Show warnings for unused variables

[aochagavia]

Right now Rust Analyzer doesn't give much feedback while writing code. There is no autocompletion and you get only syntax errors. I was thinking about a good first step toward giving more feedback and came to the idea of showing warnings for unused variables. This should be doable by reusing resolve_local_name, which was defined in #98

In the future we will probably want to extend this to detect unused struct fields, types and other stuff. But we need to begin with something.

[aochagavia]

@matklad is it a problem to keep adding things while you are migrating everything to salsa?

[aochagavia]

@matklad I would like to pick up this issue, but I am having trouble finding the right place to implement it. Do you have any suggestions? Tackling this means working at #27 as well, so a bit of mentoring would be greatly appreciated.

[matklad]

Oups, sorry, I've somehow missed all three comments on the issue :(

I think that we already can detect unused fields, yeah. There might be false-positives due to macros, but they should be rate, and we can hack around them (var is used in a macro if macro's text contains var name), for example.

The migration to salsa is mostly complete.

Do you have any suggestions?

So, #27 is an unrelated issue: validators are purely for checking syntax (in general, we try to remove checks for the parser per se to make it more tolerant to the errors).

I think the right place for this is near the resolve_local_name. So, something like

fn collect_unused_variables(
    fn_def: ast::FnDef
    scopes: &FnScopes
) -> Vec<ast::Name> {
}

should work.

[skade]

Is this issue still current?

Highlighting unused variables exists.

[lnicola]

It works by running cargo check and displaying its diagnostics, but it's not implemented natively in rust-analyzer. That might still happen, but probably not in the near future.

[matklad]

Ok, so this is handled by cargo check. Native handling is desirable, but then this issue is indistinguishable from "implement the rest of the owl compiler", so it's not useful.