Large parts of the rewrite came from contributors who had never worked on fish before.

That’s pretty useful alone.

And there’s this:

Thread Safety

Allowing background functions and concurrent functions has been a goal for many years. I have been nursing a long-lived branch which allows full threaded execution. But though the changes are small, I have been reluctant to propose them, because they will make reasoning about the shell internals too complex: it is difficult in C++ to check and enforce what crosses thread boundaries.

This is Rust’s bread and butter: we will encode thread requirements into our types, making it explicit and compiler-checked, via Send and Sync. Rust will allow turning on concurrent mode in a safe way, with a manageable increase in complexity, finally enabling this feature.