Add fieldmask rule to check for accept/reject paths

This adds support for the well-known type google.protobuf.FieldMask:
- accept: allowed paths (violation if unlisted paths are encountered)
- reject: disallowed paths (violation if any specified path is encountered)

```proto
message MyFieldMask {
  //  The `value` FieldMask must only contain paths listed in `accept`.
  google.protobuf.FieldMask value = 1 [(buf.validate.field).field_mask = {
      accept: ["a", "b", "c.a"]
  }];
}
```

```proto
message MyFieldMask {
  //  The `value` FieldMask shall not contain paths listed in `reject`.
  google.protobuf.FieldMask value = 1 [(buf.validate.field).field_mask = {
      reject: ["forbidden", "immutable", "c.a"]
  }];
}
```

Closes #428.

Signed-off-by: Christoph Hoopmann <[email protected]>

Author: choopm

proto/protovalidate-testing/buf/validate/conformance/cases/BUILD.bazel

@@ -44,6 +44,7 @@ proto_library(
         "strings.proto",
         "wkt_any.proto",
         "wkt_duration.proto",
+        "wkt_field_mask.proto",
         "wkt_nested.proto",
         "wkt_timestamp.proto",
         "wkt_wrappers.proto",
@@ -56,6 +57,7 @@ proto_library(
         "//proto/protovalidate/buf/validate:validate_proto",
         "@com_google_protobuf//:any_proto",
         "@com_google_protobuf//:duration_proto",
+        "@com_google_protobuf//:field_mask_proto",
         "@com_google_protobuf//:timestamp_proto",
         "@com_google_protobuf//:wrappers_proto",
     ],

proto/protovalidate-testing/buf/validate/conformance/cases/wkt_field_mask.proto

@@ -0,0 +1,38 @@
+// Copyright 2023-2025 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+package buf.validate.conformance.cases;
+
+import "buf/validate/validate.proto";
+import "google/protobuf/field_mask.proto";
+
+message FieldMaskNone {
+  google.protobuf.FieldMask val = 1;
+}
+message FieldMaskRequired {
+  google.protobuf.FieldMask val = 1 [(buf.validate.field).required = true];
+}
+
+message FieldMaskAccept {
+  google.protobuf.FieldMask val = 1 [(buf.validate.field).field_mask = {
+    accept: ["a", "b"]
+  }];
+}
+message FieldMaskReject {
+  google.protobuf.FieldMask val = 1 [(buf.validate.field).field_mask = {
+    reject: ["c", "d"]
+  }];
+}

proto/protovalidate/buf/validate/validate.proto

@@ -313,6 +313,7 @@ message FieldRules {
     // Well-Known Field Types
     AnyRules any = 20;
     DurationRules duration = 21;
+    FieldMaskRules field_mask = 28;
     TimestampRules timestamp = 22;
   }
 
@@ -4605,6 +4606,53 @@ message DurationRules {
   extensions 1000 to max;
 }
 
+// FieldMaskRules describe rules applied exclusively to the `google.protobuf.FieldMask` well-known type.
+message FieldMaskRules {
+  // Requires the FieldMask paths to be a subset of `accept`.
+  // If any unknown path is included, an error message is generated.
+  //
+  // ```proto
+  // message MyFieldMask {
+  //   //  The `value` FieldMask must only contain paths listed in `accept`.
+  //   google.protobuf.FieldMask value = 1 [(buf.validate.field).field_mask = {
+  //       accept: ["a", "b", "c.a"]
+  //   }];
+  // }
+  // ```
+  repeated string accept = 1 [(predefined).cel = {
+    id: "field_mask.accept"
+    expression: "!this.paths.all(f, f in getField(rules, 'accept')) ? 'value must be a subset of %s'.format([getField(rules, 'accept')]) : ''"
+  }];
+
+  // Requires the FieldMask paths to not include any subset of `reject`.
+  // If any reject path is included, an error message is generated.
+  //
+  // ```proto
+  // message MyFieldMask {
+  //   //  The `value` FieldMask shall not contain paths listed in `reject`.
+  //   google.protobuf.FieldMask value = 1 [(buf.validate.field).field_mask = {
+  //       reject: ["forbidden", "immutable", "c.a"]
+  //   }];
+  // }
+  // ```
+  repeated string reject = 2 [(predefined).cel = {
+    id: "field_mask.reject"
+    expression: "!this.paths.all(f, !(f in getField(rules, 'reject'))) ? 'value must not contain any subset of %s'.format([getField(rules, 'reject')]) : ''"
+  }];
+
+  // Extension fields in this range that have the (buf.validate.predefined)
+  // option set will be treated as predefined field rules that can then be
+  // set on the field options of other fields to apply field rules.
+  // Extension numbers 1000 to 99999 are reserved for extension numbers that are
+  // defined in the [Protobuf Global Extension Registry][1]. Extension numbers
+  // above this range are reserved for extension numbers that are not explicitly
+  // assigned. For rules defined in publicly-consumed schemas, use of extensions
+  // above 99999 is discouraged due to the risk of conflicts.
+  //
+  // [1]: https://github.com/protocolbuffers/protobuf/blob/main/docs/options.md
+  extensions 1000 to max;
+}
+
 // TimestampRules describe the rules applied exclusively to the `google.protobuf.Timestamp` well-known type.
 message TimestampRules {
   // `const` dictates that this field, of the `google.protobuf.Timestamp` type, must exactly match the specified value. If the field value doesn't correspond to the specified timestamp, an error message will be generated.