Enable the mutable oneof accessor.

PiperOrigin-RevId: 580142622

rust/test/shared/accessors_proto3_test.rs

@@ -205,3 +205,37 @@ fn test_oneof_accessors() {
 
     assert_that!(msg.oneof_field(), matches_pattern!(OneofBytes(eq(b"123"))));
 }
+
+#[test]
+fn test_oneof_mut_accessors() {
+    use TestAllTypes_::OneofFieldMut::*;
+
+    let mut msg = TestAllTypes::new();
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(not_set(_)));
+
+    msg.oneof_uint32_set(Some(7));
+
+    match msg.oneof_field_mut() {
+        OneofUint32(mut v) => {
+            assert_eq!(v.get(), 7);
+            v.set(8);
+            assert_eq!(v.get(), 8);
+        }
+        f => panic!("unexpected field_mut type! {:?}", f),
+    }
+
+    // Confirm that the mut write above applies to both the field accessor and the
+    // oneof view accessor.
+    assert_that!(msg.oneof_uint32_opt(), eq(Optional::Set(8)));
+    assert_that!(
+        msg.oneof_field(),
+        matches_pattern!(TestAllTypes_::OneofField::OneofUint32(eq(8)))
+    );
+
+    msg.oneof_uint32_set(None);
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(not_set(_)));
+
+    msg.oneof_uint32_set(Some(7));
+    msg.oneof_bytes_mut().set(b"123");
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(OneofBytes(_)));
+}

rust/test/shared/accessors_test.rs

@@ -696,6 +696,40 @@ fn test_oneof_accessors() {
     assert_that!(msg.oneof_field(), matches_pattern!(OneofBytes(eq(b"123"))));
 }
 
+#[test]
+fn test_oneof_mut_accessors() {
+    use TestAllTypes_::OneofFieldMut::*;
+
+    let mut msg = TestAllTypes::new();
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(not_set(_)));
+
+    msg.oneof_uint32_set(Some(7));
+
+    match msg.oneof_field_mut() {
+        OneofUint32(mut v) => {
+            assert_eq!(v.get(), 7);
+            v.set(8);
+            assert_eq!(v.get(), 8);
+        }
+        f => panic!("unexpected field_mut type! {:?}", f),
+    }
+
+    // Confirm that the mut write above applies to both the field accessor and the
+    // oneof view accessor.
+    assert_that!(msg.oneof_uint32_opt(), eq(Optional::Set(8)));
+    assert_that!(
+        msg.oneof_field(),
+        matches_pattern!(TestAllTypes_::OneofField::OneofUint32(eq(8)))
+    );
+
+    msg.oneof_uint32_set(None);
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(not_set(_)));
+
+    msg.oneof_uint32_set(Some(7));
+    msg.oneof_bytes_mut().set(b"123");
+    assert_that!(msg.oneof_field_mut(), matches_pattern!(OneofBytes(_)));
+}
+
 macro_rules! generate_repeated_numeric_test {
     ($(($t: ty, $field: ident)),*) => {
         paste! { $(

src/google/protobuf/compiler/rust/oneof.cc

@@ -246,15 +246,26 @@ void GenerateOneofAccessors(Context<OneofDescriptor> oneof) {
             if (rs_type.empty()) {
               continue;
             }
-            // TODO: Allow mut.
-            /*oneof.Emit({
+            oneof.Emit(
+                {
                     {"case", ToCamelCase(field->name())},
-                    {"rs_getter", field->name() + "_mut"},
+                    {"rs_mut_getter", field->name() + "_mut"},
                     {"type", rs_type},
                 },
+
+                // The flow here is:
+                // 1) First find out which oneof field is already set (if any)
+                // 2) If a field is set, call the corresponding field's _mut()
+                // and wrap that Mut<> in the SomeOneofMut eum.
+                // During step 2 this code uses try_into_mut().unwrap() instead
+                // of .or_default() so that it will panic if step 1 says that
+                // the field is set, but then the _mut() accessor for the
+                // corresponding field shows as unset; if that happened it would
+                // imply a severe error in protobuf code; .or_default() would
+                // silently continue and cause the field to become set on the
+                // message, which is not the intended behavior.
                 R"rs($Msg$_::$case_enum_name$::$case$ =>
-               $Msg$_::$mut_enum_name$::$case$(self.$rs_getter$()), )rs");
-            */
+               $Msg$_::$mut_enum_name$::$case$(self.$rs_mut_getter$().try_into_mut().unwrap()), )rs");
           }
         }},
        {"case_thunk", Thunk(oneof, "case")}},