Function rustc_query_impl::make_query::predicates_of

pub fn predicates_of<'tcx>(
    tcx: QueryCtxt<'tcx>, 
    key: predicates_of<'tcx>
) -> QueryStackFrame

Maps from the DefId of an item (trait/struct/enum/fn) to the predicates (where-clauses) that must be proven true in order to reference it. This is almost always the “predicates query” that you want.

predicates_of builds on predicates_defined_on – in fact, it is almost always the same as that query, except for the case of traits. For traits, predicates_of contains an additional Self: Trait<...> predicate that users don’t actually write. This reflects the fact that to invoke the trait (e.g., via Default::default) you must supply types that actually implement the trait. (However, this extra predicate gets in the way of some checks, which are intended to operate over only the actual where-clauses written by the user.)