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, 267 insertions, 246 deletions

diff --git a/src/libserialize/serialize.rs b/src/libserialize/serialize.rs
index 558f9e603e1..0646ee1758f 100644
--- a/src/libserialize/serialize.rs
+++ b/src/libserialize/serialize.rs
@@ -19,406 +19,424 @@ use std::rc::Rc;
 use std::cell::{Cell, RefCell};
 use std::sync::Arc;
 
-pub trait Encoder<E> {
+pub trait Encoder {
+    type Error;
+
     // Primitive types:
-    fn emit_nil(&mut self) -> Result<(), E>;
-    fn emit_uint(&mut self, v: uint) -> Result<(), E>;
-    fn emit_u64(&mut self, v: u64) -> Result<(), E>;
-    fn emit_u32(&mut self, v: u32) -> Result<(), E>;
-    fn emit_u16(&mut self, v: u16) -> Result<(), E>;
-    fn emit_u8(&mut self, v: u8) -> Result<(), E>;
-    fn emit_int(&mut self, v: int) -> Result<(), E>;
-    fn emit_i64(&mut self, v: i64) -> Result<(), E>;
-    fn emit_i32(&mut self, v: i32) -> Result<(), E>;
-    fn emit_i16(&mut self, v: i16) -> Result<(), E>;
-    fn emit_i8(&mut self, v: i8) -> Result<(), E>;
-    fn emit_bool(&mut self, v: bool) -> Result<(), E>;
-    fn emit_f64(&mut self, v: f64) -> Result<(), E>;
-    fn emit_f32(&mut self, v: f32) -> Result<(), E>;
-    fn emit_char(&mut self, v: char) -> Result<(), E>;
-    fn emit_str(&mut self, v: &str) -> Result<(), E>;
+    fn emit_nil(&mut self) -> Result<(), Self::Error>;
+    fn emit_uint(&mut self, v: uint) -> Result<(), Self::Error>;
+    fn emit_u64(&mut self, v: u64) -> Result<(), Self::Error>;
+    fn emit_u32(&mut self, v: u32) -> Result<(), Self::Error>;
+    fn emit_u16(&mut self, v: u16) -> Result<(), Self::Error>;
+    fn emit_u8(&mut self, v: u8) -> Result<(), Self::Error>;
+    fn emit_int(&mut self, v: int) -> Result<(), Self::Error>;
+    fn emit_i64(&mut self, v: i64) -> Result<(), Self::Error>;
+    fn emit_i32(&mut self, v: i32) -> Result<(), Self::Error>;
+    fn emit_i16(&mut self, v: i16) -> Result<(), Self::Error>;
+    fn emit_i8(&mut self, v: i8) -> Result<(), Self::Error>;
+    fn emit_bool(&mut self, v: bool) -> Result<(), Self::Error>;
+    fn emit_f64(&mut self, v: f64) -> Result<(), Self::Error>;
+    fn emit_f32(&mut self, v: f32) -> Result<(), Self::Error>;
+    fn emit_char(&mut self, v: char) -> Result<(), Self::Error>;
+    fn emit_str(&mut self, v: &str) -> Result<(), Self::Error>;
 
     // Compound types:
-    fn emit_enum<F>(&mut self, name: &str, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+    fn emit_enum<F>(&mut self, name: &str, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
 
     fn emit_enum_variant<F>(&mut self, v_name: &str,
                             v_id: uint,
                             len: uint,
-                            f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_enum_variant_arg<F>(&mut self, a_idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+                            f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_enum_variant_arg<F>(&mut self, a_idx: uint, f: F)
+                                -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
 
     fn emit_enum_struct_variant<F>(&mut self, v_name: &str,
                                    v_id: uint,
                                    len: uint,
-                                   f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+                                   f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
     fn emit_enum_struct_variant_field<F>(&mut self,
                                          f_name: &str,
                                          f_idx: uint,
-                                         f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+                                         f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+
+    fn emit_struct<F>(&mut self, name: &str, len: uint, f: F)
+                      -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_struct_field<F>(&mut self, f_name: &str, f_idx: uint, f: F)
+                            -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+
+    fn emit_tuple<F>(&mut self, len: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_tuple_arg<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+
+    fn emit_tuple_struct<F>(&mut self, name: &str, len: uint, f: F)
+                            -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_tuple_struct_arg<F>(&mut self, f_idx: uint, f: F)
+                                -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+
+    // Specialized types:
+    fn emit_option<F>(&mut self, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_option_none(&mut self) -> Result<(), Self::Error>;
+    fn emit_option_some<F>(&mut self, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
 
