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_span/src/def_id.rs

@@ -41,8 +41,8 @@ impl fmt::Display for CrateNum {
 /// As a local identifier, a `CrateNum` is only meaningful within its context, e.g. within a tcx.
 /// Therefore, make sure to include the context when encode a `CrateNum`.
 impl<E: Encoder> Encodable<E> for CrateNum {
-    default fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_u32(self.as_u32())
+    default fn encode(&self, s: &mut E) {
+        s.emit_u32(self.as_u32());
     }
 }
 
@@ -203,7 +203,7 @@ rustc_index::newtype_index! {
 }
 
 impl<E: Encoder> Encodable<E> for DefIndex {
-    default fn encode(&self, _: &mut E) -> Result<(), E::Error> {
+    default fn encode(&self, _: &mut E) {
         panic!("cannot encode `DefIndex` with `{}`", std::any::type_name::<E>());
     }
 }
@@ -306,9 +306,9 @@ impl DefId {
 }
 
 impl<E: Encoder> Encodable<E> for DefId {
-    default fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        self.krate.encode(s)?;
-        self.index.encode(s)
+    default fn encode(&self, s: &mut E) {
+        self.krate.encode(s);
+        self.index.encode(s);
     }
 }
 
@@ -382,8 +382,8 @@ impl fmt::Debug for LocalDefId {
 }
 
 impl<E: Encoder> Encodable<E> for LocalDefId {
-    fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        self.to_def_id().encode(s)
+    fn encode(&self, s: &mut E) {
+        self.to_def_id().encode(s);
     }
 }