How to use captured lifetime token in function parameter

Here is the code:

struct A<'a, 'b> {
    s1: &'a str,
    s2: &'b str,
}

struct B<'a> {
    s3: &'a str,
}

macro_rules! gen_fn {
    ( $name:ident, $param:ty, < $($gen:tt),+ > ) => {
        fn $name< $($gen),+ >(param: $param< $($gen),+ >) {
            todo!()
        }
    }
}

fn main() {
    gen_fn!( op_a, A, <'a, 'b>);
    gen_fn!( op_b, B, <'a> );
}

However this code can’t compile because Rust consider $param as a single argument, and $gen is another argument, so it expect token , or ).

When I try to expand the macro, the code is generated as I expect:

fn op_a<'a, 'b>(param: A<'a, 'b>) {
    $crate::panicking::panic("not yet implemented")
}
fn op_b<'a>(param: B<'a>) {
    $crate::panicking::panic("not yet implemented")
}

Am I doing wrong or it is just unsupported.

>Solution :

You can use ident instead of ty, but only if you’re not using paths (a::b):

macro_rules! gen_fn {
    ( $name:ident, $param:ident, < $($gen:tt),+ > ) => {
        fn $name< $($gen),+ >(param: $param< $($gen),+ >) {
            todo!()
        }
    }
}

If you’re using paths, you can use a repeated ident:

macro_rules! gen_fn {
    ( $name:ident, $($param:ident)::+, < $($gen:tt),+ > ) => {
        fn $name< $($gen),+ >(param: $($param)::+< $($gen),+ >) {
            todo!()
        }
    }
}