To deal with the "openness" of Protobuf enumeration types, Prost currently represents them in the message struct as i32 and generates a Rust enum that can convert to and from i32. This is a bit awkward to work with, though.

A better approach could be to use a newtype with associated constants. To use the Syntax enum as an example, it would look like this:

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ...)]
pub struct Syntax(i32);

impl Syntax {
    pub const PROTO2: Syntax = Syntax(0);
    pub const PROTO3: Syntax = Syntax(1);

    // alternatively the field could just be public
    pub fn from_raw(raw: i32) -> Syntax {
        Syntax(raw)
    }

    pub fn as_raw(self) -> i32 {
        self.0
    }
}

This approach has a couple of advantages:

• It can be embedded directly in the message struct.
• It's more type safe since you normally won't have to interact with the raw i32 value.
• You can still use it roughly like a Rust enum, and do things like match on the variants:

match syntax {
    Syntax::PROTO2 => { ... }
    Syntax::PROTO3 => { ... }
    syntax => panic!("unknown syntax variant {}", syntax.as_raw()),
}