Use delayed error handling for Encodable and Encoder infallible.

There are two impls of the `Encoder` trait: `opaque::Encoder` and
`opaque::FileEncoder`. The former encodes into memory and is infallible, the
latter writes to file and is fallible.

Currently, standard `Result`/`?`/`unwrap` error handling is used, but this is a
bit verbose and has non-trivial cost, which is annoying given how rare failures
are (especially in the infallible `opaque::Encoder` case).

This commit changes how `Encoder` fallibility is handled. All the `emit_*`
methods are now infallible. `opaque::Encoder` requires no great changes for
this. `opaque::FileEncoder` now implements a delayed error handling strategy.
If a failure occurs, it records this via the `res` field, and all subsequent
encoding operations are skipped if `res` indicates an error has occurred. Once
encoding is complete, the new `finish` method is called, which returns a
`Result`. In other words, there is now a single `Result`-producing method
instead of many of them.

This has very little effect on how any file errors are reported if
`opaque::FileEncoder` has any failures.

Much of this commit is boring mechanical changes, removing `Result` return
values and `?` or `unwrap` from expressions. The more interesting parts are as
follows.
- serialize.rs: The `Encoder` trait gains an `Ok` associated type. The
  `into_inner` method is changed into `finish`, which returns
  `Result<Vec<u8>, !>`.
- opaque.rs: The `FileEncoder` adopts the delayed error handling
  strategy. Its `Ok` type is a `usize`, returning the number of bytes
  written, replacing previous uses of `FileEncoder::position`.
- Various methods that take an encoder now consume it, rather than being
  passed a mutable reference, e.g. `serialize_query_result_cache`.

compiler/rustc_ast/src/ast.rs

@@ -2473,9 +2473,7 @@ rustc_index::newtype_index! {
 }
 
 impl<S: Encoder> rustc_serialize::Encodable<S> for AttrId {
-    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
-        Ok(())
-    }
+    fn encode(&self, _s: &mut S) {}
 }
 
 impl<D: Decoder> rustc_serialize::Decodable<D> for AttrId {

compiler/rustc_ast/src/ptr.rs

@@ -121,8 +121,8 @@ impl<D: Decoder, T: 'static + Decodable<D>> Decodable<D> for P<T> {
 }
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<T> {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        (**self).encode(s)
+    fn encode(&self, s: &mut S) {
+        (**self).encode(s);
     }
 }
 
@@ -191,8 +191,8 @@ impl<'a, T> IntoIterator for &'a P<[T]> {
 }
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for P<[T]> {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        Encodable::encode(&**self, s)
+    fn encode(&self, s: &mut S) {
+        Encodable::encode(&**self, s);
     }
 }
 

compiler/rustc_ast/src/tokenstream.rs

@@ -142,9 +142,9 @@ impl fmt::Debug for LazyTokenStream {
 }
 
 impl<S: Encoder> Encodable<S> for LazyTokenStream {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+    fn encode(&self, s: &mut S) {
         // Used by AST json printing.
-        Encodable::encode(&self.create_token_stream(), s)
+        Encodable::encode(&self.create_token_stream(), s);
     }
 }