`unknown_lints` doesn't trigger in all `cfg_attr`

[Xiphoseer]

I tried this code:

#![deny(warnings)]
#![cfg_attr(all(), allow(lint1))]

#[cfg_attr(all(), allow(lint2))]
mod baz {
    #![cfg_attr(all(), allow(lint3))]
}

#[cfg_attr(all(), allow(lint4))]
pub fn test() {}

Playground Link

I expected attributs in all 4 positions to cause compiler errors, but just the first one (inner attribute on the root module) does. When using allow without cfg_attr I get all 4 errors. Came across this while debugging why adding #![cfg_attr(feature = "name", allow(unused_macro_rules))] didn't introduce new warnings on stable in a module.

Meta

This happens in at least:

• 1.60.0
• 1.61.0-beta.7
• nightly-2022-05-15

which are the versions currently on the playground

[est31]

This seems to have been a regression of the 1.29.0 compiler: the warnings on 1.28.0 are just fine.

The issue is that LintLevelsBuilder::push, which takes care of the unknown_lints lint, gets the attributes in a pre-cfg-expanded form, as it is called very early in compilation. But Level::from_attr, which it calls, only looks for normal attributes, as in, does not recognize cfg_attr (and it shouldn't).

For some reason, allow's behind cfg_attr still work though, and I have no clue why.

unknown_lints still works on the crate root because cfg_attr there has been expanded already.

[est31]

Expectably, as the same code also handles the renamed_and_removed_lints lint, the same issue exists if you put bare_trait_object as the lint's name in the above example.

[est31]

I think the best solution for this is to move the lints into a late lint pass. Maybe the push stuff could be called after cfg expansion but this could have side effects for other lints, idk.

@rustbot claim

[est31]

So it turns out that allowing the unknown_lints lint doesn't work either (inside cfg_attr), even though it does work for while_true, which is an early lint:

#[cfg_attr(all(), allow(unknown_lints, while_true))]
mod bar {
    #[allow(noex_inside_module)] // Warns for the unknown lint name here due to the allow above not working
    fn _foo() { while true {} } // Doesn't warn for the `while true` here due to the allow working
}

This opens the possibility of just moving the unknown_lints and renamed_and_removed_lints lints into their own early lint pass.

[est31]

Oh so apparently the push function does get called later on, but the warnings are suppressed because warn_about_weird_lints is set to false. I have missed that. So one only might have to set warn_about_weird_lints to true more early on...

[est31]

I've made a PR: #97266