/// Something that implements `HashStable<CTX>` can be hashed in a way that is
/// stable across multiple compilation sessions.
+///
+/// Note that `HashStable` imposes rather more strict requirements than usual
+/// hash functions:
+///
+/// - Stable hashes are sometimes used as identifiers. Therefore they must
+/// conform to the corresponding `PartialEq` implementations:
+///
+/// - `x == y` implies `hash_stable(x) == hash_stable(y)`, and
+/// - `x != y` implies `hash_stable(x) != hash_stable(y)`.
+///
+/// That second condition is usually not required for hash functions
+/// (e.g. `Hash`). In practice this means that `hash_stable` must feed any
+/// information into the hasher that a `PartialEq` comparision takes into
+/// account. See [#49300](https://github.com/rust-lang/rust/issues/49300)
+/// for an example where violating this invariant has caused trouble in the
+/// past.
+///
+/// - `hash_stable()` must be independent of the current
+/// compilation session. E.g. they must not hash memory addresses or other
+/// things that are "randomly" assigned per compilation session.
+///
+/// - `hash_stable()` must be independent of the host architecture. The
+/// `StableHasher` takes care of endianness and `isize`/`usize` platform
+/// differences.
pub trait HashStable<CTX> {
fn hash_stable<W: StableHasherResult>(&self,
hcx: &mut CTX,