Fish 4.0.0 represents a significant update for the fish shell, marked by a complete rewrite of its core from C++ to Rust. This transition involved extensive changes, including over 110,000 lines of code being rewritten across more than 1,100 files, with the contributions made by over 200 authors. This 2-year project is noted for maintaining backward compatibility with existing features, ensuring that users can transition smoothly without major changes to their workflow. While some users appreciate the new Rust-based implementation for performance potential, others express concerns about certain non-bash-compatible features that may hinder their efficiency. Community discussions reflect both excitement about the new features and interest in the process of rewriting software in a different programming language, highlighting curiosity about collaboration in open source projects. There are indications of ongoing enhancements, with mentions of packaging updates through Homebrew. Overall, the rewrite is viewed positively by long-term users, with a focus on improved performance while keeping the user experience largely unchanged.