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 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
  24 #[stable(feature = "array_from_ref", since = "1.53.0")]
  25 pub fn from_ref<T>(s: &T) -> &[T; 1] {
  26     // SAFETY: Converting `&T` to `&[T; 1]` is sound.
  27     unsafe { &*(s as *const T).cast::<[T; 1]>() }
  28 }
  29
  30 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
  31 #[stable(feature = "array_from_ref", since = "1.53.0")]
  32 pub fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
  33     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
  34     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
  35 }
  36
  37 /// The error type returned when a conversion from a slice to an array fails.
  38 #[stable(feature = "try_from", since = "1.34.0")]
  39 #[derive(Debug, Copy, Clone)]
  40 pub struct TryFromSliceError(());
  41
  42 #[stable(feature = "core_array", since = "1.36.0")]
  43 impl fmt::Display for TryFromSliceError {
  44     #[inline]
  45     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  46         fmt::Display::fmt(self.__description(), f)
  47     }
  48 }
  49
  50 impl TryFromSliceError {
  51     #[unstable(
  52         feature = "array_error_internals",
  53         reason = "available through Error trait and this method should not \
  54                      be exposed publicly",
  55         issue = "none"
  56     )]
  57     #[inline]
  58     #[doc(hidden)]
  59     pub fn __description(&self) -> &str {
  60         "could not convert slice to array"
  61     }
  62 }
  63
  64 #[stable(feature = "try_from_slice_error", since = "1.36.0")]
  65 impl From<Infallible> for TryFromSliceError {
  66     fn from(x: Infallible) -> TryFromSliceError {
  67         match x {}
  68     }
  69 }
  70
  71 #[stable(feature = "rust1", since = "1.0.0")]
  72 impl<T, const N: usize> AsRef<[T]> for [T; N] {
  73     #[inline]
  74     fn as_ref(&self) -> &[T] {
  75         &self[..]
  76     }
  77 }
  78
  79 #[stable(feature = "rust1", since = "1.0.0")]
  80 impl<T, const N: usize> AsMut<[T]> for [T; N] {
  81     #[inline]
  82     fn as_mut(&mut self) -> &mut [T] {
  83         &mut self[..]
  84     }
  85 }
  86
  87 #[stable(feature = "array_borrow", since = "1.4.0")]
  88 impl<T, const N: usize> Borrow<[T]> for [T; N] {
  89     fn borrow(&self) -> &[T] {
  90         self
  91     }
  92 }
  93
  94 #[stable(feature = "array_borrow", since = "1.4.0")]
  95 impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
  96     fn borrow_mut(&mut self) -> &mut [T] {
  97         self
  98     }
  99 }
 100
 101 #[stable(feature = "try_from", since = "1.34.0")]
 102 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
 103 where
 104     T: Copy,
 105 {
 106     type Error = TryFromSliceError;
 107
 108     fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
 109         <&Self>::try_from(slice).map(|r| *r)
 110     }
 111 }
 112
 113 #[stable(feature = "try_from", since = "1.34.0")]
 114 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
 115     type Error = TryFromSliceError;
 116
 117     fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
 118         if slice.len() == N {
 119             let ptr = slice.as_ptr() as *const [T; N];
 120             // SAFETY: ok because we just checked that the length fits
 121             unsafe { Ok(&*ptr) }
 122         } else {
 123             Err(TryFromSliceError(()))
 124         }
 125     }
 126 }
 127
 128 #[stable(feature = "try_from", since = "1.34.0")]
 129 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
 130     type Error = TryFromSliceError;
 131
 132     fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
 133         if slice.len() == N {
 134             let ptr = slice.as_mut_ptr() as *mut [T; N];
 135             // SAFETY: ok because we just checked that the length fits
 136             unsafe { Ok(&mut *ptr) }
 137         } else {
 138             Err(TryFromSliceError(()))
 139         }
 140     }
 141 }
 142
 143 /// The hash of an array is the same as that of the corresponding slice,
 144 /// as required by the `Borrow` implementation.
 145 ///
 146 /// ```
 147 /// #![feature(build_hasher_simple_hash_one)]
 148 /// use std::hash::BuildHasher;
 149 ///
 150 /// let b = std::collections::hash_map::RandomState::new();
 151 /// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
 152 /// let s: &[u8] = &[0xa8, 0x3c, 0x09];
 153 /// assert_eq!(b.hash_one(a), b.hash_one(s));
 154 /// ```
 155 #[stable(feature = "rust1", since = "1.0.0")]
 156 impl<T: Hash, const N: usize> Hash for [T; N] {
 157     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 158         Hash::hash(&self[..], state)
 159     }
 160 }
 161
 162 #[stable(feature = "rust1", since = "1.0.0")]
 163 impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N] {
 164     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 165         fmt::Debug::fmt(&&self[..], f)
 166     }
 167 }
 168
 169 // Note: the `#[rustc_skip_array_during_method_dispatch]` on `trait IntoIterator`
 170 // hides this implementation from explicit `.into_iter()` calls on editions < 2021,
 171 // so those calls will still resolve to the slice implementation, by reference.
