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I'm working with an old Rust module that uses the extprim crate to provide a u128 type.

I'm trying to use this with a newer crate that uses Rust's primitive u128 type (available since Rust 1.26).

What's an efficient way to convert back and forth between these two types?

Update:

When your rustc version is greater than 1.26.0 the From trait is implemented and you can use into respectively from easily.

For a lower version than that see below.

As a note: "The most efficient way" is very subjective.

I would use the low64() and high64() methods to generate a rust u128.

extern crate extprim; // 1.6.0

use extprim::u128;

fn main() u128::from(number.low64());
 println!(":?", number);
 assert_eq!(number.to_string(), real_number.to_string());

 // and back
 let old_number = u128::u128::from_parts((real_number >> 64) as u64, (real_number) as u64);
 assert_eq!(number, old_number);

(playground)

Since you can't compare both directly, I used the to_string() function to convert them to a string and compare those.