> IndexMut<I> for [T; N]
 291 where
 292     [T]: IndexMut<I>,
 293 {
 294     #[inline]
 295     fn index_mut(&mut self, index: I) -> &mut Self::Output {
 296         IndexMut::index_mut(self as &mut [T], index)
 297     }
 298 }
 299
 300 #[stable(feature = "rust1", since = "1.0.0")]
 301 impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
 302     #[inline]
 303     fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
 304         PartialOrd::partial_cmp(&&self[..], &&other[..])
 305     }
 306     #[inline]
 307     fn lt(&self, other: &[T; N]) -> bool {
 308         PartialOrd::lt(&&self[..], &&other[..])
 309     }
 310     #[inline]
 311     fn le(&self, other: &[T; N]) -> bool {
 312         PartialOrd::le(&&self[..], &&other[..])
 313     }
 314     #[inline]
 315     fn ge(&self, other: &[T; N]) -> bool {
 316         PartialOrd::ge(&&self[..], &&other[..])
 317     }
 318     #[inline]
 319     fn gt(&self, other: &[T; N]) -> bool {
 320         PartialOrd::gt(&&self[..], &&other[..])
 321     }
 322 }
 323
 324 /// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
 325 #[stable(feature = "rust1", since = "1.0.0")]
 326 impl<T: Ord, const N: usize> Ord for [T; N] {
 327     #[inline]
 328     fn cmp(&self, other: &[T; N]) -> Ordering {
 329         Ord::cmp(&&self[..], &&other[..])
 330     }
 331 }
 332
 333 #[cfg(not(bootstrap))]
 334 #[stable(feature = "copy_clone_array_lib", since = "1.58.0")]
 335 impl<T: Copy, const N: usize> Copy for [T; N] {}
 336
 337 #[cfg(not(bootstrap))]
 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 #[cfg(not(bootstrap))]
 352 trait SpecArrayClone: Clone {
 353     fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
 354 }
 355
 356 #[cfg(not(bootstrap))]
 357 impl<T: Clone> SpecArrayClone for T {
 358     #[inline]
 359     default fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 360         // SAFETY: we know for certain that this iterator will yield exactly `N`
 361         // items.
 362         unsafe { collect_into_array_unchecked(&mut array.iter().cloned()) }
 363     }
 364 }
 365
 366 #[cfg(not(bootstrap))]
 367 impl<T: Copy> SpecArrayClone for T {
 368     #[inline]
 369     fn clone<const N: usize>(array: &[T; N]) -> [T; N] {
 370         *array
 371     }
 372 }
 373
 374 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 375 // require Default to be implemented, and having different impl blocks for
 376 // different numbers isn't supported yet.
 377
 378 macro_rules! array_impl_default {
 379     {$n:expr, $t:ident $($ts:ident)*} => {
 380         #[stable(since = "1.4.0", feature = "array_default")]
 381         impl<T> Default for [T; $n] where T: Default {
 382             fn default() -> [T; $n] {
 383                 [$t::default(), $($ts::default()),*]
 384             }
 385         }
 386         array_impl_default!{($n - 1), $($ts)*}
 387     };
 388     {$n:expr,} => {
 389         #[stable(since = "1.4.0", feature = "array_default")]
 390         #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 391         impl<T> const Default for [T; $n] {
 392             fn default() -> [T; $n] { [] }
 393         }
 394     };
 395 }
 396
 397 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}
 398
 399 #[lang = "array"]
 400 impl<T, const N: usize> [T; N] {
 401     /// Returns an array of the same size as `self`, with function `f` applied to each element
 402     /// in order.
 403     ///
 404     /// If you don't necessarily need a new fixed-size array, consider using
 405     /// [`Iterator::map`] instead.
 406     ///
 407     ///
 408     /// # Note on performance and stack usage
 409     ///
 410     /// Unfortunately, usages of this method are currently not always optimized
 411     /// as well as they could be. This mainly concerns large arrays, as mapping
 412     /// over small arrays seem to be optimized just fine. Also note that in
 413     /// debug mode (i.e. without any optimizations), this method can use a lot
 414     /// of stack space (a few times the size of the array or more).
