https://twitter.com/jarrednicholls/status/1029462295057977345

macro-code.rs

#![feature(custom_attribute)]

struct Something;
struct Bar;
trait SomeTrait {
    type Result;
}

macro_rules! mac {
    ($some:ident, $my:ty, $mys:expr) => {
        #[meta(result = $mys)]
        struct $some;

        impl SomeTrait<$some> for Something {
            type Result = $my;
        }
    };

    ($some:ident, $my:ty) => {
        mac!($some, $my, stringify!($my));
    };
}

mac!(Foo, Bar);

macro-expansion.rs

#![feature(prelude_import)]
#![no_std]
#![feature(custom_attribute)]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate std;

struct Something;
struct Bar;
trait SomeTrait {
    type
    Result;
}

macro_rules! mac(( $ some : ident , $ my : ty , $ mys : expr ) => {
                 # [ meta ( result = $ mys ) ] struct $ some ; impl SomeTrait
                 < $ some > for Something { type Result = $ my ; } } ; (
                 $ some : ident , $ my : ty ) => {
                 mac ! ( $ some , $ my , stringify ! ( $ my ) ) ; } ;);
#[meta(result = "Bar")]
struct Foo;

impl SomeTrait<Foo> for Something {
    type
    Result
    =


    Bar;
}