[zdimension]

> 1. add two signed 64 but it's without overflowing

  fn f(x: i64, y: i64) -> Option<i64> {
    x.checked_add(y)
  }

Option<T> being the Rust equivalent for std::optional, but with all the nice ADT features so it's ergonomic to use. You can transform it to a Result<T, E> if you want, or even just panic and exit if it overflows. In any case, the overflow handling is built-in and doesn't use 128 bit ints.

> 2. Multiply two signed 64 bit ints without overflow

  x.checked_mul(y)

(https://doc.rust-lang.org/std/primitive.i64.html#method.chec..., https://doc.rust-lang.org/std/primitive.i64.html#method.chec...)