-    fn emit_struct<F>(&mut self, name: &str, len: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_struct_field<F>(&mut self, f_name: &str, f_idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+    fn emit_seq<F>(&mut self, len: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_seq_elt<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
 
-    fn emit_tuple<F>(&mut self, len: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_tuple_arg<F>(&mut self, idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+    fn emit_map<F>(&mut self, len: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+    fn emit_map_elt_key<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
+        where F: FnMut(&mut Self) -> Result<(), Self::Error>;
+    fn emit_map_elt_val<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
+}
 
-    fn emit_tuple_struct<F>(&mut self, name: &str, len: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_tuple_struct_arg<F>(&mut self, f_idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
+pub trait Decoder {
+    type Error;
 
-    // Specialized types:
-    fn emit_option<F>(&mut self, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_option_none(&mut self) -> Result<(), E>;
-    fn emit_option_some<F>(&mut self, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-
-    fn emit_seq<F>(&mut self, len: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_seq_elt<F>(&mut self, idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-
-    fn emit_map<F>(&mut self, len: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-    fn emit_map_elt_key<F>(&mut self, idx: uint, f: F) -> Result<(), E> where
-        F: FnMut(&mut Self) -> Result<(), E>;
-    fn emit_map_elt_val<F>(&mut self, idx: uint, f: F) -> Result<(), E> where
-        F: FnOnce(&mut Self) -> Result<(), E>;
-}
-
-pub trait Decoder<E> {
     // Primitive types:
-    fn read_nil(&mut self) -> Result<(), E>;
-    fn read_uint(&mut self) -> Result<uint, E>;
-    fn read_u64(&mut self) -> Result<u64, E>;
-    fn read_u32(&mut self) -> Result<u32, E>;
-    fn read_u16(&mut self) -> Result<u16, E>;
-    fn read_u8(&mut self) -> Result<u8, E>;
-    fn read_int(&mut self) -> Result<int, E>;
-    fn read_i64(&mut self) -> Result<i64, E>;
-    fn read_i32(&mut self) -> Result<i32, E>;
-    fn read_i16(&mut self) -> Result<i16, E>;
-    fn read_i8(&mut self) -> Result<i8, E>;
-    fn read_bool(&mut self) -> Result<bool, E>;
-    fn read_f64(&mut self) -> Result<f64, E>;
-    fn read_f32(&mut self) -> Result<f32, E>;
-    fn read_char(&mut self) -> Result<char, E>;
-    fn read_str(&mut self) -> Result<String, E>;
+    fn read_nil(&mut self) -> Result<(), Self::Error>;
+    fn read_uint(&mut self) -> Result<uint, Self::Error>;
+    fn read_u64(&mut self) -> Result<u64, Self::Error>;
+    fn read_u32(&mut self) -> Result<u32, Self::Error>;
+    fn read_u16(&mut self) -> Result<u16, Self::Error>;
+    fn read_u8(&mut self) -> Result<u8, Self::Error>;
+    fn read_int(&mut self) -> Result<int, Self::Error>;
+    fn read_i64(&mut self) -> Result<i64, Self::Error>;
+    fn read_i32(&mut self) -> Result<i32, Self::Error>;
+    fn read_i16(&mut self) -> Result<i16, Self::Error>;
+    fn read_i8(&mut self) -> Result<i8, Self::Error>;
+    fn read_bool(&mut self) -> Result<bool, Self::Error>;
+    fn read_f64(&mut self) -> Result<f64, Self::Error>;
+    fn read_f32(&mut self) -> Result<f32, Self::Error>;
+    fn read_char(&mut self) -> Result<char, Self::Error>;
+    fn read_str(&mut self) -> Result<String, Self::Error>;
 
