False positive unused_attributes lint on doc(hidden) attribute on non-trait associated item

[kpreid]

Given the following code:

pub struct Foo {}

impl Foo {
    #[doc(hidden)]
    pub const FOO: i32 = 0;
}

The current output (on nightly) is:

warning: `#[doc(hidden)]` is ignored on trait impl items
 --> src/lib.rs:4:5
  |
4 |     #[doc(hidden)]
  |     ^^^^^^^^^^^^^^ help: remove this attribute
  |
  = note: `#[warn(unused_attributes)]` on by default
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: whether the impl item is `doc(hidden)` or not entirely depends on the corresponding trait item

This warning is incorrect because this is not a trait impl, but an inherent impl.

Tested with local nightly rustc 1.63.0-nightly (cd282d7f7 2022-05-18) and Playground 2022-05-19 c0672870491e84362f76.

CC #96008 and @fmease

[fmease]

Sorry, stupid oversight that could've been prevented with a more extensive test. PR is up.

[TethysSvensson]

This bug seems to affect the current beta release. Has the fix been backported?