1 files changed, 55 insertions, 4 deletions

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 9effb3790..eae0e1c76 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -7,7 +7,6 @@
 use crate::borrow::{Borrow, BorrowMut};
 use crate::cmp::Ordering;
 use crate::convert::{Infallible, TryFrom};
-#[cfg(not(bootstrap))]
 use crate::error::Error;
 use crate::fmt;
 use crate::hash::{self, Hash};
@@ -33,6 +32,10 @@ pub use iter::IntoIter;
 /// # Example
 ///
 /// ```rust
+/// // type inference is helping us here, the way `from_fn` knows how many
+/// // elements to produce is the length of array down there: only arrays of
+/// // equal lengths can be compared, so the const generic parameter `N` is
+/// // inferred to be 5, thus creating array of 5 elements.
 /// let array = core::array::from_fn(|i| i);
 /// assert_eq!(array, [0, 1, 2, 3, 4]);
 /// ```
@@ -121,7 +124,6 @@ impl fmt::Display for TryFromSliceError {
     }
 }
 
-#[cfg(not(bootstrap))]
 #[stable(feature = "try_from", since = "1.34.0")]
 impl Error for TryFromSliceError {
     #[allow(deprecated)]
@@ -184,6 +186,18 @@ impl<T, const N: usize> const BorrowMut<[T]> for [T; N] {
     }
 }
 
+/// Tries to create an array `[T; N]` by copying from a slice `&[T]`. Succeeds if
+/// `slice.len() == N`.
+///
+/// ```
+/// let bytes: [u8; 3] = [1, 0, 2];
+///
+/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&bytes[0..2]).unwrap();
+/// assert_eq!(1, u16::from_le_bytes(bytes_head));
+///
+/// let bytes_tail: [u8; 2] = bytes[1..3].try_into().unwrap();
+/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
+/// ```
 #[stable(feature = "try_from", since = "1.34.0")]
 impl<T, const N: usize> TryFrom<&[T]> for [T; N]
 where
@@ -196,6 +210,18 @@ where
     }
 }
 
+/// Tries to create an array `[T; N]` by copying from a mutable slice `&mut [T]`.
+/// Succeeds if `slice.len() == N`.
+///
+/// ```
+/// let mut bytes: [u8; 3] = [1, 0, 2];
+///
+/// let bytes_head: [u8; 2] = <[u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
+/// assert_eq!(1, u16::from_le_bytes(bytes_head));
+///
+/// let bytes_tail: [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
+/// assert_eq!(512, u16::from_le_bytes(bytes_tail));
+/// ```
 #[stable(feature = "try_from_mut_slice_to_array", since = "1.59.0")]
 impl<T, const N: usize> TryFrom<&mut [T]> for [T; N]
 where
@@ -208,6 +234,18 @@ where
     }
 }
 
+/// Tries to create an array ref `&[T; N]` from a slice ref `&[T]`. Succeeds if
+/// `slice.len() == N`.
+///
+/// ```
+/// let bytes: [u8; 3] = [1, 0, 2];
+///
+/// let bytes_head: &[u8; 2] = <&[u8; 2]>::try_from(&bytes[0..2]).unwrap();
+/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
+///
+/// let bytes_tail: &[u8; 2] = bytes[1..3].try_into().unwrap();
+/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
+/// ```
 #[stable(feature = "try_from", since = "1.34.0")]
 impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
     type Error = TryFromSliceError;
@@ -223,6 +261,18 @@ impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
     }
 }
 
+/// Tries to create a mutable array ref `&mut [T; N]` from a mutable slice ref
+/// `&mut [T]`. Succeeds if `slice.len() == N`.
+///
+/// ```
+/// let mut bytes: [u8; 3] = [1, 0, 2];
+///
+/// let bytes_head: &mut [u8; 2] = <&mut [u8; 2]>::try_from(&mut bytes[0..2]).unwrap();
+/// assert_eq!(1, u16::from_le_bytes(*bytes_head));
+///
+/// let bytes_tail: &mut [u8; 2] = (&mut bytes[1..3]).try_into().unwrap();
+/// assert_eq!(512, u16::from_le_bytes(*bytes_tail));
+/// ```
 #[stable(feature = "try_from", since = "1.34.0")]
 impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
     type Error = TryFromSliceError;
@@ -386,7 +436,8 @@ impl<T: Copy> SpecArrayClone for T {
 macro_rules! array_impl_default {
     {$n:expr, $t:ident $($ts:ident)*} => {
         #[stable(since = "1.4.0", feature = "array_default")]
-        impl<T> Default for [T; $n] where T: Default {
+        #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+        impl<T> const Default for [T; $n] where T: ~const Default {
             fn default() -> [T; $n] {
                 [$t::default(), $($ts::default()),*]
             }
@@ -865,7 +916,7 @@ where
 
     mem::forget(guard);
     // SAFETY: All elements of the array were populated in the loop above.
-    let output = unsafe { MaybeUninit::array_assume_init(array) };
+    let output = unsafe { array.transpose().assume_init() };
     Ok(Try::from_output(output))
 }