Function rustc_resolve::late::lifetimes::is_late_bound_map

fn is_late_bound_map(
    tcx: TyCtxt<'_>,
    def_id: LocalDefId
) -> Option<&FxIndexSet<LocalDefId>>

Detects late-bound lifetimes and inserts them into late_bound.

A region declared on a fn is late-bound if:

• it is constrained by an argument type;
• it does not appear in a where-clause.

“Constrained” basically means that it appears in any type but not amongst the inputs to a projection. In other words, <&'a T as Trait<''b>>::Foo does not constrain 'a or 'b.