 172 #[stable(feature = "array_into_iter_impl", since = "1.53.0")]
 173 impl<T, const N: usize> IntoIterator for [T; N] {
 174     type Item = T;
 175     type IntoIter = IntoIter<T, N>;
 176
 177     /// Creates a consuming iterator, that is, one that moves each value out of
 178     /// the array (from start to end). The array cannot be used after calling
 179     /// this unless `T` implements `Copy`, so the whole array is copied.
 180     ///
 181     /// Arrays have special behavior when calling `.into_iter()` prior to the
 182     /// 2021 edition -- see the [array] Editions section for more information.
 183     ///
 184     /// [array]: prim@array
 185     fn into_iter(self) -> Self::IntoIter {
 186         IntoIter::new(self)
 187     }
 188 }
 189
 190 #[stable(feature = "rust1", since = "1.0.0")]
 191 impl<'a, T, const N: usize> IntoIterator for &'a [T; N] {
 192     type Item = &'a T;
 193     type IntoIter = Iter<'a, T>;
 194
 195     fn into_iter(self) -> Iter<'a, T> {
 196         self.iter()
 197     }
 198 }
 199
 200 #[stable(feature = "rust1", since = "1.0.0")]
 201 impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
 202     type Item = &'a mut T;
 203     type IntoIter = IterMut<'a, T>;
 204
 205     fn into_iter(self) -> IterMut<'a, T> {
 206         self.iter_mut()
 207     }
 208 }
 209
 210 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 211 impl<T, I, const N: usize> Index<I> for [T; N]
 212 where
 213     [T]: Index<I>,
 214 {
 215     type Output = <[T] as Index<I>>::Output;
 216
 217     #[inline]
 218     fn index(&self, index: I) -> &Self::Output {
 219         Index::index(self as &[T], index)
 220     }
 221 }
 222
 223 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
 224 impl<T, I, const N: usize> IndexMut<I> for [T; N]
 225 where
 226     [T]: IndexMut<I>,
 227 {
 228     #[inline]
 229     fn index_mut(&mut self, index: I) -> &mut Self::Output {
 230         IndexMut::index_mut(self as &mut [T], index)
 231     }
 232 }
 233
 234 #[stable(feature = "rust1", since = "1.0.0")]
 235 impl<T: PartialOrd, const N: usize> PartialOrd for [T; N] {
 236     #[inline]
 237     fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
 238         PartialOrd::partial_cmp(&&self[..], &&other[..])
 239     }
 240     #[inline]
 241     fn lt(&self, other: &[T; N]) -> bool {
 242         PartialOrd::lt(&&self[..], &&other[..])
 243     }
 244     #[inline]
 245     fn le(&self, other: &[T; N]) -> bool {
 246         PartialOrd::le(&&self[..], &&other[..])
 247     }
 248     #[inline]
 249     fn ge(&self, other: &[T; N]) -> bool {
 250         PartialOrd::ge(&&self[..], &&other[..])
 251     }
 252     #[inline]
 253     fn gt(&self, other: &[T; N]) -> bool {
 254         PartialOrd::gt(&&self[..], &&other[..])
 255     }
 256 }
 257
 258 /// Implements comparison of arrays [lexicographically](Ord#lexicographical-comparison).
 259 #[stable(feature = "rust1", since = "1.0.0")]
 260 impl<T: Ord, const N: usize> Ord for [T; N] {
 261     #[inline]
 262     fn cmp(&self, other: &[T; N]) -> Ordering {
 263         Ord::cmp(&&self[..], &&other[..])
 264     }
 265 }
 266
 267 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 268 // require Default to be implemented, and having different impl blocks for
 269 // different numbers isn't supported yet.
 270
 271 macro_rules! array_impl_default {
 272     {$n:expr, $t:ident $($ts:ident)*} => {
 273         #[stable(since = "1.4.0", feature = "array_default")]
 274         impl<T> Default for [T; $n] where T: Default {
 275             fn default() -> [T; $n] {
 276                 [$t::default(), $($ts::default()),*]
 277             }
 278         }
 279         array_impl_default!{($n - 1), $($ts)*}
 280     };
 281     {$n:expr,} => {
 282         #[stable(since = "1.4.0", feature = "array_default")]
 283         impl<T> Default for [T; $n] {
 284             fn default() -> [T; $n] { [] }
 285         }
 286     };
 287 }
 288
 289 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}
 290
 291 #[lang = "array"]
 292 impl<T, const N: usize> [T; N] {
 293     /// Returns an array of the same size as `self`, with function `f` applied to each element
 294     /// in order.