 415     ///
 416     /// Therefore, in performance-critical code, try to avoid using this method
 417     /// on large arrays or check the emitted code. Also try to avoid chained
 418     /// maps (e.g. `arr.map(...).map(...)`).
 419     ///
 420     /// In many cases, you can instead use [`Iterator::map`] by calling `.iter()`
 421     /// or `.into_iter()` on your array. `[T; N]::map` is only necessary if you
 422     /// really need a new array of the same size as the result. Rust's lazy
 423     /// iterators tend to get optimized very well.
 424     ///
 425     ///
 426     /// # Examples
 427     ///
 428     /// ```
 429     /// let x = [1, 2, 3];
 430     /// let y = x.map(|v| v + 1);
 431     /// assert_eq!(y, [2, 3, 4]);
 432     ///
 433     /// let x = [1, 2, 3];
 434     /// let mut temp = 0;
 435     /// let y = x.map(|v| { temp += 1; v * temp });
 436     /// assert_eq!(y, [1, 4, 9]);
 437     ///
 438     /// let x = ["Ferris", "Bueller's", "Day", "Off"];
 439     /// let y = x.map(|v| v.len());
 440     /// assert_eq!(y, [6, 9, 3, 3]);
 441     /// ```
 442     #[stable(feature = "array_map", since = "1.55.0")]
 443     pub fn map<F, U>(self, f: F) -> [U; N]
 444     where
 445         F: FnMut(T) -> U,
 446     {
 447         // SAFETY: we know for certain that this iterator will yield exactly `N`
 448         // items.
 449         unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 450     }
 451
 452     /// 'Zips up' two arrays into a single array of pairs.
 453     ///
 454     /// `zip()` returns a new array where every element is a tuple where the
 455     /// first element comes from the first array, and the second element comes
 456     /// from the second array. In other words, it zips two arrays together,
 457     /// into a single one.
 458     ///
 459     /// # Examples
 460     ///
 461     /// ```
 462     /// #![feature(array_zip)]
 463     /// let x = [1, 2, 3];
 464     /// let y = [4, 5, 6];
 465     /// let z = x.zip(y);
 466     /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
 467     /// ```
 468     #[unstable(feature = "array_zip", issue = "80094")]
 469     pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
 470         let mut iter = IntoIterator::into_iter(self).zip(rhs);
 471
 472         // SAFETY: we know for certain that this iterator will yield exactly `N`
 473         // items.
 474         unsafe { collect_into_array_unchecked(&mut iter) }
 475     }
 476
 477     /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
 478     #[stable(feature = "array_as_slice", since = "1.57.0")]
 479     pub const fn as_slice(&self) -> &[T] {
 480         self
 481     }
 482
 483     /// Returns a mutable slice containing the entire array. Equivalent to
 484     /// `&mut s[..]`.
 485     #[stable(feature = "array_as_slice", since = "1.57.0")]
 486     pub fn as_mut_slice(&mut self) -> &mut [T] {
 487         self
 488     }
 489
 490     /// Borrows each element and returns an array of references with the same
 491     /// size as `self`.
 492     ///
 493     ///
 494     /// # Example
 495     ///
 496     /// ```
 497     /// #![feature(array_methods)]
 498     ///
 499     /// let floats = [3.1, 2.7, -1.0];
 500     /// let float_refs: [&f64; 3] = floats.each_ref();
 501     /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
 502     /// ```
 503     ///
 504     /// This method is particularly useful if combined with other methods, like
 505     /// [`map`](#method.map). This way, you can avoid moving the original
 506     /// array if its elements are not [`Copy`].
 507     ///
 508     /// ```
 509     /// #![feature(array_methods)]
 510     ///
 511     /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
 512     /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
 513     /// assert_eq!(is_ascii, [true, false, true]);
 514     ///
 515     /// // We can still access the original array: it has not been moved.
 516     /// assert_eq!(strings.len(), 3);
 517     /// ```
 518     #[unstable(feature = "array_methods", issue = "76118")]
 519     pub fn each_ref(&self) -> [&T; N] {
 520         // SAFETY: we know for certain that this iterator will yield exactly `N`
 521         // items.
