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`.

1 files changed, 3 insertions, 4 deletions

diff --git a/compiler/rustc_serialize/tests/opaque.rs b/compiler/rustc_serialize/tests/opaque.rs
index ec3c34f8bdb..703b7f5e7a5 100644
--- a/compiler/rustc_serialize/tests/opaque.rs
+++ b/compiler/rustc_serialize/tests/opaque.rs
@@ -2,7 +2,7 @@
 
 use rustc_macros::{Decodable, Encodable};
 use rustc_serialize::opaque::{Decoder, Encoder};
-use rustc_serialize::{Decodable, Encodable};
+use rustc_serialize::{Decodable, Encodable, Encoder as EncoderTrait};
 use std::fmt::Debug;
 
 #[derive(PartialEq, Clone, Debug, Encodable, Decodable)]
@@ -32,12 +32,11 @@ fn check_round_trip<T: Encodable<Encoder> + for<'a> Decodable<Decoder<'a>> + Par
     values: Vec<T>,
 ) {
     let mut encoder = Encoder::new();
-
     for value in &values {
-        Encodable::encode(value, &mut encoder).unwrap();
+        Encodable::encode(value, &mut encoder);
     }
 
-    let data = encoder.into_inner();
+    let data = encoder.finish().unwrap();
     let mut decoder = Decoder::new(&data[..], 0);
 
     for value in values {