Function handle_opaque_type_uses 

pub(crate) fn handle_opaque_type_uses<'tcx>(
    root_cx: &mut BorrowCheckRootCtxt<'tcx>,
    infcx: &BorrowckInferCtxt<'tcx>,
    body: &Body<'tcx>,
    universal_region_relations: &Frozen<UniversalRegionRelations<'tcx>>,
    region_bound_pairs: &RegionBoundPairs<'tcx>,
    known_type_outlives_obligations: &[PolyTypeOutlivesPredicate<'tcx>],
    location_map: &Rc<DenseLocationMap>,
    constraints: &mut MirTypeckRegionConstraints<'tcx>,
) -> Vec<DeferredOpaqueTypeError<'tcx>>

This looks at all uses of opaque types in their defining scope inside of this function.

It first uses all defining uses to compute the actual concrete type of each opaque type definition.

We then apply this inferred type to actually check all uses of the opaque.