Macro hygiene

[Tamschi]

I noticed that the #[trace] macro uses let-bindings in unspanned and call-site-spanned quotes and also refers to items without fully qualified path.

This can lead to hard-to-work-around identifier collisions.
Additionally, (on stable) only mixed-site spans suppress warnings in macro-generated code (as long as the macro is defined outside the current workspace, at least).

[Tamschi]

I'm interested in contributing this improvement, since I have experience with writing hygienic procedural macros, but I'd like to make sure it's wanted first.

[andylokandy]

Good idea! I'd like to review it if you're willing to submit a PR.

[Tamschi]

I'll see when I have some spare time. I should be able to do this within a month or so, though.

[Tamschi]

I made an attempt, but it seems neither async-trait nor proc-macro-error2 are hygienic, so I can't test this properly.
proc-macro-error2 being somewhat unhygienic also makes it moot to fix these issues here.

I don't currently have the bandwidth to implement macro hygiene for all three crates, so I won't work on it until/unless it becomes a blocker for me. (Consider the attempt abandoned. I'll let you know before I retry.)