There's some good discussion of how it happened here: https://www.reddit.com/r/rust/comments/7cwpbq/fearless_concurrency_in_firefox_quantum/dpt9gq0/ https://www

There's some good discussion of how it happened here: https://www.reddit.com/r/rust/comments/7cwpbq/fearless_concurrency_in_firefox_quantum/dpt9gq0/ https://www.reddit.com/r/rust/comments/7cwpbq/fearless_concu... One of the biggest contributors is "custom derive," which cuts out a lot of boilerplate. Another is that Gecko C++ is also pretty old and so doesn't rely on a lot of what is now standard. It's also just a ground-up rewrite, which has the advantage of hindsight. It's not a trick of moving code around.