Centralize on using Binder to introduce new binding levels, rather than having FnSig carry an implicit binding level. This means that we be more typesafe in general, since things that instantiate bound regions can drop the Binder to reflect that.

src/librustc_driver/test.rs

@@ -275,11 +275,11 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
             onceness: ast::Many,
             store: ty::RegionTraitStore(region_bound, ast::MutMutable),
             bounds: ty::region_existential_bound(region_bound),
-            sig: ty::FnSig {
+            sig: ty::Binder(ty::FnSig {
                 inputs: input_tys.to_vec(),
                 output: ty::FnConverging(output_ty),
                 variadic: false,
-            },
+            }),
             abi: abi::Rust,
         })
     }