[const-prop] Handle ProjectionElem::Deref

Author: wesleywiser

src/librustc_mir/transform/const_prop.rs

@@ -333,6 +333,12 @@ impl<'a, 'mir, 'tcx> ConstPropagator<'a, 'mir, 'tcx> {
                             this.ecx.operand_field(eval, field.index() as u64)
                         })?;
                     },
+                    ProjectionElem::Deref => {
+                        trace!("processing deref");
+                        eval = self.use_ecx(source_info, |this| {
+                            this.ecx.deref_operand(eval)
+                        })?.into();
+                    }
                     // We could get more projections by using e.g., `operand_projection`,
                     // but we do not even have the stack frame set up properly so
                     // an `Index` projection would throw us off-track.

src/test/mir-opt/const_prop/ref_deref.rs

@@ -0,0 +1,21 @@
+fn main() {
+    *(&4);
+}
+
+// END RUST SOURCE
+// START rustc.main.ConstProp.before.mir
+// bb0: {
+//     ...
+//     _2 = &(promoted[0]: i32);
+//     _1 = (*_2);
+//     ...
+//}
+// END rustc.main.ConstProp.before.mir
+// START rustc.main.ConstProp.after.mir
+// bb0: {
+//     ...
+//     _2 = const Scalar(AllocId(0).0x0) : &i32;
+//     _1 = const 4i32;
+//     ...
+// }
+// END rustc.main.ConstProp.after.mir