 522         unsafe { collect_into_array_unchecked(&mut self.iter()) }
 523     }
 524
 525     /// Borrows each element mutably and returns an array of mutable references
 526     /// with the same size as `self`.
 527     ///
 528     ///
 529     /// # Example
 530     ///
 531     /// ```
 532     /// #![feature(array_methods)]
 533     ///
 534     /// let mut floats = [3.1, 2.7, -1.0];
 535     /// let float_refs: [&mut f64; 3] = floats.each_mut();
 536     /// *float_refs[0] = 0.0;
 537     /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
 538     /// assert_eq!(floats, [0.0, 2.7, -1.0]);
 539     /// ```
 540     #[unstable(feature = "array_methods", issue = "76118")]
 541     pub fn each_mut(&mut self) -> [&mut T; N] {
 542         // SAFETY: we know for certain that this iterator will yield exactly `N`
 543         // items.
 544         unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
 545     }
 546
 547     /// Divides one array reference into two at an index.
 548     ///
 549     /// The first will contain all indices from `[0, M)` (excluding
 550     /// the index `M` itself) and the second will contain all
 551     /// indices from `[M, N)` (excluding the index `N` itself).
 552     ///
 553     /// # Panics
 554     ///
 555     /// Panics if `M > N`.
 556     ///
 557     /// # Examples
 558     ///
 559     /// ```
 560     /// #![feature(split_array)]
 561     ///
 562     /// let v = [1, 2, 3, 4, 5, 6];
 563     ///
 564     /// {
 565     ///    let (left, right) = v.split_array_ref::<0>();
 566     ///    assert_eq!(left, &[]);
 567     ///    assert_eq!(right, &[1, 2, 3, 4, 5, 6]);
 568     /// }
 569     ///
 570     /// {
 571     ///     let (left, right) = v.split_array_ref::<2>();
 572     ///     assert_eq!(left, &[1, 2]);
 573     ///     assert_eq!(right, &[3, 4, 5, 6]);
 574     /// }
 575     ///
 576     /// {
 577     ///     let (left, right) = v.split_array_ref::<6>();
 578     ///     assert_eq!(left, &[1, 2, 3, 4, 5, 6]);
 579     ///     assert_eq!(right, &[]);
 580     /// }
 581     /// ```
 582     #[unstable(
 583         feature = "split_array",
 584         reason = "return type should have array as 2nd element",
 585         issue = "90091"
 586     )]
 587     #[inline]
 588     pub fn split_array_ref<const M: usize>(&self) -> (&[T; M], &[T]) {
 589         (&self[..]).split_array_ref::<M>()
 590     }
 591
 592     /// Divides one mutable array reference into two at an index.
 593     ///
 594     /// The first will contain all indices from `[0, M)` (excluding
 595     /// the index `M` itself) and the second will contain all
 596     /// indices from `[M, N)` (excluding the index `N` itself).
 597     ///
 598     /// # Panics
 599     ///
 600     /// Panics if `M > N`.
 601     ///
 602     /// # Examples
 603     ///
 604     /// ```
 605     /// #![feature(split_array)]
 606     ///
 607     /// let mut v = [1, 0, 3, 0, 5, 6];
 608     /// let (left, right) = v.split_array_mut::<2>();
 609     /// assert_eq!(left, &mut [1, 0][..]);
 610     /// assert_eq!(right, &mut [3, 0, 5, 6]);
 611     /// left[1] = 2;
 612     /// right[1] = 4;
 613     /// assert_eq!(v, [1, 2, 3, 4, 5, 6]);
 614     /// ```
 615     #[unstable(
 616         feature = "split_array",
 617         reason = "return type should have array as 2nd element",
 618         issue = "90091"
 619     )]
 620     #[inline]
 621     pub fn split_array_mut<const M: usize>(&mut self) -> (&mut [T; M], &mut [T]) {
 622         (&mut self[..]).split_array_mut::<M>()
 623     }
 624 }
 625
 626 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 627 /// yields fewer than `N` items, this function exhibits undefined behavior.
 628 ///
 629 /// See [`collect_into_array`] for more information.
