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, 7 insertions, 15 deletions

diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index e99fa6c113b..82e6972d027 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -146,21 +146,17 @@ fn encodable_body(
                     .map(|binding| {
                         let bind_ident = &binding.binding;
                         let result = quote! {
-                            match ::rustc_serialize::Encodable::<#encoder_ty>::encode(
+                            ::rustc_serialize::Encodable::<#encoder_ty>::encode(
                                 #bind_ident,
                                 __encoder,
-                            ) {
-                                ::std::result::Result::Ok(()) => (),
-                                ::std::result::Result::Err(__err)
-                                    => return ::std::result::Result::Err(__err),
-                            }
+                            );
                         };
                         result
                     })
                     .collect::<TokenStream>()
             });
             quote! {
-                ::std::result::Result::Ok(match *self { #encode_inner })
+                match *self { #encode_inner }
             }
         }
         _ => {
@@ -172,14 +168,10 @@ fn encodable_body(
                     .map(|binding| {
                         let bind_ident = &binding.binding;
                         let result = quote! {
-                            match ::rustc_serialize::Encodable::<#encoder_ty>::encode(
+                            ::rustc_serialize::Encodable::<#encoder_ty>::encode(
                                 #bind_ident,
                                 __encoder,
-                            ) {
-                                ::std::result::Result::Ok(()) => (),
-                                ::std::result::Result::Err(__err)
-                                    => return ::std::result::Result::Err(__err),
-                            }
+                            );
                         };
                         result
                     })
@@ -190,7 +182,7 @@ fn encodable_body(
                         ::rustc_serialize::Encoder::emit_enum_variant(
                             __encoder,
                             #variant_idx,
-                            |__encoder| { ::std::result::Result::Ok({ #encode_fields }) }
+                            |__encoder| { #encode_fields }
                         )
                     }
                 } else {
@@ -223,7 +215,7 @@ fn encodable_body(
             fn encode(
                 &self,
                 __encoder: &mut #encoder_ty,
-            ) -> ::std::result::Result<(), <#encoder_ty as ::rustc_serialize::Encoder>::Error> {
+            ) {
                 #lints
                 #encode_body
             }