Migration Macro

macro.rs

macro_rules! build_migration_chain {
    ($type:ident = $ver:literal) => {
        impl $crate::Migrate for $type {
            type From = Self;
            const VERSION: i64 = $ver;
        }
    };
    ($first_type:ident = $first_ver:literal, $($rest:tt)*) => {
        build_migration_chain!($first_type = $first_ver);

        build_migration_chain!(@internal $first_type, $($rest)*);
    };
    (@internal $prev_type:ident, $type:ident = $ver:literal, $($rest:tt)*) => {
        impl $crate::Migrate for $type {
            type From = $prev_type;
            const VERSION: i64 = $ver;
        }

        build_migration_chain!(@internal $type, $($rest)*);
    };
    (@internal $prev_type:ident, $type:ident = $ver:literal) => {
        impl $crate::Migrate for $type {
            type From = $prev_type;
            const VERSION: i64 = $ver;
        }
    };
}

build_migration_chain!(TypeA = 0, TypeB = 1, TypeC = 2, TypeD = 3);

// Results in:
impl Migrate for TypeA {
    type From = Self;
    const VERSION: i64 = 0;
}
impl Migrate for TypeB {
    type From = TypeA;
    const VERSION: i64 = 1;
}
impl Migrate for TypeC {
    type From = TypeB;
    const VERSION: i64 = 2;
}
impl Migrate for TypeD {
    type From = TypeC;
    const VERSION: i64 = 3;
}