     // Compound types:
-    fn read_enum<T, F>(&mut self, name: &str, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-
-    fn read_enum_variant<T, F>(&mut self, names: &[&str], f: F) -> Result<T, E> where
-        F: FnMut(&mut Self, uint) -> Result<T, E>;
-    fn read_enum_variant_arg<T, F>(&mut self, a_idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-
-    fn read_enum_struct_variant<T, F>(&mut self, names: &[&str], f: F) -> Result<T, E> where
-        F: FnMut(&mut Self, uint) -> Result<T, E>;
+    fn read_enum<T, F>(&mut self, name: &str, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+
+    fn read_enum_variant<T, F>(&mut self, names: &[&str], f: F)
+                               -> Result<T, Self::Error>
+        where F: FnMut(&mut Self, uint) -> Result<T, Self::Error>;
+    fn read_enum_variant_arg<T, F>(&mut self, a_idx: uint, f: F)
+                                   -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+
+    fn read_enum_struct_variant<T, F>(&mut self, names: &[&str], f: F)
+                                      -> Result<T, Self::Error>
+        where F: FnMut(&mut Self, uint) -> Result<T, Self::Error>;
     fn read_enum_struct_variant_field<T, F>(&mut self,
                                             &f_name: &str,
                                             f_idx: uint,
                                             f: F)
-                                            -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
+                                            -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
 
-    fn read_struct<T, F>(&mut self, s_name: &str, len: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
+    fn read_struct<T, F>(&mut self, s_name: &str, len: uint, f: F)
+                         -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
     fn read_struct_field<T, F>(&mut self,
                                f_name: &str,
                                f_idx: uint,
                                f: F)
-                               -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-
-    fn read_tuple<T, F>(&mut self, len: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-    fn read_tuple_arg<T, F>(&mut self, a_idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-
-    fn read_tuple_struct<T, F>(&mut self, s_name: &str, len: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-    fn read_tuple_struct_arg<T, F>(&mut self, a_idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
+                               -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+
+    fn read_tuple<T, F>(&mut self, len: uint, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+    fn read_tuple_arg<T, F>(&mut self, a_idx: uint, f: F)
+                            -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+
+    fn read_tuple_struct<T, F>(&mut self, s_name: &str, len: uint, f: F)
+                               -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+    fn read_tuple_struct_arg<T, F>(&mut self, a_idx: uint, f: F)
+                                   -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
 
     // Specialized types:
-    fn read_option<T, F>(&mut self, f: F) -> Result<T, E> where
-        F: FnMut(&mut Self, bool) -> Result<T, E>;
-
-    fn read_seq<T, F>(&mut self, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self, uint) -> Result<T, E>;
-    fn read_seq_elt<T, F>(&mut self, idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-
-    fn read_map<T, F>(&mut self, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self, uint) -> Result<T, E>;
-    fn read_map_elt_key<T, F>(&mut self, idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
-    fn read_map_elt_val<T, F>(&mut self, idx: uint, f: F) -> Result<T, E> where
-        F: FnOnce(&mut Self) -> Result<T, E>;
+    fn read_option<T, F>(&mut self, f: F) -> Result<T, Self::Error>
+        where F: FnMut(&mut Self, bool) -> Result<T, Self::Error>;
+
+    fn read_seq<T, F>(&mut self, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self, uint) -> Result<T, Self::Error>;
+    fn read_seq_elt<T, F>(&mut self, idx: uint, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+
+    fn read_map<T, F>(&mut self, f: F) -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self, uint) -> Result<T, Self::Error>;
+    fn read_map_elt_key<T, F>(&mut self, idx: uint, f: F)
+                              -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
+    fn read_map_elt_val<T, F>(&mut self, idx: uint, f: F)
+                              -> Result<T, Self::Error>
+        where F: FnOnce(&mut Self) -> Result<T, Self::Error>;
 
