`forbid` overwritten by later `allow` on the same "scope level"

[RalfJung]

forbid is not honored when it is followed by an allow on the same "scope level". This compiles, but should fail:

#![forbid(unused)]
#![allow(unused)]

fn square(num: i32) -> i32 {
    num * num
}

If we move the allow down to the function level, it fails to compile as expected:

#![forbid(unused)]

#[allow(unused)]
fn square(num: i32) -> i32 {
    num * num
}

The same issue also arises with CLI arguments, which all share the "scope level". That was worked around with in #70918, but once the above is fixed, that hack can likely be removed.

Original description

According to my understanding, the purpose of -F/forbid for lints is that they can not be allowed any more. Thus I would expect that calling rustc with -Funused -Aunused will fail when there is unused code in the file.

However, that is not the case: with that sequence of arguments, unused code is silently accepted.

Cc Manishearth/compiletest-rs#216

[RalfJung]

This is in fact a stable-to-beta regression: with Rust 1.42, the code still gets rejected as expected.

[RalfJung]

Likely regression candidate: #67885

[RalfJung]

Interestingly, doing the same via flags in the source code also does not work as expected -- the allow overwrites the forbid!

#![forbid(unused)]
#![allow(unused)]

fn square(num: i32) -> i32 {
    num * num
}

This is also a regression, but not a recent one: with Rust 1.20, the code gets rejected as expected; with Rust 1.21 and later it gets accepted. It is also in direct contradiction with the documentation which says

'forbid' is a special lint level that's stronger than 'deny'. It's the same as 'deny' in that a lint at this level will produce an error, but unlike the 'deny' level, the 'forbid' level can not be overridden to be anything lower than an error.

[spastorino]

According to @RalfJung this has possibly regressed on #67885.
I guess this needs confirmation ...

@rustbot ping cleanup

[rustbot]

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
instructions for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @chrissimpkins @contrun @DutchGhost @elshize @ethanboxx @h-michael @HallerPatrick @hdhoang @hellow554 @imtsuki @jakevossen5 @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @senden9 @shekohex @sinato @spastorino @turboladen @woshilapin @yerke