Add check for required on oneof fields in buf lint (#2528)

This adds a check in `PROTOVALIDATE` rules in `buf lint` that checks
oneof fields do not have `(buf.validate.field).required`.

private/bufpkg/bufcheck/buflint/buflint_test.go

@@ -633,6 +633,8 @@ func TestRunProtovalidateRules(t *testing.T) {
 		bufanalysistesting.NewFileAnnotation(t, "number.proto", 134, 5, 134, 56, "PROTOVALIDATE"),
 		bufanalysistesting.NewFileAnnotation(t, "number.proto", 139, 5, 139, 50, "PROTOVALIDATE"),
 		bufanalysistesting.NewFileAnnotation(t, "number.proto", 142, 5, 142, 52, "PROTOVALIDATE"),
+		bufanalysistesting.NewFileAnnotation(t, "oneof.proto", 13, 7, 13, 43, "PROTOVALIDATE"),
+		bufanalysistesting.NewFileAnnotation(t, "oneof.proto", 19, 7, 19, 43, "PROTOVALIDATE"),
 		bufanalysistesting.NewFileAnnotation(t, "repeated.proto", 25, 5, 25, 48, "PROTOVALIDATE"),
 		bufanalysistesting.NewFileAnnotation(t, "repeated.proto", 27, 5, 27, 48, "PROTOVALIDATE"),
 		bufanalysistesting.NewFileAnnotation(t, "repeated.proto", 45, 5, 45, 48, "PROTOVALIDATE"),

private/bufpkg/bufcheck/buflint/internal/buflintvalidate/field.go

@@ -178,6 +178,16 @@ func checkConstraintsForField(
 	if fieldDescriptor.IsExtension() {
 		checkConstraintsForExtension(adder, fieldConstraints)
 	}
+	if fieldDescriptor.ContainingOneof() != nil && fieldConstraints.GetRequired() {
+		adder.addForPathf(
+			[]int32{requiredFieldNumber},
+			"Field %q has %s but is in a oneof (%s). Oneof fields must not have %s.",
+			adder.fieldName(),
+			adder.getFieldRuleName(requiredFieldNumber),
+			fieldDescriptor.ContainingOneof().Name(),
+			adder.getFieldRuleName(requiredFieldNumber),
+		)
+	}
 	checkFieldFlags(adder, fieldConstraints)
 	if err := checkCELForField(
 		adder,

private/bufpkg/bufcheck/buflint/testdata/protovalidate_rules/oneof.proto

@@ -0,0 +1,25 @@
+syntax = "proto3";
+
+package what;
+
+import "buf/validate/validate.proto";
+import "google/protobuf/duration.proto";
+
+message OneofTest {
+  oneof foo {
+    int32 v1 = 1 [
+      (buf.validate.field).int32.lt = 3,
+      // must not have required
+      (buf.validate.field).required = true
+    ];
+    string v2 = 2;
+    google.protobuf.Duration v3 = 3 [
+      (buf.validate.field).duration.gt = {seconds: 5},
+      // must not have required
+      (buf.validate.field).required = true,
+      (buf.validate.field).duration.lt = {seconds: 10}
+    ];
+  }
+  // required is OK for non-oneof fields
+  string f4 = 4 [(buf.validate.field).required = true];
+}