#[macro_export]
macro_rules! my_macro {
    ($new_type:ident, $passed_type:ty => $body:expr) => {
        type $new_type = [$passed_type; 1];
        $body
    };
}

trait Trait<T> {
    fn trait_fn(&mut self) -> String;
}

struct Struct<T> {
    t: std::marker::PhantomData<T>,
}

impl<T> Trait<T> for Struct<T> {
    fn trait_fn(&mut self) -> String {
        my_macro! {
            A, T => {
                let x: A = [0];
                format!("{:?}", x)
            }
        }
    }
}

fn main() {
    let mut s = Struct::<u8> { t: std::marker::PhantomData };
    println!("{}", s.trait_fn());
}