[Best practice for optional in proto or default value

Question

We are setting up our rust services and using prost-build to bridge between proto <-> rust land. Our proto definitions are in proto3 Lets take the following proto message:

message Test {
string id = 1;
string body = 2;
string maybe_nullable_thing = 3;
}

This generates a struct like so:

pub struct Test {
    #[prost(string, tag="1")]
    pub id: ::prost::alloc::string::String,
    #[prost(string, tag="2")]
    pub body: ::prost::alloc::string::String,
    #[prost(string, tag="3")]
    pub maybe_nullable_thing: ::prost::alloc::string::String,
}

In other languagues where we have tried this, the fields of a proto message are optional by design and can be left out. In the example there can be cases where maybe_nullable_thing can not be set.

I can work around this issue by using the optional keyword. Altho I remember that it was not the best practice to do so(maybe I am mistaken?)

In terms of best practice with proto3 and rust in general, is it okay to use the optional keyword? If I use serde along with my Test struct I can see the default values of all the fields begin set to "".to_owned() (or empty string).

So I am not sure whats the best practice here? Would love to get some pointers on the best way forward here.

Answer 1

Looking at the readme for Tokio's PROST! tool, it appears their advice is to wrap any non-repeated and non-scalar fields, or any optional fields with Option<T>. This may or may not be different for prost-build, but it should give you a good reference to what's expected when using proto3 and Rust.

In general, however, you should wrap any value you want to be optional in Option<T>. This is not a bad practice, this is the default, standard way to represent "maybe nullable things" in Rust.