Better error message when bare type in impl parameter list

[bb1yd]

Fixes #147889

Marking this PR ready for feedback on this problem.

The approach I'm trying is to create a snapshot parser and attempt to re-parse the input in another way. If it succeeds, we successfully predicted the user's intention and emit the related help message. If it fails, we cancel the errors produced during the attempt and report the original error.

The problem is that parse functions may internally recover from parse failures and emit errors directly, which means we can't cancel them later. I saw that we have a recovery field to control the recovery mechanism, but it seems there are (maybe) a lot of places in the parser that don't check the recovery field before doing their recovery logic.

Should we always use look_ahead instead of using a snapshot parser and calling another parse function?

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @adwinwhite

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