 295     ///
 296     /// # Examples
 297     ///
 298     /// ```
 299     /// let x = [1, 2, 3];
 300     /// let y = x.map(|v| v + 1);
 301     /// assert_eq!(y, [2, 3, 4]);
 302     ///
 303     /// let x = [1, 2, 3];
 304     /// let mut temp = 0;
 305     /// let y = x.map(|v| { temp += 1; v * temp });
 306     /// assert_eq!(y, [1, 4, 9]);
 307     ///
 308     /// let x = ["Ferris", "Bueller's", "Day", "Off"];
 309     /// let y = x.map(|v| v.len());
 310     /// assert_eq!(y, [6, 9, 3, 3]);
 311     /// ```
 312     #[stable(feature = "array_map", since = "1.55.0")]
 313     pub fn map<F, U>(self, f: F) -> [U; N]
 314     where
 315         F: FnMut(T) -> U,
 316     {
 317         // SAFETY: we know for certain that this iterator will yield exactly `N`
 318         // items.
 319         unsafe { collect_into_array_unchecked(&mut IntoIterator::into_iter(self).map(f)) }
 320     }
 321
 322     /// 'Zips up' two arrays into a single array of pairs.
 323     ///
 324     /// `zip()` returns a new array where every element is a tuple where the
 325     /// first element comes from the first array, and the second element comes
 326     /// from the second array. In other words, it zips two arrays together,
 327     /// into a single one.
 328     ///
 329     /// # Examples
 330     ///
 331     /// ```
 332     /// #![feature(array_zip)]
 333     /// let x = [1, 2, 3];
 334     /// let y = [4, 5, 6];
 335     /// let z = x.zip(y);
 336     /// assert_eq!(z, [(1, 4), (2, 5), (3, 6)]);
 337     /// ```
 338     #[unstable(feature = "array_zip", issue = "80094")]
 339     pub fn zip<U>(self, rhs: [U; N]) -> [(T, U); N] {
 340         let mut iter = IntoIterator::into_iter(self).zip(rhs);
 341
 342         // SAFETY: we know for certain that this iterator will yield exactly `N`
 343         // items.
 344         unsafe { collect_into_array_unchecked(&mut iter) }
 345     }
 346
 347     /// Returns a slice containing the entire array. Equivalent to `&s[..]`.
 348     #[unstable(feature = "array_methods", issue = "76118")]
 349     pub fn as_slice(&self) -> &[T] {
 350         self
 351     }
 352
 353     /// Returns a mutable slice containing the entire array. Equivalent to
 354     /// `&mut s[..]`.
 355     #[unstable(feature = "array_methods", issue = "76118")]
 356     pub fn as_mut_slice(&mut self) -> &mut [T] {
 357         self
 358     }
 359
 360     /// Borrows each element and returns an array of references with the same
 361     /// size as `self`.
 362     ///
 363     ///
 364     /// # Example
 365     ///
 366     /// ```
 367     /// #![feature(array_methods)]
 368     ///
 369     /// let floats = [3.1, 2.7, -1.0];
 370     /// let float_refs: [&f64; 3] = floats.each_ref();
 371     /// assert_eq!(float_refs, [&3.1, &2.7, &-1.0]);
 372     /// ```
 373     ///
 374     /// This method is particularly useful if combined with other methods, like
 375     /// [`map`](#method.map). This way, you can avoid moving the original
 376     /// array if its elements are not `Copy`.
 377     ///
 378     /// ```
 379     /// #![feature(array_methods)]
 380     ///
 381     /// let strings = ["Ferris".to_string(), "♥".to_string(), "Rust".to_string()];
 382     /// let is_ascii = strings.each_ref().map(|s| s.is_ascii());
 383     /// assert_eq!(is_ascii, [true, false, true]);
 384     ///
 385     /// // We can still access the original array: it has not been moved.
 386     /// assert_eq!(strings.len(), 3);
 387     /// ```
 388     #[unstable(feature = "array_methods", issue = "76118")]
 389     pub fn each_ref(&self) -> [&T; N] {
 390         // SAFETY: we know for certain that this iterator will yield exactly `N`
 391         // items.
 392         unsafe { collect_into_array_unchecked(&mut self.iter()) }
 393     }
 394
 395     /// Borrows each element mutably and returns an array of mutable references
 396     /// with the same size as `self`.
