0_what_we_used_to_do.rs

// This is the structure of the code #[derive(Queryable)]
// generates in diesel_derives. It's tricky, because we can't assume
// that you did `extern crate diesel;` or that anything named `diesel`
// is in scope. In a normal macro you have `$crate` for this, but
// custom derives don't have access to that so we have to do hacks to make this work.
const _IMPL_QUERYABLE_FOR_FOO: () {
    // This is kinda like if we had access to $crate
    // but this doesn't work if we try to use this code within diesel itself
    extern crate diesel;
    
    impl diesel::query_source::Queryable for Foo {
        // ...
    }
}

1_what_we_do_now.rs

// This is in Diesel itself
#[macro_export]
#[doc(hidden)]
macro_rules! __diesel_use_everything {
    () => {
        pub use $crate::*;
    };
}

// This is the generated code
const _IMPL_QUERYABLE_FOR_FOO {
    mod diesel {
        __diesel_use_everything!();
    }
    
    impl diesel::query_source::Queryable for Foo {
        // ...
    }
}