Auto merge of #97905 - nnethercote:revert-infallible-encoder, r=bjorn3

Revert part of #94372 to improve performance

#94732 was supposed to give small but widespread performance improvements, as judged from three per-merge performance runs. But the performance run that occurred after merging included a roughly equal number of improvements and regressions, for unclear reasons.

This PR is for a test run reverting those changes, to see what happens.

r? `@ghost`

4 files changed, 20 insertions, 15 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 03ac82b467b..775ebb48402 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -26,8 +26,7 @@ use rustc_middle::ty::codec::TyDecoder;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::GeneratorDiagnosticData;
 use rustc_middle::ty::{self, ParameterizedOverTcx, Ty, TyCtxt, Visibility};
-use rustc_serialize::opaque::MemDecoder;
-use rustc_serialize::{Decodable, Decoder};
+use rustc_serialize::{opaque, Decodable, Decoder};
 use rustc_session::cstore::{
     CrateSource, ExternCrate, ForeignModule, LinkagePreference, NativeLib,
 };
@@ -155,7 +154,7 @@ struct ImportedSourceFile {
 }
 
 pub(super) struct DecodeContext<'a, 'tcx> {
-    opaque: MemDecoder<'a>,
+    opaque: opaque::Decoder<'a>,
     cdata: Option<CrateMetadataRef<'a>>,
     blob: &'a MetadataBlob,
     sess: Option<&'tcx Session>,
@@ -187,7 +186,7 @@ pub(super) trait Metadata<'a, 'tcx>: Copy {
     fn decoder(self, pos: usize) -> DecodeContext<'a, 'tcx> {
         let tcx = self.tcx();
         DecodeContext {
-            opaque: MemDecoder::new(self.blob(), pos),
+            opaque: opaque::Decoder::new(self.blob(), pos),
             cdata: self.cdata(),
             blob: self.blob(),
             sess: self.sess().or(tcx.map(|tcx| tcx.sess)),
@@ -419,7 +418,7 @@ impl<'a, 'tcx> TyDecoder for DecodeContext<'a, 'tcx> {
     where
         F: FnOnce(&mut Self) -> R,
     {
-        let new_opaque = MemDecoder::new(self.opaque.data, pos);
+        let new_opaque = opaque::Decoder::new(self.opaque.data, pos);
         let old_opaque = mem::replace(&mut self.opaque, new_opaque);
         let old_state = mem::replace(&mut self.lazy_state, LazyState::NoNode);
         let r = f(self);
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 8867e008e42..3285273ba90 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -27,8 +27,7 @@ use rustc_middle::ty::codec::TyEncoder;
 use rustc_middle::ty::fast_reject::{self, SimplifiedType, TreatParams};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, SymbolName, Ty, TyCtxt};
-use rustc_serialize::opaque::MemEncoder;
-use rustc_serialize::{Encodable, Encoder};
+use rustc_serialize::{opaque, Encodable, Encoder};
 use rustc_session::config::CrateType;
 use rustc_session::cstore::{ForeignModule, LinkagePreference, NativeLib};
 use rustc_span::hygiene::{ExpnIndex, HygieneEncodeContext, MacroKind};
@@ -44,7 +43,7 @@ use std::num::NonZeroUsize;
 use tracing::{debug, trace};
 
 pub(super) struct EncodeContext<'a, 'tcx> {
-    opaque: MemEncoder,
+    opaque: opaque::Encoder,
     tcx: TyCtxt<'tcx>,
     feat: &'tcx rustc_feature::Features,
 
@@ -94,6 +93,9 @@ macro_rules! encoder_methods {
 }
 
 impl<'a, 'tcx> Encoder for EncodeContext<'a, 'tcx> {
+    type Ok = <opaque::Encoder as Encoder>::Ok;
+    type Err = <opaque::Encoder as Encoder>::Err;
+
     encoder_methods! {
         emit_usize(usize);
         emit_u128(u128);
@@ -116,6 +118,10 @@ impl<'a, 'tcx> Encoder for EncodeContext<'a, 'tcx> {
         emit_str(&str);
         emit_raw_bytes(&[u8]);
     }
+
+    fn finish(self) -> Result<Self::Ok, Self::Err> {
+        self.opaque.finish()
+    }
 }
 
 impl<'a, 'tcx, T> Encodable<EncodeContext<'a, 'tcx>> for LazyValue<T> {
@@ -2182,7 +2188,7 @@ pub fn encode_metadata(tcx: TyCtxt<'_>) -> EncodedMetadata {
 }
 
 fn encode_metadata_impl(tcx: TyCtxt<'_>) -> EncodedMetadata {
-    let mut encoder = MemEncoder::new();
+    let mut encoder = opaque::Encoder::new();
     encoder.emit_raw_bytes(METADATA_HEADER);
 
     // Will be filled with the root position after encoding everything.
@@ -2217,7 +2223,7 @@ fn encode_metadata_impl(tcx: TyCtxt<'_>) -> EncodedMetadata {
     // culminating in the `CrateRoot` which points to all of it.
     let root = ecx.encode_crate_root();
 
-    let mut result = ecx.opaque.finish();
+    let mut result = ecx.opaque.finish().unwrap();
 
     // Encode the root position.
     let header = METADATA_HEADER.len();
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 04f0847f5cc..fb2ffe1d73d 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -22,7 +22,7 @@ use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, ReprOptions, Ty};
 use rustc_middle::ty::{GeneratorDiagnosticData, ParameterizedOverTcx, TyCtxt};
-use rustc_serialize::opaque::MemEncoder;
+use rustc_serialize::opaque::Encoder;
 use rustc_session::config::SymbolManglingVersion;
 use rustc_session::cstore::{CrateDepKind, ForeignModule, LinkagePreference, NativeLib};
 use rustc_span::edition::Edition;
@@ -323,7 +323,7 @@ macro_rules! define_tables {
         }
 
         impl TableBuilders {
-            fn encode(&self, buf: &mut MemEncoder) -> LazyTables {
+            fn encode(&self, buf: &mut Encoder) -> LazyTables {
                 LazyTables {
                     $($name: self.$name.encode(buf)),+
                 }
diff --git a/compiler/rustc_metadata/src/rmeta/table.rs b/compiler/rustc_metadata/src/rmeta/table.rs
index 5ab4269ae99..8baa67a8f9f 100644
--- a/compiler/rustc_metadata/src/rmeta/table.rs
+++ b/compiler/rustc_metadata/src/rmeta/table.rs
@@ -4,8 +4,8 @@ use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_hir::def::{CtorKind, CtorOf};
 use rustc_index::vec::Idx;
 use rustc_middle::ty::ParameterizedOverTcx;
-use rustc_serialize::opaque::MemEncoder;
-use rustc_serialize::Encoder;
+use rustc_serialize::opaque::Encoder;
+use rustc_serialize::Encoder as _;
 use rustc_span::hygiene::MacroKind;
 use std::convert::TryInto;
 use std::marker::PhantomData;
@@ -281,7 +281,7 @@ where
         Some(value).write_to_bytes(&mut self.blocks[i]);
     }
 
-    pub(crate) fn encode<const N: usize>(&self, buf: &mut MemEncoder) -> LazyTable<I, T>
+    pub(crate) fn encode<const N: usize>(&self, buf: &mut Encoder) -> LazyTable<I, T>
     where
         Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
     {