 397     ///
 398     ///
 399     /// # Example
 400     ///
 401     /// ```
 402     /// #![feature(array_methods)]
 403     ///
 404     /// let mut floats = [3.1, 2.7, -1.0];
 405     /// let float_refs: [&mut f64; 3] = floats.each_mut();
 406     /// *float_refs[0] = 0.0;
 407     /// assert_eq!(float_refs, [&mut 0.0, &mut 2.7, &mut -1.0]);
 408     /// assert_eq!(floats, [0.0, 2.7, -1.0]);
 409     /// ```
 410     #[unstable(feature = "array_methods", issue = "76118")]
 411     pub fn each_mut(&mut self) -> [&mut T; N] {
 412         // SAFETY: we know for certain that this iterator will yield exactly `N`
 413         // items.
 414         unsafe { collect_into_array_unchecked(&mut self.iter_mut()) }
 415     }
 416 }
 417
 418 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 419 /// yields fewer than `N` items, this function exhibits undefined behavior.
 420 ///
 421 /// See [`collect_into_array`] for more information.
 422 ///
 423 ///
 424 /// # Safety
 425 ///
 426 /// It is up to the caller to guarantee that `iter` yields at least `N` items.
 427 /// Violating this condition causes undefined behavior.
 428 unsafe fn collect_into_array_unchecked<I, const N: usize>(iter: &mut I) -> [I::Item; N]
 429 where
 430     // Note: `TrustedLen` here is somewhat of an experiment. This is just an
 431     // internal function, so feel free to remove if this bound turns out to be a
 432     // bad idea. In that case, remember to also remove the lower bound
 433     // `debug_assert!` below!
 434     I: Iterator + TrustedLen,
 435 {
 436     debug_assert!(N <= iter.size_hint().1.unwrap_or(usize::MAX));
 437     debug_assert!(N <= iter.size_hint().0);
 438
 439     match collect_into_array(iter) {
 440         Some(array) => array,
 441         // SAFETY: covered by the function contract.
 442         None => unsafe { crate::hint::unreachable_unchecked() },
 443     }
 444 }
 445
 446 /// Pulls `N` items from `iter` and returns them as an array. If the iterator
 447 /// yields fewer than `N` items, `None` is returned and all already yielded
 448 /// items are dropped.
 449 ///
 450 /// Since the iterator is passed as a mutable reference and this function calls
 451 /// `next` at most `N` times, the iterator can still be used afterwards to
 452 /// retrieve the remaining items.
 453 ///
 454 /// If `iter.next()` panicks, all items already yielded by the iterator are
 455 /// dropped.
 456 fn collect_into_array<I, const N: usize>(iter: &mut I) -> Option<[I::Item; N]>
 457 where
 458     I: Iterator,
 459 {
 460     if N == 0 {
 461         // SAFETY: An empty array is always inhabited and has no validity invariants.
 462         return unsafe { Some(mem::zeroed()) };
 463     }
 464
 465     struct Guard<T, const N: usize> {
 466         ptr: *mut T,
 467         initialized: usize,
 468     }
 469
 470     impl<T, const N: usize> Drop for Guard<T, N> {
 471         fn drop(&mut self) {
 472             debug_assert!(self.initialized <= N);
 473
 474             let initialized_part = crate::ptr::slice_from_raw_parts_mut(self.ptr, self.initialized);
 475
 476             // SAFETY: this raw slice will contain only initialized objects.
 477             unsafe {
 478                 crate::ptr::drop_in_place(initialized_part);
 479             }
 480         }
 481     }
 482
 483     let mut array = MaybeUninit::uninit_array::<N>();
 484     let mut guard: Guard<_, N> =
 485         Guard { ptr: MaybeUninit::slice_as_mut_ptr(&mut array), initialized: 0 };
 486
 487     while let Some(item) = iter.next() {
 488         // SAFETY: `guard.initialized` starts at 0, is increased by one in the
 489         // loop and the loop is aborted once it reaches N (which is
 490         // `array.len()`).
 491         unsafe {
 492             array.get_unchecked_mut(guard.initialized).write(item);
 493         }
 494         guard.initialized += 1;
 495
 496         // Check if the whole array was initialized.
 497         if guard.initialized == N {
 498             mem::forget(guard);
 499
 500             // SAFETY: the condition above asserts that all elements are
 501             // initialized.
 502             let out = unsafe { MaybeUninit::array_assume_init(array) };
 503             return Some(out);
 504         }
 505     }
 506
 507     // This is only reached if the iterator is exhausted before
 508     // `guard.initialized` reaches `N`. Also note that `guard` is dropped here,
 509     // dropping all already initialized elements.
 510     None
 511 }