no_implicit_prelude attribute is not validated properly with unused_attributes lint

[ehuss]

The no_implicit_prelude attribute can only be applied to modules and the crate root. However, it seems to be silently ignored in all other positions:

#[no_implicit_prelude]
fn f() {
    #[no_implicit_prelude]
    {
        #[no_implicit_prelude]
        let s = String::new(); // OK
    }
}

I expected to see this happen: The UNUSED_ATTRIBUTES lint should trigger for all positions where it is ignored (with a future-incompatible note, since this should never have been allowed), possibly transitioned to a hard error in the future.

Instead, this happened: Attribute is ignored without a warning.

NOTE: There is a test which checks this, but it is only firing because there is already a crate-level no_implicit_prelude attribute.

rustc 1.51.0-nightly (c97f11af7 2021-01-10)