Fish Shell 4 is written in Rust

zsh can run pretty much any POSIX command unaltered but there are definitely some of the more advanced features in zsh interactive use you couldn't just copy into a bash script without modification (a lot of the globbing stuff comes to mind).

There are also some subtleties that make it unwise to test if a command in a bash script behaves exactly as desired in zsh instead of bash, particularly around sub shells and undefined variables.