Why is Rust's usize to u128 conversion considered failable?

Question

Take a look:

use std::convert::{From, TryFrom};

fn main() {
  let size: usize = 42;
  let good: u128  = u128::try_from(size).unwrap(); // works fine
  let bad:  u128  = u128::from(size);              // doesn't compile!
}

As far as I know, usize is an integral type and those are never larger than 128 bits. Therefore, I see no way the usize -> u128 conversion can fail. So, why doesn't u128 implement From<usize>?

Update: Rust's documentation says:

From T for U implies Into U for T

Though usize -> u128 seems fine, u128 -> usize doesn't. OK, but why isn't Into<u128> implemented for usize instead?

Answer 1

Though usize -> u128 seems fine, u128 -> usize doesn't. OK, but why isn't Into implemented for usize instead?

Because while usize is guaranteed to always ever be at least 16 bits as far as Rust is concerned, it's not guaranteed to always ever be at most 64 bits.

It seems unlikely to ever be useful but technically nothing precludes 256 bits pointers, and since usize is guaranteed to be pointer-sized it would make the usize -> u128 conversion failible.