 630 ///
 631 ///
 632 /// # Safety
 633 ///
 634 /// It is up to the caller to guarantee that `iter` yields at least `N` items.
 635 /// Violating this condition causes undefined behavior.
 636 unsafe fn collect_into_array_rslt_unchecked<E, I, T, const N: usize>(
 637     iter: &mut I,
 638 ) -> Result<[T; N], E>
 639 where
 640     // Note: `TrustedLen` here is somewhat of an experiment. This is just an
 641     // internal function, so feel free to remove if this bound turns out to be a
 642     // bad idea. In that case, remember to also remove the lower bound
 643     // `debug_assert!` below!
 644     I: Iterator<Item = Result<T, E>> + TrustedLen,
 645 {
 646     debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
 647     debug_assert!(N <= iter.size_hint().0);
 648
 649     // SAFETY: covered by the function contract.
 650     unsafe { collect_into_array(iter).unwrap_unchecked() }
 651 }
 652
 653 // Infallible version of `collect_into_array_rslt_unchecked`.
 654 unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
 655 where
 656     I: Iterator + TrustedLen,
 657 {
 658     let mut map = iter.map(Ok::<_, Infallible>);
 659
 660     // SAFETY: The same safety considerations w.r.t. the iterator length
 661     // apply for `collect_into_array_rslt_unchecked` as for
 662     // `collect_into_array_unchecked`
 663     match unsafe { collect_into_array_rslt_unchecked(&mut map) } {
 664         Ok(array) => array,
 665     }
 666 }
 667
 668 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 669 /// yields fewer than `N` items, `None` is returned and all already yielded
 670 /// items are dropped.
 671 ///
 672 /// Since the iterator is passed as a mutable reference and this function calls
 673 /// `next` at most `N` times, the iterator can still be used afterwards to
 674 /// retrieve the remaining items.
 675 ///
 676 /// If `iter.next()` panicks, all items already yielded by the iterator are
 677 /// dropped.
 678 fn collect_into_array<E, I, T, const N: usize>(iter: &mut I) -> Option<Result<[T; N], E>>
 679 where
 680     I: Iterator<Item = Result<T, E>>,
 681 {
 682     if N == 0 {
 683         // SAFETY: An empty array is always inhabited and has no validity invariants.
 684         return unsafe { Some(Ok(mem::zeroed())) };
 685     }
 686
 687     struct Guard<'a, T, const N: usize> {
 688         array_mut: &'a mut [MaybeUninit<T>; N],
 689         initialized: usize,
 690     }
 691
 692     impl<T, const N: usize> Drop for Guard<'_, T, N> {
 693         fn drop(&mut self) {
 694             debug_assert!(self.initialized <= N);
 695
 696             // SAFETY: this slice will contain only initialized objects.
 697             unsafe {
 698                 crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
 699                     &mut self.array_mut.get_unchecked_mut(..self.initialized),
 700                 ));
 701             }
 702         }
 703     }
 704
 705     let mut array = MaybeUninit::uninit_array::<N>();
 706     let mut guard = Guard { array_mut: &mut array, initialized: 0 };
 707
 708     while let Some(item_rslt) = iter.next() {
 709         let item = match item_rslt {
 710             Err(err) => {
 711                 return Some(Err(err));
 712             }
 713             Ok(elem) => elem,
 714         };
 715
 716         // SAFETY: `guard.initialized` starts at 0, is increased by one in the
 717         // loop and the loop is aborted once it reaches N (which is
 718         // `array.len()`).
 719         unsafe {
 720             guard.array_mut.get_unchecked_mut(guard.initialized).write(item);
 721         }
 722         guard.initialized += 1;
 723
 724         // Check if the whole array was initialized.
 725         if guard.initialized == N {
 726             mem::forget(guard);
 727
 728             // SAFETY: the condition above asserts that all elements are
 729             // initialized.
 730             let out = unsafe { MaybeUninit::array_assume_init(array) };
 731             return Some(Ok(out));
 732         }
 733     }
 734
 735     // This is only reached if the iterator is exhausted before
 736     // `guard.initialized` reaches `N`. Also note that `guard` is dropped here,
 737     // dropping all already initialized elements.
 738     None
 739 }