Make `Decodable` and `Decoder` infallible. by nnethercote · Pull Request #93066...

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 PR 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.

r? @bjorn3