Function generic_param_impl_layouts 

fn generic_param_impl_layouts<'tcx>(
    tcx: TyCtxt<'tcx>,
    caller: DefId,
    ty: Ty<'tcx>,
) -> Vec<TyAndLayout<'tcx, Ty<'tcx>>>

Collect the layouts of every concrete implementor of a generic type parameter’s trait bounds (empty for non-param types).