Support CopyForDeref.

Author: cjgillot

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -119,6 +119,12 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                     self.assign_operand(state, target, operand);
                 }
             }
+            Rvalue::CopyForDeref(rhs) => {
+                state.flood(target.as_ref(), self.map());
+                if let Some(target) = self.map.find(target.as_ref()) {
+                    self.assign_operand(state, target, &Operand::Copy(*rhs));
+                }
+            }
             Rvalue::Aggregate(kind, operands) => {
                 // If we assign `target = Enum::Variant#0(operand)`,
                 // we must make sure that all `target as Variant#i` are `Top`.

tests/mir-opt/dataflow-const-prop/struct.main.DataflowConstProp.diff

@@ -88,16 +88,20 @@
           _17 = deref_copy (*_16);
           StorageLive(_12);
           _18 = deref_copy (*_16);
-          _12 = ((*_18).0: S);
+-         _12 = ((*_18).0: S);
++         _12 = const S(1_i32);
           StorageLive(_13);
           _19 = deref_copy (*_16);
-          _13 = ((*_19).1: u8);
+-         _13 = ((*_19).1: u8);
++         _13 = const 5_u8;
           StorageLive(_14);
           _20 = deref_copy (*_16);
-          _14 = ((*_20).2: f32);
+-         _14 = ((*_20).2: f32);
++         _14 = const 7f32;
           StorageLive(_15);
           _21 = deref_copy (*_16);
-          _15 = ((*_21).3: S);
+-         _15 = ((*_21).3: S);
++         _15 = const S(13_i32);
           StorageDead(_16);
           _0 = const ();
           StorageDead(_15);