Rust macro generate enum from another enum

Question

I am trying to solve a problem with code duplication to make it less error-prone.

I'm working with different Data Sources each with some specific properties. Here is how my data model looks like:

mod sources {
    pub struct Ds1;
    pub struct Ds2;
    //hundreds more

    pub trait DsTypeTrait{
        type Input;
        type Output;
    }

    impl DsTypeTrait for Ds1 {
        type Input = u32;
        type Output = u32;
    }

    impl DsTypeTrait for Ds2 {
        type Input = String;
        type Output = String;
    }

    //etc...

    enum DataSource{
        Ds1(Ds1),
        Ds2(Ds2),
        //...
    }
}

So any time somebody wants to add support for a new data source they need to add it to the enum DataSource. The PROBLEM with the solution is that if another module contains some custom data format for, e.g. DataSource communications it would be required to add the DataSource in the 2 places which is extremely error-prone. For example:

mod some_other {
    use super::sources::*;

    struct DataSourceDataInputOutput<Ds: DsTypeTrait>{
        input: <Ds as DsTypeTrait>::Input,
        output: <Ds as DsTypeTrait>::Output
    }

    enum DataSourceIO{
        Ds1(DataSourceDataInputOutput<Ds1>),
        Ds2(DataSourceDataInputOutput<Ds2>)
        //Extremely easy to forget to add here
    }
}

QUESTION: Given the enum DataSource is it possible to write a macro to generate enum DataSourceIO automatically and avoid modifying enum DataSourceIO manually each time a new DataSource is added?

Some Name Some Name · Accepted Answer · 2020-09-17T18:15:01

It seems enum parsing is a complicated issue so it is reasonable to move the DataSource actual declaration in the macro itself. Here is what I came up with:

#[macro_use]
mod sources {
    macro_rules! data_sources {
        ($name:ident, $type:tt) => {
            enum $name{
                Ds1($type<Ds1>),
                Ds2($type<Ds2>)
            }
        }
    }

    //...
}

mod some_other {

    struct DataSourceDataInputOutput<Ds: DsTypeTrait>{
        input: <Ds as DsTypeTrait>::Input,
        output: <Ds as DsTypeTrait>::Output
    }
    
    data_sources!(Test, DataSourceDataInputOutput);

    fn test_fn() {
        let ds1io: DataSourceDataInputOutput<Ds1> = DataSourceDataInputOutput{
            input: 1,
            output: 2
        };
        let test = DataSourceIO::Ds1(ds1io);  //works ok
    }

}

Rust macro generate enum from another enum

1 Answers