Function trace_to_callee_arg 

fn trace_to_callee_arg<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &Body<'tcx>,
    operand: &Operand<'_>,
) -> Option<usize>

Trace backward from an operand (inner call arg) through Copy/Move/Cast assignments to the outer callee’s argument local, returning its index.