Hygiene-based ident generator

playground.rs

#![feature(trace_macros)] trace_macros!(true);
/// Given N tokens, recurses N times to generate N unique identifiers
/// then calls you back at the given macro
macro_rules! idents {
    // finished recursing, do the callback
    (@rec [] -> $ids:tt $mac:ident!($($params:tt)*)) => {
        $mac!($($params)* $ids)
    };
    
    // consume one token and generate one identifier
    (@rec [$tok:tt $($toks:tt)*] -> [$($id:ident)*] $($thru:tt)*) => {
        idents!(@rec [$($toks)*] -> [$($id)* id] $($thru)*)
    };
    
    // entry point
    ([$($toks:tt)*] $mac:ident!$params:tt) => {
        idents!(@rec [$($toks)*] -> [] $mac!$params)
    }
}

macro_rules! foo {
    // callback rule: when idents! finishes recursing, we come back here
    (@cb [$($num:tt)*], [$($id:ident)*]) => {
        $(let $id = $num;)*
        println!("{:?}", [$($id),*]);
    };
    
    // macro entry point
    ($($num:tt)*) => {
        idents!([$($num)*] foo!(@cb [$($num)*],))
        //                 ^ callback specification
        //      ^ tokens for counting the recursion
    }
}

fn main() {
    foo!(1 2 3);
}