Function rewrite_binary_call_balanced 

pub(crate) fn rewrite_binary_call_balanced<F>(
    src: &str,
    name: &str,
    make: F,
) -> String
where
    F: Fn(&str, &str) -> String,

Generic paren-balanced rewriter for two-argument calls. Walks the source, matches <name>( on a word boundary, finds the top-level ,, and replaces the whole call with the closure’s output.