auto merge of #20514 : alexcrichton/rust/serialize-associated-type, r=aturon

This commit moves the libserialize crate (and will force the hand of the
rustc-serialize crate) to not require the `old_orphan_check` feature gate as
well as using associated types wherever possible. Concretely, the following
changes were made:

* The error type of `Encoder` and `Decoder` is now an associated type, meaning
  that these traits have no type parameters.

* The `Encoder` and `Decoder` type parameters on the `Encodable` and `Decodable`
  traits have moved to the corresponding method of the trait. This movement
  alleviates the dependency on `old_orphan_check` but implies that
  implementations can no longer be specialized for the type of encoder/decoder
  being implemented.

Due to the trait definitions changing, this is a:

[breaking-change]

1 files changed, 16 insertions, 0 deletions

diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index 6eee1d903ea..13eda7bb88f 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -111,14 +111,30 @@ impl<S, T: Hash<S>> Hash<S> for P<T> {
     }
 }
 
+#[cfg(stage0)]
 impl<E, D: Decoder<E>, T: 'static + Decodable<D, E>> Decodable<D, E> for P<T> {
     fn decode(d: &mut D) -> Result<P<T>, E> {
         Decodable::decode(d).map(P)
     }
 }
 
+#[cfg(not(stage0))]
+impl<T: 'static + Decodable> Decodable for P<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
+        Decodable::decode(d).map(P)
+    }
+}
+
+#[cfg(stage0)]
 impl<E, S: Encoder<E>, T: Encodable<S, E>> Encodable<S, E> for P<T> {
     fn encode(&self, s: &mut S) -> Result<(), E> {
         (**self).encode(s)
     }
 }
+
+#[cfg(not(stage0))]
+impl<T: Encodable> Encodable for P<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        (**self).encode(s)
+    }
+}