How do I convert a Rust numeric type but only if it fits? [duplicate]

Question

I want the following

(2u128.pow(x)) as u64

but such that it succeeds for x < 64 and fails for x >= 64.

Answer 1

As you correctly pointed out yourself, you should use TryFrom, but you should also make sure the exponentiation itself doesn't overflow, by using u128::checked_pow instead of u128::pow:

use std::convert::TryFrom;

let x = 129;
let y = 2u128.checked_pow(x).expect("Number too big for exponentiation");
let z = u64::try_from(y).expect("Number too big for cast");

(playground link)