macro_rules! spam {
    (spam: { $($t:tt)* }) => { stringify!($($t)*) };
}

macro_rules! super_spam {
    ($x:ident) => { spam: { $x $x $x } };
}

fn main() {
    // Doesn't work:
    println!(spam! { super_spam!(spam) });
}

(Playground)

Trying to do something like in downcast-rs.

Edit: The expanded colon I said refers to the one in the definition of super_spam!

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