Renaming builtin attributes does not work, but name resolution thinks it does

[jonas-schievink]

Forgive my sins, but:

use allow as deny;

#[deny(unused)]
fn f() {}

The #[deny] still acts as a #[deny], but the import should make it behave like #[allow]. No "unused import" lint is emitted, so name resolution thinks that the import has an effect.

[petrochenkov]

This should be an error, we have a special check for this in resolve.
I'll check why the error is not reported.

[petrochenkov]

Compare with

use global_allocator as allow;

#[allow(unused)]
fn f() {}

error[E0659]: `allow` is ambiguous (built-in attribute vs any other name)
 --> src/lib.rs:3:3
  |
3 | #[allow(unused)]
  |   ^^^^^ ambiguous name
  |
  = note: `allow` could refer to a built-in attribute
note: `allow` could also refer to the attribute macro imported here
 --> src/lib.rs:1:5
  |
1 | use global_allocator as allow;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: use `crate::allow` to refer to this attribute macro unambiguously

Looks like we just consider all (inert) built-in attributes same during this check.

[petrochenkov]

Fixed in #80031.