When lint attributes in the same scope overlap, all but the last declared one are dropped

[FaultyRAM]

#![forbid(warnings)]
#![allow(dead_code)]

struct Foo;

fn main() {}

Playground link

The above code should fail to compile with an error about trying to allow a forbidden lint, but successfully builds without any warnings. Tested on latest stable/beta/nightly.

[zackmdavis]

At least one thing that's going on here is that warnings isn't (as one might intuitively think) an ordinary lint group that happens to contain all lints. Rather, it's a special magic setting that makes lints deny when they otherwise would have warned. This is pretty confusing and should at least be better documented.

It looks like it might also be the case that directives at the same scope can clobber each other? So

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

struct Foo;

fn main() {}

surprisingly works, but

#![forbid(dead_code)]

#[allow(dead_code)]
struct Foo;

fn main() {}

fails as expected. This is also pretty confusing.

[FaultyRAM]

That does seem to be the case, since this compiles as well:

#[forbid(dead_code)]
#[allow(dead_code)]
struct Foo;

fn main() {}

Wait, scratch that. This doesn't compile, but doesn't mention the allow:

#[allow(dead_code)]
#[forbid(dead_code)]
struct Foo;

fn main() {}

And this compiles with a warning:

#[allow(dead_code)]
#[forbid(dead_code)]
#[warn(dead_code)]
struct Foo;

fn main() {}

So it looks like when we have overlapping lint settings in the same scope, we're now dropping all but that last one.

[Enselic]

This issue can be closed because this was fixed by #81556 which added plenty of tests.

$ rustc +1.71 main.rs
warning: allow(dead_code) incompatible with previous forbid
 --> /home/martin/src/bin/src/main.rs:2:10
  |
1 | #![forbid(warnings)]
  |           -------- `forbid` level set here
2 | #![allow(dead_code)]
  |          ^^^^^^^^^ overruled by previous forbid
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
  = note: `#[warn(forbidden_lint_groups)]` on by default

warning: 1 warning emitted