This has been a constant annoyance of mine whenever using Protovalidate that I'd like to clean up in every language. It should be easy in other languages, this is just for Go.

Whenever I define a CEL rule, I am constantly annoyed that I have to define an id and a message when the expression contains all the context I need. I want to be able to do this:

message Invoice {
  option (buf.validate.message).cel = {expression: "this.sub_total.currency_code == this.taxes.currency_code"};

  money.v1.Money sub_total = 5 [(buf.validate.field).required = true];
  money.v1.Money taxes = 6 [(buf.validate.field).required = true];
}

Instead, I'm all but forced to do this:

message Invoice {
  option (buf.validate.message).cel = {
    id: "same_currency_code"
    message: "sub_total and taxes cannot have different currency codes"
    expression: "this.sub_total.currency_code == this.taxes.currency_code"
  };

  money.v1.Money sub_total = 5 [(buf.validate.field).required = true];
  money.v1.Money taxes = 6 [(buf.validate.field).required = true];
}

I don't understand why I have to come up with a unique ID, and why I have to pay the mental overhead of coming up with a human-readable message when all the context I need is right in the expression.

Right now, if I don't set a message or ID, I'll get a Violation, but the String of the violation is []. This PR fixes that to default to returning the expression, and the error message is now exactly what I want:

"this.sub_total.currency_code == this.taxes.currency_code" returned false

Corollary, but both out of scope in this PR and not perfectly unfixable without a breaking change (although we can make improvements):

It's super annoying to me that (buf.validate.field).cel isn't CEL. It's a message that then has a field expression that is CEL. All I actually want to do is this:

message Invoice {
  option (buf.validate.message).cel = "this.sub_total.currency_code == this.taxes.currency_code";
}

As a next-best, I'd want to do this:

message Invoice {
  option (buf.validate.message).cel.expression = "this.sub_total.currency_code == this.taxes.currency_code";
}

But this is a Protobuf compile error because (buf.validate.message).cel is a repeated message.

In hindsight, I wish we did:

message MessageRules {
  repeated string cel = 1;
  repeated Rule rule = 2;
}

And then I could do the full version with (buf.validate.message).rule and the simple version with (buf.validate.message).cel, which also matches what I'd expect (rule is a Rule, cel is a CEL expression). But we can't do that anymore.

Perhaps we could do:

message MessageRules {
  // Or perhaps leave it non-deprecated so that we don't freak people out.
  repeated Rule cel = 3 [deprecated = true];
  repeated string cel_expr = 5;
  repeated Rule rule = 6;
}

Then I could do:

message Invoice {
  option (buf.validate.message).cel_expr = "this.sub_total.currency_code == this.taxes.currency_code";
}

But that is for a future PR. For now, I just want this to work without id or message.