Don't create an intermediate place when reading discriminants

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>>,
@@ -522,7 +522,8 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
                 let op = self.eval_place_to_op(&proj.base, None)?;
                 self.operand_projection(op, &proj.elem)?
             }
-
+            // FIXME(oli-obk): statics and promoteds have preevaluated `val` fields nowadays
+            // add nonallocating variants here
             _ => self.eval_place_to_mplace(mir_place)?.into(),
         };
 

src/librustc_mir/interpret/place.rs

@@ -611,7 +611,7 @@ where
     }
 
     /// Computes a place. You should only use this if you intend to write into this
-    /// place; for reading, a more efficient alternative is `eval_place_for_read`.
+    /// place; for reading, a more efficient alternative is `eval_place_to_op`.
     pub fn eval_place(
         &mut self,
         mir_place: &mir::Place<'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)?;
             }