Function call_arg_to_outer_param 

fn call_arg_to_outer_param(op: &Operand<'_>, body: &Body<'_>) -> Option<usize>

Trace a call-site argument operand back to the enclosing callee’s parameter index (0-based), following direct Copy/Move assignment chains. Returns None for constants, field projections, call results, or values that do not trace to a parameter (those references cannot be restated on the enclosing callee).