diff --git a/src/librustc_mir/interpret/mod.rs b/src/librustc_mir/interpret/mod.rs
index 82fe08fa038a4..96ea0d5094966 100644
--- a/src/librustc_mir/interpret/mod.rs
+++ b/src/librustc_mir/interpret/mod.rs
@@ -39,6 +39,6 @@ pub use self::machine::{Machine, AllocMap, MayLeak};
 
 pub use self::operand::{ScalarMaybeUndef, Immediate, ImmTy, Operand, OpTy};
 
-pub use self::visitor::ValueVisitor;
+pub use self::visitor::{ValueVisitor, MutValueVisitor};
 
 pub use self::validity::RefTracking;
diff --git a/src/librustc_mir/interpret/validity.rs b/src/librustc_mir/interpret/validity.rs
index a342cf6f64766..ad7ffd291bed1 100644
--- a/src/librustc_mir/interpret/validity.rs
+++ b/src/librustc_mir/interpret/validity.rs
@@ -21,7 +21,7 @@ use rustc::mir::interpret::{
 };
 
 use super::{
-    OpTy, MPlaceTy, ImmTy, Machine, EvalContext, ValueVisitor
+    OpTy, MPlaceTy, Machine, EvalContext, ValueVisitor
 };
 
 macro_rules! validation_failure {
@@ -281,8 +281,9 @@ impl<'rt, 'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>>
         }
     }
 
-    fn visit_primitive(&mut self, value: ImmTy<'tcx, M::PointerTag>) -> EvalResult<'tcx>
+    fn visit_primitive(&mut self, value: OpTy<'tcx, M::PointerTag>) -> EvalResult<'tcx>
     {
+        let value = self.ecx.read_immediate(value)?;
         // Go over all the primitive types
         let ty = value.layout.ty;
         match ty.sty {
diff --git a/src/librustc_mir/interpret/visitor.rs b/src/librustc_mir/interpret/visitor.rs
index 4a47045643253..f0a71242599bf 100644
--- a/src/librustc_mir/interpret/visitor.rs
+++ b/src/librustc_mir/interpret/visitor.rs
@@ -8,7 +8,7 @@ use rustc::mir::interpret::{
 };
 
 use super::{
-    Machine, EvalContext, MPlaceTy, OpTy, ImmTy,
+    Machine, EvalContext, MPlaceTy, OpTy,
 };
 
 // A thing that we can project into, and that has a layout.
@@ -201,9 +201,11 @@ macro_rules! make_value_visitor {
             { Ok(()) }
 
             /// Called whenever we reach a value of primitive type.  There can be no recursion
-            /// below such a value.  This is the leave function.
+            /// below such a value.  This is the leaf function.
+            /// We do *not* provide an `ImmTy` here because some implementations might want
+            /// to write to the place this primitive lives in.
             #[inline(always)]
-            fn visit_primitive(&mut self, _val: ImmTy<'tcx, M::PointerTag>) -> EvalResult<'tcx>
+            fn visit_primitive(&mut self, _v: Self::V) -> EvalResult<'tcx>
             { Ok(()) }
 
             // Default recursors. Not meant to be overloaded.
@@ -279,9 +281,7 @@ macro_rules! make_value_visitor {
                     _ => v.layout().ty.builtin_deref(true).is_some(),
                 };
                 if primitive {
-                    let op = v.to_op(self.ecx())?;
-                    let val = self.ecx().read_immediate(op)?;
-                    return self.visit_primitive(val);
+                    return self.visit_primitive(v);
                 }
 
                 // Proceed into the fields.