And add a new diagnostic for non-`Fn` parenthesized generic args.
Path lowering started to look like a mess, with each function carrying additional parameters for the diagnostic callback (since paths can occur both in type and in expression/pattern position, and their diagnostic handling is different) and the segment index, for the diagnostics report. So I refactored it from stateless functions on `TyLoweringContext` into stateful struct, `PathLoweringContext`, that tracks the process of lowering a path from resolution til assoc types selection.
This is not documented (and I discovered that from experimenting and looking at the compiler's source code), but cfg_attrs *on the same level* as the attribute macro should be processed before it is expanded. cfg_attrs *below* should not (and this is contrary to what happens with derive macros, where both should be processed).
More correctly, *also* per-token. Because as it turns out, while the top-level edition affects parsing (I think), the per-token edition affects escaping of identifiers/keywords.
I expected this to be faster (due to less allocations and better cache locality), but benchmarked it is not (neither it is slower). Memory usage, however, drops by ~50mb (of `analysis-stats .`). I guess tt construction is just not hot.
This also simplifies using even less memory for token trees by compressing equal span, which I plan to do right after.
Some workflows are more easily expressed with a flat tt, while some are better expressed with a tree. With the right helpers, though (which was mostly a matter of trial and error), even the worst workflows become very easy indeed.
Which caused the macros of the popular `tracing` crate to not offer completions.
The reason is rather complicated: it boils down to macro ignoring their input and completion always choosing the first expansion.
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.
