Deeply check that method signatures match, and allow for nested RPITITs

compiler/rustc_middle/src/arena.rs

@@ -101,6 +101,8 @@ macro_rules! arena_types {
             [decode] impl_source: rustc_middle::traits::ImplSource<'tcx, ()>,
 
             [] dep_kind: rustc_middle::dep_graph::DepKindStruct<'tcx>,
+
+            [] trait_impl_trait_tys: rustc_data_structures::fx::FxHashMap<rustc_hir::def_id::DefId, rustc_middle::ty::Ty<'tcx>>,
         ]);
     )
 }