Function associated_type_for_impl_trait_in_impl 

fn associated_type_for_impl_trait_in_impl(
    tcx: TyCtxt<'_>,
    trait_assoc_def_id: DefId,
    impl_fn: &ImplItem<'_>,
    disambiguator: &mut PerParentDisambiguatorState,
) -> LocalDefId

Given an trait_assoc_def_id corresponding to an associated item synthesized from an impl Trait in an associated function from a trait, and an impl_fn that represents an implementation of the associated function that the impl Trait comes from, synthesize an associated type for that impl Trait that inherits properties that we infer from the method and the associated type.