The diagnostic implemented is a simple one (E0109). It serves as a test for the new foundation.
This commit only implements diagnostics for type in bodies and body-carrying signatures; the next commit will include diagnostics in the rest of the things.
Also fix one weird bug that was detected when implementing this that caused `Fn::(A, B) -> C` (which is a valid, if bizarre, alternative syntax to `Fn(A, B) -> C` to lower incorrectly.
And also fix a maybe-bug where parentheses were sneaked into a code string needlessly; this was not detected until now because the parentheses were removed (by the make-AST family API), but with a change in this commit they are now inserted. So fix that too.
Thanks to the observation (supported by counting) that the vast majority paths have neither generics no type anchors, and thanks to a new datastructure `ThinVecWithHeader` that is essentially `(T, Box<[U]>)` but with the size of a single pointer, we are able to reach this feat.
This (together with `ThinVecWithHeader`) makes the possibility to shrink `TypeRef`, because most types are paths.
So that given a `TypeRef` we will be able to trace it back to source code.
This is necessary to be able to provide diagnostics for lowering to chalk tys, since the input to that is `TypeRef`.
This means that `TypeRef`s now have an identity, which means storing them in arena and not interning them, which is an unfortunate (but necessary) loss but also a pretty massive change. Luckily, because of the separation layer we have for IDE and HIR, this change never crosses the IDE boundary.
This PR touches a lot of parts. But the main changes are changing
`hir_expand::Name` to be raw edition-dependently and only when necessary
(unrelated to how the user originally wrote the identifier),
and changing `is_keyword()` and `is_raw_identifier()` to be edition-aware
(this was done in #17896, but the FIXMEs were fixed here).
It is possible that I missed some cases, but most IDE parts should properly
escape (or not escape) identifiers now.
The rules of thumb are:
- If we show the identifier to the user, its rawness should be determined
by the edition of the edited crate. This is nice for IDE features,
but really important for changes we insert to the source code.
- For tests, I chose `Edition::CURRENT` (so we only have to (maybe) update
tests when an edition becomes stable, to avoid churn).
- For debugging tools (helper methods and logs), I used `Edition::LATEST`.
