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 540160bc4c2a4daadc4d040c58dc529b3d758fe4..53de8b42c059f219b586be1a9a6daf0a906db6c5 100644 (file)
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -164,6 +164,19 @@
 /// `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
@@ -589,7 +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> {