Contracts core intrinsics.

These are hooks to:

  1. control whether contract checks are run
  2. allow 3rd party tools to intercept and reintepret the results of running contracts.

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -537,6 +537,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 ImmTy::from_uint(val, usize_layout())
             }
             UbChecks => ImmTy::from_bool(M::ub_checks(self)?, *self.tcx),
+            ContractChecks => ImmTy::from_bool(M::contract_checks(self)?, *self.tcx),
         })
     }
 }