     // Failure
-    fn error(&mut self, err: &str) -> E;
+    fn error(&mut self, err: &str) -> Self::Error;
 }
 
-pub trait Encodable<S:Encoder<E>, E> for Sized? {
-    fn encode(&self, s: &mut S) -> Result<(), E>;
+pub trait Encodable for Sized? {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error>;
 }
 
-pub trait Decodable<D:Decoder<E>, E> {
-    fn decode(d: &mut D) -> Result<Self, E>;
+pub trait Decodable {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error>;
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for uint {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for uint {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_uint(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for uint {
-    fn decode(d: &mut D) -> Result<uint, E> {
+impl Decodable for uint {
+    fn decode<D: Decoder>(d: &mut D) -> Result<uint, D::Error> {
         d.read_uint()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for u8 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for u8 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_u8(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for u8 {
-    fn decode(d: &mut D) -> Result<u8, E> {
+impl Decodable for u8 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<u8, D::Error> {
         d.read_u8()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for u16 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for u16 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_u16(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for u16 {
-    fn decode(d: &mut D) -> Result<u16, E> {
+impl Decodable for u16 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<u16, D::Error> {
         d.read_u16()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for u32 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for u32 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_u32(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for u32 {
-    fn decode(d: &mut D) -> Result<u32, E> {
+impl Decodable for u32 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<u32, D::Error> {
         d.read_u32()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for u64 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for u64 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_u64(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for u64 {
-    fn decode(d: &mut D) -> Result<u64, E> {
+impl Decodable for u64 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<u64, D::Error> {
         d.read_u64()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for int {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for int {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_int(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for int {
-    fn decode(d: &mut D) -> Result<int, E> {
+impl Decodable for int {
+    fn decode<D: Decoder>(d: &mut D) -> Result<int, D::Error> {
         d.read_int()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for i8 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for i8 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_i8(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for i8 {
-    fn decode(d: &mut D) -> Result<i8, E> {
+impl Decodable for i8 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<i8, D::Error> {
         d.read_i8()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for i16 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for i16 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_i16(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for i16 {
-    fn decode(d: &mut D) -> Result<i16, E> {
+impl Decodable for i16 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<i16, D::Error> {
         d.read_i16()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for i32 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for i32 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_i32(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for i32 {
-    fn decode(d: &mut D) -> Result<i32, E> {
+impl Decodable for i32 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<i32, D::Error> {
         d.read_i32()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for i64 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for i64 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_i64(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for i64 {
-    fn decode(d: &mut D) -> Result<i64, E> {
+impl Decodable for i64 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<i64, D::Error> {
         d.read_i64()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for str {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for str {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_str(self)
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for String {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for String {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_str(self[])
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for String {
-    fn decode(d: &mut D) -> Result<String, E> {
+impl Decodable for String {
+    fn decode<D: Decoder>(d: &mut D) -> Result<String, D::Error> {
         d.read_str()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for f32 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for f32 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_f32(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for f32 {
-    fn decode(d: &mut D) -> Result<f32, E> {
+impl Decodable for f32 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<f32, D::Error> {
         d.read_f32()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for f64 {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for f64 {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_f64(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for f64 {
-    fn decode(d: &mut D) -> Result<f64, E> {
+impl Decodable for f64 {
+    fn decode<D: Decoder>(d: &mut D) -> Result<f64, D::Error> {
         d.read_f64()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for bool {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for bool {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_bool(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for bool {
-    fn decode(d: &mut D) -> Result<bool, E> {
+impl Decodable for bool {
+    fn decode<D: Decoder>(d: &mut D) -> Result<bool, D::Error> {
         d.read_bool()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for char {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for char {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_char(*self)
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for char {
-    fn decode(d: &mut D) -> Result<char, E> {
+impl Decodable for char {
+    fn decode<D: Decoder>(d: &mut D) -> Result<char, D::Error> {
         d.read_char()
     }
 }
 
-impl<E, S:Encoder<E>> Encodable<S, E> for () {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl Encodable for () {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_nil()
     }
 }
 
