make ValueVisitor mut-polymorphic

src/librustc_mir/const_eval.rs

@@ -535,7 +535,7 @@ fn validate_const<'a, 'tcx>(
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
 ) -> ::rustc::mir::interpret::ConstEvalResult<'tcx> {
     let cid = key.value;
-    let mut ecx = mk_eval_cx(tcx, cid.instance, key.param_env).unwrap();
+    let ecx = mk_eval_cx(tcx, cid.instance, key.param_env).unwrap();
     let val = (|| {
         let op = ecx.const_to_op(constant)?;
         let mut ref_tracking = RefTracking::new(op);

src/librustc_mir/interpret/validity.rs

@@ -129,7 +129,7 @@ struct ValidityVisitor<'rt, 'a: 'rt, 'mir: 'rt, 'tcx: 'a+'rt+'mir, M: Machine<'a
     path: Vec<PathElem>,
     ref_tracking: Option<&'rt mut RefTracking<'tcx, M::PointerTag>>,
     const_mode: bool,
-    ecx: &'rt mut EvalContext<'a, 'mir, 'tcx, M>,
+    ecx: &'rt EvalContext<'a, 'mir, 'tcx, M>,
 }
 
 impl<'rt, 'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> ValidityVisitor<'rt, 'a, 'mir, 'tcx, M> {
@@ -188,8 +188,8 @@ impl<'rt, 'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>>
     type V = OpTy<'tcx, M::PointerTag>;
 
     #[inline(always)]
-    fn ecx(&mut self) -> &mut EvalContext<'a, 'mir, 'tcx, M> {
-        &mut self.ecx
+    fn ecx(&self) -> &EvalContext<'a, 'mir, 'tcx, M> {
+        &self.ecx
     }
 
     #[inline]
@@ -557,7 +557,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     /// This also toggles between "run-time" (no recursion) and "compile-time" (with recursion)
     /// validation (e.g., pointer values are fine in integers at runtime).
     pub fn validate_operand(
-        &mut self,
+        &self,
         op: OpTy<'tcx, M::PointerTag>,
         path: Vec<PathElem>,
         ref_tracking: Option<&mut RefTracking<'tcx, M::PointerTag>>,