`#![allow(unknown_lints)]` warns about unknown lints in another module

[wtfsck]

It seems like #![allow(unknown_lints)] only works in the same file where it was added, and not in any sub modules in other files.

lib.rs:

#![allow(unknown_lints)]
#![allow(dead_code)]
// no warning
#![allow(not_a_real_lint)]

mod other;

// no warning
#[allow(not_a_real_lint)]
fn m() {}

mod mm {
    // no warning
    #[allow(not_a_real_lint)]
    fn m() {}
}

other.rs

// warning: unknown lint: `not_a_real_lint`
#![allow(not_a_real_lint)]

// warning: unknown lint: `not_a_real_lint`
#[allow(not_a_real_lint)]
fn m() {}

I expected to see this happen:

No warnings in other.rs about unknown lints

Instead, this happened:

Warnings about unknown lints in other.rs

warning: unknown lint: `not_a_real_lint`
 --> src\other.rs:2:10
  |
2 | #![allow(not_a_real_lint)]
  |          ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unknown_lints)]` on by default

warning: unknown lint: `not_a_real_lint`
 --> src\other.rs:5:9
  |
5 | #[allow(not_a_real_lint)]
  |         ^^^^^^^^^^^^^^^

warning: unknown lint: `not_a_real_lint`
 --> src\other.rs:2:10
  |
2 | #![allow(not_a_real_lint)]
  |          ^^^^^^^^^^^^^^^

warning: 3 warnings emitted

Meta

rustc --version --verbose:

rustc 1.51.0 (2fd73fabe 2021-03-23)
binary: rustc
commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0
commit-date: 2021-03-23
host: x86_64-pc-windows-msvc
release: 1.51.0
LLVM version: 11.0.1

Also repros with 1.52.0 beta.

[wtfsck]

It works correctly in 1.43.1 and reports warnings in 1.44.0 and later.

[ehuss]

I'm pretty sure this was an inadvertent regression from #69838.