std: Stabilize the `hash` module

This commit is an implementation of [RFC 823][rfc] which is another pass over
the `std::hash` module for stabilization. The contents of the module were not
entirely marked stable, but some portions which remained quite similar to the
previous incarnation are now marked `#[stable]`. Specifically:

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0823-hash-simplification.md

* `std::hash` is now stable (the name)
* `Hash` is now stable
* `Hash::hash` is now stable
* `Hasher` is now stable
* `SipHasher` is now stable
* `SipHasher::new` and `new_with_keys` are now stable
* `Hasher for SipHasher` is now stable
* Many `Hash` implementations are now stable

All other portions of the `hash` module remain `#[unstable]` as they are less
commonly used and were recently redesigned.

This commit is a breaking change due to the modifications to the `std::hash` API
and more details can be found on the [RFC][rfc].

Closes #22467
[breaking-change]

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index 01f3839b039..adb5383a8fd 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -111,11 +111,18 @@ impl<T: Display> Display for P<T> {
     }
 }
 
+#[cfg(stage0)]
 impl<S: Hasher, T: Hash<S>> Hash<S> for P<T> {
     fn hash(&self, state: &mut S) {
         (**self).hash(state);
     }
 }
+#[cfg(not(stage0))]
+impl<T: Hash> Hash for P<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        (**self).hash(state);
+    }
+}
 
 impl<T: 'static + Decodable> Decodable for P<T> {
     fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {