Rust meta / compound structs with marker traits

Question

I have multiple existing structs that I want to share between code contexts using a pubsub mechanism with shared "topics" (where each topic publishes a single type). Many topics reuse the same struct types, so I'd like to somehow mark the original structs to reuse them for those topics. Something like:

trait TopicMeta {
    const TOPIC: &'static str;
}

struct Point {
    x: u32,
    y: u32,
}

impl TopicMeta for Point as HomeLocation  {
    const TOPIC: &'static str = "home";
}

impl TopicMeta for Point as RoverLocation  {
    const TOPIC: &'static str = "rover";
}

However, as far as I can tell there's no way to do this?

Other options:

• I could create a new type for each topic, identical to the original Point struct except for its name.
• I could create a wrapper object for each topic with an inner field that contains the Point.

Any suggestions?

Answer 1

A common way to do this is to add a field to your type and use unit struct as tags:

trait TopicMeta {
    const TOPIC: &'static str;
}

struct Point<T> {
    x: u32,
    y: u32,
    tag: T,
}

struct HomeLocation;
struct RoverLocation;

impl TopicMeta for Point<HomeLocation> {
    const TOPIC: &'static str = "home";
}

impl TopicMeta for Point<RoverLocation> {
    const TOPIC: &'static str = "rover";
}