[Feature Request] Support standard field rules on nested message fields

[obeattie]

Feature description:

As discussed on Slack, add syntax sugar to apply standard field rules (like string.min_len) to nested message fields without requiring CEL expressions.

Problem it solves or use case:
Currently, if you want to validate a nested field in a context-specific way (e.g., require a field only in certain request messages), you must use CEL expressions. This is verbose and less discoverable than the standard field rules, and requires a manually-written message each time:

For example, to validate that user.id is not empty only in CreateUserRequest:

message User {
  string id = 1;
  string name = 2;
}

message CreateUserRequest {
  User user = 1 [
    (buf.validate.field).cel = {
      id: "user.id"
      message: "user.id must not be empty"
      expression: "this.id.size() > 0"
    }
  ];
}

This works, but requires developers to write CEL expressions that essentially duplicate existing standard rules.

Proposed implementation or solution:

Allow dot notation or nested syntax to apply standard rules to nested fields. Something like this, perhaps:

message CreateUserRequest {
  User user = 1 [
    (buf.validate.field).nested.id.string.min_len = 1
  ];
}

Contribution

Happy to provide feedback during design, but not able to implement at this time.

Examples or references

JSON Schema allows similar nested validation. I believe GraphQL input validation also supports nested field constraints.

[rodaine]

Hi, @obeattie, and thanks for the write-up!

Unfortunately, the protobuf language does not support options with dynamic types like that. The type of nested must be known to the extension where it's defined.

A workable solution is to use a map<string,FieldRules>, with keys mapping to the message's fields. Your example would look like:

message CreateUserRequest {
  User user = 1 [
    (buf.validate.field).nested = { "id": { string: { min_len: 1 } } }
  ];
}

Ergonomics aren't great, but it avoids redefining the CEL expression.