Function rustc_incremental::persist::dirty_clean::check_config

pub(in persist::dirty_clean) fn check_config(
    tcx: TyCtxt<'_>, 
    attr: &Attribute
) -> bool

Given a #[rustc_dirty] or #[rustc_clean] attribute, scan for a cfg="foo" attribute and check whether we have a cfg flag called foo.

Also make sure that the label and except fields do not both exist.