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`.

6 files changed, 16 insertions, 23 deletions

diff --git a/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs b/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs
index e2f3d6e078f..096bf8cbc15 100644
--- a/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs
+++ b/compiler/rustc_middle/src/mir/graph_cyclic_cache.rs
@@ -38,8 +38,8 @@ impl GraphIsCyclicCache {
 
 impl<S: serialize::Encoder> serialize::Encodable<S> for GraphIsCyclicCache {
     #[inline]
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        serialize::Encodable::encode(&(), s)
+    fn encode(&self, s: &mut S) {
+        serialize::Encodable::encode(&(), s);
     }
 }
 
diff --git a/compiler/rustc_middle/src/mir/interpret/mod.rs b/compiler/rustc_middle/src/mir/interpret/mod.rs
index 06cd6a66e39..214b919e24d 100644
--- a/compiler/rustc_middle/src/mir/interpret/mod.rs
+++ b/compiler/rustc_middle/src/mir/interpret/mod.rs
@@ -207,27 +207,26 @@ pub fn specialized_encode_alloc_id<'tcx, E: TyEncoder<I = TyCtxt<'tcx>>>(
     encoder: &mut E,
     tcx: TyCtxt<'tcx>,
     alloc_id: AllocId,
-) -> Result<(), E::Error> {
+) {
     match tcx.global_alloc(alloc_id) {
         GlobalAlloc::Memory(alloc) => {
             trace!("encoding {:?} with {:#?}", alloc_id, alloc);
-            AllocDiscriminant::Alloc.encode(encoder)?;
-            alloc.encode(encoder)?;
+            AllocDiscriminant::Alloc.encode(encoder);
+            alloc.encode(encoder);
         }
         GlobalAlloc::Function(fn_instance) => {
             trace!("encoding {:?} with {:#?}", alloc_id, fn_instance);
-            AllocDiscriminant::Fn.encode(encoder)?;
-            fn_instance.encode(encoder)?;
+            AllocDiscriminant::Fn.encode(encoder);
+            fn_instance.encode(encoder);
         }
         GlobalAlloc::Static(did) => {
             assert!(!tcx.is_thread_local_static(did));
             // References to statics doesn't need to know about their allocations,
             // just about its `DefId`.
-            AllocDiscriminant::Static.encode(encoder)?;
-            did.encode(encoder)?;
+            AllocDiscriminant::Static.encode(encoder);
+            did.encode(encoder);
         }
     }
-    Ok(())
 }
 
 // Used to avoid infinite recursion when decoding cyclic allocations.
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index f3db359ec33..581b3257c73 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -672,16 +672,16 @@ const TAG_CLEAR_CROSS_CRATE_SET: u8 = 1;
 
 impl<E: TyEncoder, T: Encodable<E>> Encodable<E> for ClearCrossCrate<T> {
     #[inline]
-    fn encode(&self, e: &mut E) -> Result<(), E::Error> {
+    fn encode(&self, e: &mut E) {
         if E::CLEAR_CROSS_CRATE {
-            return Ok(());
+            return;
         }
 
         match *self {
             ClearCrossCrate::Clear => TAG_CLEAR_CROSS_CRATE_CLEAR.encode(e),
             ClearCrossCrate::Set(ref val) => {
-                TAG_CLEAR_CROSS_CRATE_SET.encode(e)?;
-                val.encode(e)
+                TAG_CLEAR_CROSS_CRATE_SET.encode(e);
+                val.encode(e);
             }
         }
     }
diff --git a/compiler/rustc_middle/src/mir/predecessors.rs b/compiler/rustc_middle/src/mir/predecessors.rs
index 0b9ddaf64d4..9bc0cb1138f 100644
--- a/compiler/rustc_middle/src/mir/predecessors.rs
+++ b/compiler/rustc_middle/src/mir/predecessors.rs
@@ -56,9 +56,7 @@ impl PredecessorCache {
 
 impl<S: serialize::Encoder> serialize::Encodable<S> for PredecessorCache {
     #[inline]
-    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
-        Ok(())
-    }
+    fn encode(&self, _s: &mut S) {}
 }
 
 impl<D: serialize::Decoder> serialize::Decodable<D> for PredecessorCache {
diff --git a/compiler/rustc_middle/src/mir/switch_sources.rs b/compiler/rustc_middle/src/mir/switch_sources.rs
index fbb26800e29..4872a7835e3 100644
--- a/compiler/rustc_middle/src/mir/switch_sources.rs
+++ b/compiler/rustc_middle/src/mir/switch_sources.rs
@@ -56,9 +56,7 @@ impl SwitchSourceCache {
 
 impl<S: serialize::Encoder> serialize::Encodable<S> for SwitchSourceCache {
     #[inline]
-    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
-        Ok(())
-    }
+    fn encode(&self, _s: &mut S) {}
 }
 
 impl<D: serialize::Decoder> serialize::Decodable<D> for SwitchSourceCache {
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index 7e395902ad3..f745e55307a 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -367,9 +367,7 @@ impl PostorderCache {
 
 impl<S: serialize::Encoder> serialize::Encodable<S> for PostorderCache {
     #[inline]
-    fn encode(&self, _s: &mut S) -> Result<(), S::Error> {
-        Ok(())
-    }
+    fn encode(&self, _s: &mut S) {}
 }
 
 impl<D: serialize::Decoder> serialize::Decodable<D> for PostorderCache {