-impl<E, D:Decoder<E>> Decodable<D, E> for () {
-    fn decode(d: &mut D) -> Result<(), E> {
+impl Decodable for () {
+    fn decode<D: Decoder>(d: &mut D) -> Result<(), D::Error> {
         d.read_nil()
     }
 }
 
-impl<'a, E, S: Encoder<E>, Sized? T: Encodable<S, E>> Encodable<S, E> for &'a T {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<'a, Sized? T: Encodable> Encodable for &'a T {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         (**self).encode(s)
     }
 }
 
-impl<E, S: Encoder<E>, Sized? T: Encodable<S, E>> Encodable<S, E> for Box<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<Sized? T: Encodable> Encodable for Box<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         (**self).encode(s)
     }
 }
 
-impl<E, D:Decoder<E>, T: Decodable<D, E>> Decodable<D, E> for Box<T> {
-    fn decode(d: &mut D) -> Result<Box<T>, E> {
+impl< T: Decodable> Decodable for Box<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Box<T>, D::Error> {
         Ok(box try!(Decodable::decode(d)))
     }
 }
 
-impl<E, D:Decoder<E>, T: Decodable<D, E>> Decodable<D, E> for Box<[T]> {
-    fn decode(d: &mut D) -> Result<Box<[T]>, E> {
+impl< T: Decodable> Decodable for Box<[T]> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Box<[T]>, D::Error> {
         let v: Vec<T> = try!(Decodable::decode(d));
         Ok(v.into_boxed_slice())
     }
 }
 
-impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for Rc<T> {
+impl<T:Encodable> Encodable for Rc<T> {
     #[inline]
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         (**self).encode(s)
     }
 }
 
-impl<E, D:Decoder<E>,T:Decodable<D, E>> Decodable<D, E> for Rc<T> {
+impl<T:Decodable> Decodable for Rc<T> {
     #[inline]
-    fn decode(d: &mut D) -> Result<Rc<T>, E> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Rc<T>, D::Error> {
         Ok(Rc::new(try!(Decodable::decode(d))))
     }
 }
 
-impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for [T] {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T:Encodable> Encodable for [T] {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_seq(self.len(), |s| {
             for (i, e) in self.iter().enumerate() {
                 try!(s.emit_seq_elt(i, |s| e.encode(s)))
@@ -428,8 +446,8 @@ impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for [T] {
     }
 }
 
-impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for Vec<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T:Encodable> Encodable for Vec<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_seq(self.len(), |s| {
             for (i, e) in self.iter().enumerate() {
                 try!(s.emit_seq_elt(i, |s| e.encode(s)))
@@ -439,8 +457,8 @@ impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for Vec<T> {
     }
 }
 
-impl<E, D:Decoder<E>,T:Decodable<D, E>> Decodable<D, E> for Vec<T> {
-    fn decode(d: &mut D) -> Result<Vec<T>, E> {
+impl<T:Decodable> Decodable for Vec<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Vec<T>, D::Error> {
         d.read_seq(|d, len| {
             let mut v = Vec::with_capacity(len);
             for i in range(0, len) {
@@ -451,8 +469,8 @@ impl<E, D:Decoder<E>,T:Decodable<D, E>> Decodable<D, E> for Vec<T> {
     }
 }
 
-impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for Option<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T:Encodable> Encodable for Option<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_option(|s| {
             match *self {
                 None => s.emit_option_none(),
@@ -462,8 +480,8 @@ impl<E, S:Encoder<E>,T:Encodable<S, E>> Encodable<S, E> for Option<T> {
     }
 }
 
