Function trait_method_body_lines 

fn trait_method_body_lines(
    call_expr: &str,
    indent: &str,
    raise_default: &str,
) -> Vec<String>

Generate R function body lines that capture the .Call() result in .val, check for a tagged rust_condition_value, and return .val. raise_default is the raise helper’s fallback call (CallAttribution::raise_default).