Make `Decodable` and `Decoder` infallible.

`Decoder` has two impls:
- opaque: this impl is already partly infallible, i.e. in some places it
  currently panics on failure (e.g. if the input is too short, or on a
  bad `Result` discriminant), and in some places it returns an error
  (e.g. on a bad `Option` discriminant). The number of places where
  either happens is surprisingly small, just because the binary
  representation has very little redundancy and a lot of input reading
  can occur even on malformed data.
- json: this impl is fully fallible, but it's only used (a) for the
  `.rlink` file production, and there's a `FIXME` comment suggesting it
  should change to a binary format, and (b) in a few tests in
  non-fundamental ways. Indeed #85993 is open to remove it entirely.

And the top-level places in the compiler that call into decoding just
abort on error anyway. So the fallibility is providing little value, and
getting rid of it leads to some non-trivial performance improvements.

Much of this commit is pretty boring and mechanical. Some notes about
a few interesting parts:
- The commit removes `Decoder::{Error,error}`.
- `InternIteratorElement::intern_with`: the impl for `T` now has the same
  optimization for small counts that the impl for `Result<T, E>` has,
  because it's now much hotter.
- Decodable impls for SmallVec, LinkedList, VecDeque now all use
  `collect`, which is nice; the one for `Vec` uses unsafe code, because
  that gave better perf on some benchmarks.

4 files changed, 48 insertions, 53 deletions

diff --git a/compiler/rustc_span/src/def_id.rs b/compiler/rustc_span/src/def_id.rs
index 5390eed89fa..147c1f9e043 100644
--- a/compiler/rustc_span/src/def_id.rs
+++ b/compiler/rustc_span/src/def_id.rs
@@ -47,8 +47,8 @@ impl<E: Encoder> Encodable<E> for CrateNum {
 }
 
 impl<D: Decoder> Decodable<D> for CrateNum {
-    default fn decode(d: &mut D) -> Result<CrateNum, D::Error> {
-        Ok(CrateNum::from_u32(d.read_u32()?))
+    default fn decode(d: &mut D) -> CrateNum {
+        CrateNum::from_u32(d.read_u32())
     }
 }
 
@@ -209,7 +209,7 @@ impl<E: Encoder> Encodable<E> for DefIndex {
 }
 
 impl<D: Decoder> Decodable<D> for DefIndex {
-    default fn decode(_: &mut D) -> Result<DefIndex, D::Error> {
+    default fn decode(_: &mut D) -> DefIndex {
         panic!("cannot decode `DefIndex` with `{}`", std::any::type_name::<D>());
     }
 }
@@ -298,12 +298,10 @@ impl<E: Encoder> Encodable<E> for DefId {
 }
 
 impl<D: Decoder> Decodable<D> for DefId {
-    default fn decode(d: &mut D) -> Result<DefId, D::Error> {
-        d.read_struct(|d| {
-            Ok(DefId {
-                krate: d.read_struct_field("krate", Decodable::decode)?,
-                index: d.read_struct_field("index", Decodable::decode)?,
-            })
+    default fn decode(d: &mut D) -> DefId {
+        d.read_struct(|d| DefId {
+            krate: d.read_struct_field("krate", Decodable::decode),
+            index: d.read_struct_field("index", Decodable::decode),
         })
     }
 }
@@ -378,8 +376,8 @@ impl<E: Encoder> Encodable<E> for LocalDefId {
 }
 
 impl<D: Decoder> Decodable<D> for LocalDefId {
-    fn decode(d: &mut D) -> Result<LocalDefId, D::Error> {
-        DefId::decode(d).map(|d| d.expect_local())
+    fn decode(d: &mut D) -> LocalDefId {
+        DefId::decode(d).expect_local()
     }
 }
 
diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 7b70c20d307..e0d6bd8cb7b 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1314,19 +1314,16 @@ pub fn decode_expn_id(
 // to track which `SyntaxContext`s we have already decoded.
 // The provided closure will be invoked to deserialize a `SyntaxContextData`
 // if we haven't already seen the id of the `SyntaxContext` we are deserializing.
-pub fn decode_syntax_context<
-    D: Decoder,
-    F: FnOnce(&mut D, u32) -> Result<SyntaxContextData, D::Error>,
->(
+pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContextData>(
     d: &mut D,
     context: &HygieneDecodeContext,
     decode_data: F,
-) -> Result<SyntaxContext, D::Error> {
-    let raw_id: u32 = Decodable::decode(d)?;
+) -> SyntaxContext {
+    let raw_id: u32 = Decodable::decode(d);
     if raw_id == 0 {
         debug!("decode_syntax_context: deserialized root");
         // The root is special
-        return Ok(SyntaxContext::root());
+        return SyntaxContext::root();
     }
 