-impl<E, D:Decoder<E>,T:Decodable<D, E>> Decodable<D, E> for Option<T> {
-    fn decode(d: &mut D) -> Result<Option<T>, E> {
+impl<T:Decodable> Decodable for Option<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Option<T>, D::Error> {
         d.read_option(|d, b| {
             if b {
                 Ok(Some(try!(Decodable::decode(d))))
@@ -487,22 +505,23 @@ macro_rules! count_idents {
 macro_rules! tuple {
     () => ();
     ( $($name:ident,)+ ) => (
-        impl<E, D:Decoder<E>,$($name:Decodable<D, E>),*> Decodable<D,E> for ($($name,)*) {
+        impl<$($name:Decodable),*> Decodable for ($($name,)*) {
             #[allow(non_snake_case)]
-            fn decode(d: &mut D) -> Result<($($name,)*), E> {
+            fn decode<D: Decoder>(d: &mut D) -> Result<($($name,)*), D::Error> {
                 let len: uint = count_idents!($($name),*);
                 d.read_tuple(len, |d| {
                     let mut i = 0;
-                    let ret = ($(try!(d.read_tuple_arg({ i+=1; i-1 }, |d| -> Result<$name,E> {
+                    let ret = ($(try!(d.read_tuple_arg({ i+=1; i-1 },
+                                                       |d| -> Result<$name,D::Error> {
                         Decodable::decode(d)
                     })),)*);
                     return Ok(ret);
                 })
             }
         }
-        impl<E, S:Encoder<E>,$($name:Encodable<S, E>),*> Encodable<S, E> for ($($name,)*) {
+        impl<$($name:Encodable),*> Encodable for ($($name,)*) {
             #[allow(non_snake_case)]
-            fn encode(&self, s: &mut S) -> Result<(), E> {
+            fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
                 let ($(ref $name,)*) = *self;
                 let mut n = 0;
                 $(let $name = $name; n += 1;)*
@@ -519,40 +538,40 @@ macro_rules! tuple {
 
 tuple! { T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, }
 
-impl<E, S: Encoder<E>> Encodable<S, E> for path::posix::Path {
-    fn encode(&self, e: &mut S) -> Result<(), E> {
+impl Encodable for path::posix::Path {
+    fn encode<S: Encoder>(&self, e: &mut S) -> Result<(), S::Error> {
         self.as_vec().encode(e)
     }
 }
 
-impl<E, D: Decoder<E>> Decodable<D, E> for path::posix::Path {
-    fn decode(d: &mut D) -> Result<path::posix::Path, E> {
+impl Decodable for path::posix::Path {
+    fn decode<D: Decoder>(d: &mut D) -> Result<path::posix::Path, D::Error> {
         let bytes: Vec<u8> = try!(Decodable::decode(d));
         Ok(path::posix::Path::new(bytes))
     }
 }
 
-impl<E, S: Encoder<E>> Encodable<S, E> for path::windows::Path {
-    fn encode(&self, e: &mut S) -> Result<(), E> {
+impl Encodable for path::windows::Path {
+    fn encode<S: Encoder>(&self, e: &mut S) -> Result<(), S::Error> {
         self.as_vec().encode(e)
     }
 }
 
-impl<E, D: Decoder<E>> Decodable<D, E> for path::windows::Path {
-    fn decode(d: &mut D) -> Result<path::windows::Path, E> {
+impl Decodable for path::windows::Path {
+    fn decode<D: Decoder>(d: &mut D) -> Result<path::windows::Path, D::Error> {
         let bytes: Vec<u8> = try!(Decodable::decode(d));
         Ok(path::windows::Path::new(bytes))
     }
 }
 
-impl<E, S: Encoder<E>, T: Encodable<S, E> + Copy> Encodable<S, E> for Cell<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T: Encodable + Copy> Encodable for Cell<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         self.get().encode(s)
     }
 }
 
-impl<E, D: Decoder<E>, T: Decodable<D, E> + Copy> Decodable<D, E> for Cell<T> {
-    fn decode(d: &mut D) -> Result<Cell<T>, E> {
+impl<T: Decodable + Copy> Decodable for Cell<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Cell<T>, D::Error> {
         Ok(Cell::new(try!(Decodable::decode(d))))
     }
 }
@@ -562,26 +581,26 @@ impl<E, D: Decoder<E>, T: Decodable<D, E> + Copy> Decodable<D, E> for Cell<T> {
 // `encoder.error("attempting to Encode borrowed RefCell")`
 // from `encode` when `try_borrow` returns `None`.
 
