Suggestions are often invalid for proc-macro produced code

[m-ou-se]

The tokens generated by proc_macro (e.g. from quote!()) use Span::call_site(), which makes some suggestions produce invalid output:

error[E0308]: mismatched types
 --> src/main.rs:3:1
  |
3 | #[hello]
  | ^^^^^^^^
  | |
  | expected `&mut i32`, found integer
  | help: consider mutably borrowing here: `&mut #[hello]`

It's a bit tricky, because this call_site span does point into the current crate (to #[hello]), so this situaiton isn't detected in the same way as external macro_rules.

[m-ou-se]

Looks like rustc_middle::lint::in_external_macro should be used in more places. That one detects this call_site span by checking if def_site is a dummy or imported.

[m-ou-se]

I'm working on a fix specifically for these typeck/borrowing suggestions. Not sure how many other suggestions have the same problem.

[estebank]

@m-ou-se this is a prevalent problem. We need to add metadata to spans to distinguish call sites explicitly in an easy way and always ignore suggestions in them.