+/// Trait for marking a type as having a sort order that is
+/// stable across compilation session boundaries. More formally:
+///
+/// ```txt
+/// Ord::cmp(a1, b1) == Ord:cmp(a2, b2)
+/// where a2 = decode(encode(a1, context1), context2)
+/// b2 = decode(encode(b1, context1), context2)
+/// ```
+///
+/// i.e. the result of `Ord::cmp` is not influenced by encoding
+/// the values in one session and then decoding them in another
+/// session.
+///
+/// This is trivially true for types where encoding and decoding
+/// don't change the bytes of the values that are used during
+/// comparison and comparison only depends on these bytes (as
+/// opposed to some non-local state). Examples are u32, String,
+/// Path, etc.
+///
+/// But it is not true for:
+/// - `*const T` and `*mut T` because the values of these pointers
+/// will change between sessions.
+/// - `DefIndex`, `CrateNum`, `LocalDefId`, because their concrete
+/// values depend on state that might be different between
+/// compilation sessions.
+pub unsafe trait StableOrd: Ord {}
+
+/// Implement HashStable by just calling `Hash::hash()`. Also implement `StableOrd` for the type since
+/// that has the same requirements.