-impl<E, S: Encoder<E>, T: Encodable<S, E>> Encodable<S, E> for RefCell<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T: Encodable> Encodable for RefCell<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         self.borrow().encode(s)
     }
 }
 
-impl<E, D: Decoder<E>, T: Decodable<D, E>> Decodable<D, E> for RefCell<T> {
-    fn decode(d: &mut D) -> Result<RefCell<T>, E> {
+impl<T: Decodable> Decodable for RefCell<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<RefCell<T>, D::Error> {
         Ok(RefCell::new(try!(Decodable::decode(d))))
     }
 }
 
-impl<E, S:Encoder<E>, T:Encodable<S, E>> Encodable<S, E> for Arc<T> {
-    fn encode(&self, s: &mut S) -> Result<(), E> {
+impl<T:Encodable> Encodable for Arc<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         (**self).encode(s)
     }
 }
 
-impl<E, D:Decoder<E>,T:Decodable<D, E>+Send+Sync> Decodable<D, E> for Arc<T> {
-    fn decode(d: &mut D) -> Result<Arc<T>, E> {
+impl<T:Decodable+Send+Sync> Decodable for Arc<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<Arc<T>, D::Error> {
         Ok(Arc::new(try!(Decodable::decode(d))))
     }
 }
@@ -589,14 +608,15 @@ impl<E, D:Decoder<E>,T:Decodable<D, E>+Send+Sync> Decodable<D, E> for Arc<T> {
 // ___________________________________________________________________________
 // Helper routines
 
-pub trait EncoderHelpers<E> {
-    fn emit_from_vec<T, F>(&mut self, v: &[T], f: F) -> Result<(), E> where
-        F: FnMut(&mut Self, &T) -> Result<(), E>;
+pub trait EncoderHelpers: Encoder {
+    fn emit_from_vec<T, F>(&mut self, v: &[T], f: F)
+                           -> Result<(), <Self as Encoder>::Error>
+        where F: FnMut(&mut Self, &T) -> Result<(), <Self as Encoder>::Error>;
 }
 
-impl<E, S:Encoder<E>> EncoderHelpers<E> for S {
-    fn emit_from_vec<T, F>(&mut self, v: &[T], mut f: F) -> Result<(), E> where
-        F: FnMut(&mut S, &T) -> Result<(), E>,
+impl<S:Encoder> EncoderHelpers for S {
+    fn emit_from_vec<T, F>(&mut self, v: &[T], mut f: F) -> Result<(), S::Error> where
+        F: FnMut(&mut S, &T) -> Result<(), S::Error>,
     {
         self.emit_seq(v.len(), |this| {
             for (i, e) in v.iter().enumerate() {
@@ -609,14 +629,15 @@ impl<E, S:Encoder<E>> EncoderHelpers<E> for S {
     }
 }
 
-pub trait DecoderHelpers<E> {
-    fn read_to_vec<T, F>(&mut self, f: F) -> Result<Vec<T>, E> where
-        F: FnMut(&mut Self) -> Result<T, E>;
+pub trait DecoderHelpers: Decoder {
+    fn read_to_vec<T, F>(&mut self, f: F)
+                         -> Result<Vec<T>, <Self as Decoder>::Error> where
+        F: FnMut(&mut Self) -> Result<T, <Self as Decoder>::Error>;
 }
 
-impl<E, D:Decoder<E>> DecoderHelpers<E> for D {
-    fn read_to_vec<T, F>(&mut self, mut f: F) -> Result<Vec<T>, E> where F:
-        FnMut(&mut D) -> Result<T, E>,
+impl<D: Decoder> DecoderHelpers for D {
+    fn read_to_vec<T, F>(&mut self, mut f: F) -> Result<Vec<T>, D::Error> where F:
+        FnMut(&mut D) -> Result<T, D::Error>,
     {
         self.read_seq(|this, len| {
             let mut v = Vec::with_capacity(len);