Rollup merge of #86140 - scottmcm:array-hash-facepalm, r=kennytm

Mention the `Borrow` guarantee on the `Hash` implementations for Arrays and `Vec`

To remind people like me who forget about it and send PRs to make them different, and to (probably) get a test failure if the code is changed to no longer uphold it.

1 files changed, 17 insertions, 0 deletions

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 37af3557fdd..f44e22b3dbd 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -139,6 +139,23 @@ impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
     }
 }
 
+/// The hash of an array is the same as that of the corresponding slice,
+/// as required by the `Borrow` implementation.
+///
+/// ```
+/// use std::hash::{BuildHasher, Hash, Hasher};
+///
+/// fn hash_of(x: impl Hash, b: &impl BuildHasher) -> u64 {
+///     let mut h = b.build_hasher();
+///     x.hash(&mut h);
+///     h.finish()
+/// }
+///
+/// let b = std::collections::hash_map::RandomState::new();
+/// let a: [u8; 3] = [0xa8, 0x3c, 0x09];
+/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
+/// assert_eq!(hash_of(a, &b), hash_of(s, &b));
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Hash, const N: usize> Hash for [T; N] {
     fn hash<H: hash::Hasher>(&self, state: &mut H) {