Extend allow/warn/etc attributes to be applicable to individual use declarations

[pnkfelix]

Right now, the allow/warn/deny/forbid attributes can only be applied to entire modules.

It would be nice to be able to apply the attributes to individual use declarations, to be able to say, e.g., "I know that this particular import is not currently used in the module, and I do not want to be warned about it; but I do not want to turn off warnings for other imports in the module."

Concrete example:

mod a { pub static x: int = 3; pub static y: int = 4; }

mod b {
    use a::x; // I want to be warned about this one ...
    #[allow(unused_imports)]
    use a::y; // ... but not about this one.
}

mod c {
    #[allow(unused_imports)];
    use a::x; // Insufficient because I get no warning about `x` here.
    use a::y;
}

fn main() {}

(Also, the fact that the syntax is currently accepted when applied to individual use declarations, as above, but seems to be a no-op, is probably also suboptimal. But of course, I would fix the problem by making it have an effect, as described by this ticket.)

[emberian]

What's the main usecase here?

[pnkfelix]

@cmr The particular instance that I encountered was that I had some code that I commented out temporarily, and that commented out code was the only use of some import among a large set of declarations. I wanted to stop seeing the warnings about that import.

[dcrewi]

There is also some issue of consistency about where the setting of lint levels is permitted. Due to the way it's currently implemented, lints that are checked outside of rustc::middle::lint can only have their levels modified by command-line arguments or crate attributes. Lints that are checked from code inside of rustc::middle::lint can, in addition, have their lint levels modified by attributes on any view item, any item or any method. It's probably a good idea for the rules to be the same for all lint checks.

[emberian]

Still an issue.