fn abs(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    let lhs = self_val.as_float().unwrap();
    Ok(Value::float(lhs.abs()))
}
abs -> Float
magnitude -> Float

https://docs.ruby-lang.org/ja/latest/method/Float/i/abs.html
ruruby/src/builtin/float.rs