pub fn impl_of_method(tcx: &ctxt, def_id: DefId) -> Option<DefId>
If the given def ID describes a method belonging to an impl, return the
ID of the impl that the method belongs to. Otherwise, return None.
Prefix searches with a type followed by a colon (e.g.
fn:) to restrict the search to a given type.
Accepted types are: fn, mod,
struct (or str), enum,
trait, typedef (or
tdef).