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.

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>());
     }
 }