Defining metadata types in rust

A key feature of the API is that metadata is specified on a per-table basis. In other words, a type to be used as node metadata implements the tskit::metadata::NodeMetadata trait.

Using the tskit cargo feature derive, we can use procedural macros to define metadata types. Here, we define a metadata type for a mutation table:

    #[derive(serde::Serialize, serde::Deserialize, tskit::metadata::MutationMetadata)]
    struct MutationMetadata {
        effect_size: f64,
        dominance: f64,

We require that you also manually specify the serde derive macros because the metadata API itself does not depend on serde. Rather, it expects raw bytes and serde happens to be a good way to get them from your data types.

The derive macro also enforces some helpful behavior at compile time. You will get a compile-time error if you try to derive two different metadata types for the same rust type. The error is due to conflicting implementations for a supertrait.