diff --git a/library/core/src/hash/mod.rs b/library/core/src/hash/mod.rs
index 540160bc4c2a4..3ff84cc9672eb 100644
--- a/library/core/src/hash/mod.rs
+++ b/library/core/src/hash/mod.rs
@@ -164,6 +164,19 @@ mod sip;
 /// `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