Include scope names in diagnostics

[da-x]

(reference implementation: rust-lang/rust#49898 )

So far, rustc has only printed about filenames and line numbers for warnings and errors. I think it is rather missed, compared to gcc and other compilers, that useful context information such as function
names and structs is not included.

We can improve the information that rustc prints for warnings and errors in this regard, by adding a new line to precede the warning or the error.

For example:

In function `test`:
warning: unused variable: `a`

In enum `MyEnum`:
warning: variant is never constructed: `ItemB`
  --> various-warnings.rs:8:5
   |
 8 |     ItemB,
   |     ^^

[Ixrec]

An issue on the rfcs repo isn't an RFC. See https://github.com/rust-lang/rfcs#what-the-process-is for how to create an RFC.

👍 on the actual idea you're proposing

[da-x]

Thanks - I meant this issue to serve as a pre-RFC.

[Centril]

If you wish to create a pre-RFC, I think it would be better to do that on internals.rust-lang.org as you'll have more reviewers there. If you do file a pre-RFC over there, please close this issue.

[da-x]

Moved the pre-RFC discussion here: https://internals.rust-lang.org/t/pre-rfc-include-scope-names-in-diagnostics/7304