Remove an intermediate value from discriminant reading

Author: oli-obk

src/librustc_mir/interpret/operand.rs

@@ -508,7 +508,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
 
     // Evaluate a place with the goal of reading from it.  This lets us sometimes
     // avoid allocations.
-    fn eval_place_to_op(
+    pub(super) fn eval_place_to_op(
         &self,
         mir_place: &mir::Place<'tcx>,
         layout: Option<TyLayout<'tcx>>,

src/librustc_mir/interpret/step.rs

@@ -266,8 +266,8 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             }
 
             Discriminant(ref place) => {
-                let place = self.eval_place(place)?;
-                let discr_val = self.read_discriminant(self.place_to_op(place)?)?.0;
+                let op = self.eval_place_to_op(place, None)?;
+                let discr_val = self.read_discriminant(op)?.0;
                 let size = dest.layout.size;
                 self.write_scalar(Scalar::from_uint(discr_val, size), dest)?;
             }