     let outer_ctxts = &context.remapped_ctxts;
@@ -1334,7 +1331,7 @@ pub fn decode_syntax_context<
     // Ensure that the lock() temporary is dropped early
     {
         if let Some(ctxt) = outer_ctxts.lock().get(raw_id as usize).copied().flatten() {
-            return Ok(ctxt);
+            return ctxt;
         }
     }
 
@@ -1364,7 +1361,7 @@ pub fn decode_syntax_context<
 
     // Don't try to decode data while holding the lock, since we need to
     // be able to recursively decode a SyntaxContext
-    let mut ctxt_data = decode_data(d, raw_id)?;
+    let mut ctxt_data = decode_data(d, raw_id);
     // Reset `dollar_crate_name` so that it will be updated by `update_dollar_crate_names`
     // We don't care what the encoding crate set this to - we want to resolve it
     // from the perspective of the current compilation session
@@ -1380,7 +1377,7 @@ pub fn decode_syntax_context<
         assert_eq!(dummy.dollar_crate_name, kw::Empty);
     });
 
-    Ok(new_ctxt)
+    new_ctxt
 }
 
 fn for_all_ctxts_in<E, F: FnMut(u32, SyntaxContext, &SyntaxContextData) -> Result<(), E>>(
@@ -1422,13 +1419,13 @@ impl<E: Encoder> Encodable<E> for ExpnId {
 }
 
 impl<D: Decoder> Decodable<D> for LocalExpnId {
-    fn decode(d: &mut D) -> Result<Self, D::Error> {
-        ExpnId::decode(d).map(ExpnId::expect_local)
+    fn decode(d: &mut D) -> Self {
+        ExpnId::expect_local(ExpnId::decode(d))
     }
 }
 
 impl<D: Decoder> Decodable<D> for ExpnId {
-    default fn decode(_: &mut D) -> Result<Self, D::Error> {
+    default fn decode(_: &mut D) -> Self {
         panic!("cannot decode `ExpnId` with `{}`", std::any::type_name::<D>());
     }
 }
@@ -1451,7 +1448,7 @@ impl<E: Encoder> Encodable<E> for SyntaxContext {
 }
 
 impl<D: Decoder> Decodable<D> for SyntaxContext {
-    default fn decode(_: &mut D) -> Result<Self, D::Error> {
+    default fn decode(_: &mut D) -> Self {
         panic!("cannot decode `SyntaxContext` with `{}`", std::any::type_name::<D>());
     }
 }
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 9602bc5d0b7..92360164a01 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -975,12 +975,12 @@ impl<E: Encoder> Encodable<E> for Span {
     }
 }
 impl<D: Decoder> Decodable<D> for Span {
-    default fn decode(s: &mut D) -> Result<Span, D::Error> {
+    default fn decode(s: &mut D) -> Span {
         s.read_struct(|d| {
-            let lo = d.read_struct_field("lo", Decodable::decode)?;
-            let hi = d.read_struct_field("hi", Decodable::decode)?;
+            let lo = d.read_struct_field("lo", Decodable::decode);
+            let hi = d.read_struct_field("hi", Decodable::decode);
 
-            Ok(Span::new(lo, hi, SyntaxContext::root(), None))
+            Span::new(lo, hi, SyntaxContext::root(), None)
         })
     }
 }
@@ -1448,30 +1448,30 @@ impl<S: Encoder> Encodable<S> for SourceFile {
 }
 
