Warn about lints that are disabled but not currently activated

[johnthagen]

It would be nice if rustc could catch an unnecessary use of #[allow(unused_variables)]:

#[allow(unused_variables)]
fn fun(x: u32) -> u32{
    x
}

Often times #[allow(unused_variables)] is added during development and later should be removed after all parameters are in use so that it doesn't mask a real warning in the future.

Originally this was brought up as a lint for clippy (rust-lang/rust-clippy#1552) but was directed here because clippy does not handle allow/warn/deny itself.

[sanmai-NL]

Interesting idea, but what makes such an attribute unnecessary?

[frewsxcv]

Interesting idea, but what makes such an attribute unneccessary?

Presumably because the existence of the attribute in that particular scenario doesn't result in any fewer warnings upon compilation.

[sanmai-NL]

@frewsxcv: yeah, but that context may (likely will) change between compilation runs. I would expect the programmer to put those lint check attributes in with a reason. In general, it is wise/good to check for unneeded allow attributes though. I personally see value in an unnecessary_unused attribute, just trying to point out the trade-off here. Giving warnings when the allow attribute coincidentally happens to have no effect at some point in the evolution of the source code is perhaps not desired?

[sanxiyn]

This was also discussed in internals.rust-lang.org: Warning about disabled lints that don't need disabling?