References refer to allocated objects

library/core/src/primitive_docs.rs

@@ -1389,6 +1389,25 @@ mod prim_usize {}
 /// work on references as well as they do on owned values! The implementations described here are
 /// meant for generic contexts, where the final type `T` is a type parameter or otherwise not
 /// locally known.
+///
+/// # Safety
+///
+/// For all types, `T: ?Sized`, and for all `t: &T` or `t: &mut T`, unsafe code may assume that
+/// the following properties hold. Rust programmers must assume that, unless explicitly stated
+/// otherwise, any Rust code they did not author themselves may rely on these properties, and that
+/// violating them may cause that code to exhibit undefined behavior.
+///
+/// * `t` is aligned to `align_of_val(t)`
+/// * `t` is dereferenceable for `size_of_val(t)` many bytes
+///
+/// If `t` points at address `a`, being "dereferenceable" for N bytes means that the memory range
+/// `[a, a + N)` is all contained within a single [allocated object].
+///
+/// Note that the precise validity invariants for reference types are a work in progress. In the
+/// future, new guarantees may be added. However, the guarantees documented in this section will
+/// never be removed.
+///
+/// [allocated object]: ptr#allocated-object
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_ref {}