 impl<D: Decoder> Decodable<D> for SourceFile {
-    fn decode(d: &mut D) -> Result<SourceFile, D::Error> {
+    fn decode(d: &mut D) -> SourceFile {
         d.read_struct(|d| {
-            let name: FileName = d.read_struct_field("name", |d| Decodable::decode(d))?;
+            let name: FileName = d.read_struct_field("name", |d| Decodable::decode(d));
             let src_hash: SourceFileHash =
-                d.read_struct_field("src_hash", |d| Decodable::decode(d))?;
-            let start_pos: BytePos = d.read_struct_field("start_pos", |d| Decodable::decode(d))?;
-            let end_pos: BytePos = d.read_struct_field("end_pos", |d| Decodable::decode(d))?;
+                d.read_struct_field("src_hash", |d| Decodable::decode(d));
+            let start_pos: BytePos = d.read_struct_field("start_pos", |d| Decodable::decode(d));
+            let end_pos: BytePos = d.read_struct_field("end_pos", |d| Decodable::decode(d));
             let lines: Vec<BytePos> = d.read_struct_field("lines", |d| {
-                let num_lines: u32 = Decodable::decode(d)?;
+                let num_lines: u32 = Decodable::decode(d);
                 let mut lines = Vec::with_capacity(num_lines as usize);
 
                 if num_lines > 0 {
                     // Read the number of bytes used per diff.
-                    let bytes_per_diff: u8 = Decodable::decode(d)?;
+                    let bytes_per_diff: u8 = Decodable::decode(d);
 
                     // Read the first element.
-                    let mut line_start: BytePos = Decodable::decode(d)?;
+                    let mut line_start: BytePos = Decodable::decode(d);
                     lines.push(line_start);
 
                     for _ in 1..num_lines {
                         let diff = match bytes_per_diff {
-                            1 => d.read_u8()? as u32,
-                            2 => d.read_u16()? as u32,
-                            4 => d.read_u32()?,
+                            1 => d.read_u8() as u32,
+                            2 => d.read_u16() as u32,
+                            4 => d.read_u32(),
                             _ => unreachable!(),
                         };
 
@@ -1481,17 +1481,17 @@ impl<D: Decoder> Decodable<D> for SourceFile {
                     }
                 }
 
-                Ok(lines)
-            })?;
+                lines
+            });
             let multibyte_chars: Vec<MultiByteChar> =
-                d.read_struct_field("multibyte_chars", |d| Decodable::decode(d))?;
+                d.read_struct_field("multibyte_chars", |d| Decodable::decode(d));
             let non_narrow_chars: Vec<NonNarrowChar> =
-                d.read_struct_field("non_narrow_chars", |d| Decodable::decode(d))?;
-            let name_hash: u128 = d.read_struct_field("name_hash", |d| Decodable::decode(d))?;
+                d.read_struct_field("non_narrow_chars", |d| Decodable::decode(d));
+            let name_hash: u128 = d.read_struct_field("name_hash", |d| Decodable::decode(d));
             let normalized_pos: Vec<NormalizedPos> =
-                d.read_struct_field("normalized_pos", |d| Decodable::decode(d))?;
-            let cnum: CrateNum = d.read_struct_field("cnum", |d| Decodable::decode(d))?;
-            Ok(SourceFile {
+                d.read_struct_field("normalized_pos", |d| Decodable::decode(d));
+            let cnum: CrateNum = d.read_struct_field("cnum", |d| Decodable::decode(d));
+            SourceFile {
                 name,
                 start_pos,
                 end_pos,
@@ -1506,7 +1506,7 @@ impl<D: Decoder> Decodable<D> for SourceFile {
                 normalized_pos,
                 name_hash,
                 cnum,
-            })
+            }
         })
     }
 }
@@ -1949,8 +1949,8 @@ impl<S: rustc_serialize::Encoder> Encodable<S> for BytePos {
 }
 
 impl<D: rustc_serialize::Decoder> Decodable<D> for BytePos {
-    fn decode(d: &mut D) -> Result<BytePos, D::Error> {
-        Ok(BytePos(d.read_u32()?))
+    fn decode(d: &mut D) -> BytePos {
+        BytePos(d.read_u32())
     }
 }
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 702e3594660..42fe05b52cc 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1755,8 +1755,8 @@ impl<S: Encoder> Encodable<S> for Symbol {
 
 impl<D: Decoder> Decodable<D> for Symbol {
     #[inline]
-    fn decode(d: &mut D) -> Result<Symbol, D::Error> {
-        Ok(Symbol::intern(&d.read_str()?))
+    fn decode(d: &mut D) -> Symbol {
+        Symbol::intern(&d.read_str())
     }
 }