Function apply_definition_site_hidden_types 

pub(crate) fn apply_definition_site_hidden_types<'tcx>(
    infcx: &BorrowckInferCtxt<'tcx>,
    body: &Body<'tcx>,
    universal_regions: &UniversalRegions<'tcx>,
    region_bound_pairs: &RegionBoundPairs<'tcx>,
    known_type_outlives_obligations: &[PolyTypeOutlivesPredicate<'tcx>],
    constraints: &mut MirTypeckRegionConstraints<'tcx>,
    hidden_types: &mut DefinitionSiteHiddenTypes<'tcx>,
    opaque_types: &[(OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)],
) -> Vec<DeferredOpaqueTypeError<'tcx>>

This function is what actually applies member constraints to the borrowck state. It is also responsible to check all uses of the opaques in their defining scope.

It does this by equating the hidden type of each use with the instantiated final hidden type of the opaque.