Shared via Rust Playground

playground.rs

/// This auxiliary macro generates an ident for each argument passed to it,
/// and then calls another macro back with the generated idents.
macro_rules! gensym {
    // base case: we send all the collected idents back to the other macro
    (@go $callback:ident ($($args:tt)*) ($($i:ident)*)) => {
        $callback!($($args)* $($i)*)
    };
    
    // recursion: here, we add in a new ident named "x" (but distinct from every other "x")
    (@go $callback:ident $args:tt ($($i:ident)*) $head:tt $($tail:tt)*) => {
        gensym!(@go $callback $args ($($i)* x) $($tail)*)
        //                                     ^ notice I pass through one fewer arg ($head is gone)
        //                                  ^ new ident!!!
    };

    // entry point: here, you pass in N tokens for $scratch, and the macro recurses that many times
    ($callback:ident!$args:tt $($scratch:tt)*) => {
        gensym!(@go $callback $args () $($scratch)*)
        //      | | |      |  |   | || ^----------^ at the end are the arguments telling us how many times to recurse
        //      | | |      |  |   | ^^ in this list we'll collect generated idents
        //      | | |      |  ^---^ arguments to pass along to the other macro
        //      | | ^------^ the macro to call back
        //      ^-^ jump to inner macro rule
    }
}

/// Silly macro that uses N idents to declare some variables, init them to zero, and print them
macro_rules! thing {
    // this is the callback rule (gensym! calls this after generating idents)
    (@go [$($t:ty)*] $($i:ident)*) => {{
        $(let $i: $t = 0;)*
        
        format!("{:?}", ($($i + <$t as Default>::default()),*))
    }};

    // entry point rule
    ($($t:ty)*) => {
        gensym!(thing!(@go [$($t)*]) $(($t))*)
        //      |                  | ^------^ pass along all the types, wrapped in parens just to make sure they are one tt each
        //      ^------------------^ first part tells gensym! how to call us back
    }
}

fn main() {
    println!("{}", thing!(u8 u32 u16));
}