change PhantomData type for BuildHasherDefault

[rust.git] / library / core / src / hash / mod.rs

diff --git a/library/core/src/hash/mod.rs b/library/core/src/hash/mod.rs
index da3f20d18e5108f9b5cc1a6ce10e014df78a8f2b..53de8b42c059f219b586be1a9a6daf0a906db6c5 100644 (file)
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -153,9 +153,34 @@
 /// Thankfully, you won't need to worry about upholding this property when
 /// deriving both [`Eq`] and `Hash` with `#[derive(PartialEq, Eq, Hash)]`.
 ///
+/// ## Prefix collisions
+///
+/// Implementations of `hash` should ensure that the data they
+/// pass to the `Hasher` are prefix-free. That is,
+/// unequal values should cause two different sequences of values to be written,
+/// and neither of the two sequences should be a prefix of the other.
+///
+/// For example, the standard implementation of [`Hash` for `&str`][impl] passes an extra
+/// `0xFF` byte to the `Hasher` so that the values `("ab", "c")` and `("a",
+/// "bc")` hash differently.
+///
+/// ## Portability
+///
+/// Due to differences in endianness and type sizes, data fed by `Hash` to a `Hasher`
+/// should not be considered portable across platforms. Additionally the data passed by most
+/// standard library types should not be considered stable between compiler versions.
+///
+/// This means tests shouldn't probe hard-coded hash values or data fed to a `Hasher` and
+/// instead should check consistency with `Eq`.
+///
+/// Serialization formats intended to be portable between platforms or compiler versions should
+/// either avoid encoding hashes or only rely on `Hash` and `Hasher` implementations that
+/// provide additional guarantees.
+///
 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
 /// [`HashSet`]: ../../std/collections/struct.HashSet.html
 /// [`hash`]: Hash::hash
+/// [impl]: ../../std/primitive.str.html#impl-Hash
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Hash"]
 pub trait Hash {
@@ -577,7 +602,7 @@ fn hash_one<T: Hash>(&self, x: T) -> u64
 /// [`HashSet`]: ../../std/collections/struct.HashSet.html
 /// [zero-sized]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts
 #[stable(since = "1.7.0", feature = "build_hasher")]
-pub struct BuildHasherDefault<H>(marker::PhantomData<H>);
+pub struct BuildHasherDefault<H>(marker::PhantomData<fn() -> H>);
 
 #[stable(since = "1.9.0", feature = "core_impl_debug")]
 impl<H> fmt::Debug for BuildHasherDefault<H> {