Function rustc_ty_utils::assoc::associated_types_for_impl_traits_in_associated_fn

fn associated_types_for_impl_traits_in_associated_fn(
    tcx: TyCtxt<'_>,
    fn_def_id: LocalDefId
) -> &[DefId]

Given an fn_def_id of a trait or a trait implementation:

if fn_def_id is a function defined inside a trait, then it synthesizes a new def id corresponding to a new associated type for each return- position impl Trait in the signature.

if fn_def_id is a function inside of an impl, then for each synthetic associated type generated for the corresponding trait function described above, synthesize a corresponding associated type in the impl.