Add a pass to mark doc attributes as used

[sfackler]

The unused-attribute lint currently globally whitelists all doc attributes as used, even if they're placed in contexts that rustdoc would never look at (e.g. match arms). There should be a separate pass over the AST that marks the attributes in the correct contexts.

#![warn(unused_attributes)]

fn f() {
    match 10 {
        /// test
        _ => {}
    }
}

[steveklabnik]

Triage: no change that I'm aware of.

[steveklabnik]

Triage: same as last year

[Havvy]

Triage: There's a new lint unused_doc_comments that triggers in these cases. IMO, this is good enough to close the issue as fixed. cc @QuietMisdreavus for confirmation.

[QuietMisdreavus]

@Havvy When/where was this lint added? I'm not familiar with it.

[euclio]

Added in #43009, where this case is specifically tested.

[Mark-Simulacrum]

Closing as per the above, looks like this was implemented in the unused_doc_comments lint.