Shared via Rust Playground

playground.rs

macro_rules! foo {
    ($trait_:ident) => { foo! { $trait_<> } };
    ($trait_:ident < $($args:ident),* $(,)* >) => {
        impl $trait_ {
            pub fn bar<T: $trait_<$($args)*>>(&self) {}
        }
    };
}

trait Trait1 {}
trait Trait2<T> {}

foo!(Trait1);
foo!(Trait2<T>);  // error: no rules expected the token `<`

fn main() {}