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, 2 insertions, 4 deletions

diff --git a/compiler/rustc_query_impl/src/plumbing.rs b/compiler/rustc_query_impl/src/plumbing.rs
index 634236c0dac..87aedc6542d 100644
--- a/compiler/rustc_query_impl/src/plumbing.rs
+++ b/compiler/rustc_query_impl/src/plumbing.rs
@@ -142,7 +142,7 @@ impl<'tcx> QueryCtxt<'tcx> {
         self,
         encoder: &mut on_disk_cache::CacheEncoder<'_, 'tcx, opaque::FileEncoder>,
         query_result_index: &mut on_disk_cache::EncodedDepNodeIndex,
-    ) -> opaque::FileEncodeResult {
+    ) {
         macro_rules! encode_queries {
             ($($query:ident,)*) => {
                 $(
@@ -150,14 +150,12 @@ impl<'tcx> QueryCtxt<'tcx> {
                         self,
                         encoder,
                         query_result_index
-                    )?;
+                    );
                 )*
             }
         }
 
         rustc_cached_queries!(encode_queries!);
-
-        Ok(())
     }
 
     pub fn try_print_query_stack(