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.

1 files changed, 1 insertions, 4 deletions

diff --git a/compiler/rustc_driver/src/lib.rs b/compiler/rustc_driver/src/lib.rs
index 19fa6812b45..2b64312dbef 100644
--- a/compiler/rustc_driver/src/lib.rs
+++ b/compiler/rustc_driver/src/lib.rs
@@ -597,10 +597,7 @@ impl RustcDefaultCalls {
                 let rlink_data = fs::read_to_string(file).unwrap_or_else(|err| {
                     sess.fatal(&format!("failed to read rlink file: {}", err));
                 });
-                let codegen_results: CodegenResults =
-                    json::decode(&rlink_data).unwrap_or_else(|err| {
-                        sess.fatal(&format!("failed to decode rlink: {}", err));
-                    });
+                let codegen_results: CodegenResults = json::decode(&rlink_data);
                 let result = compiler.codegen_backend().link(sess, codegen_results, &outputs);
                 abort_on_err(result, sess);
             } else {