“Depending on how you count, we’re at anywhere between 60% (c++ sloc removed) and 74% (rust sloc vs c++ sloc) done.”
The fish shell is rewriting itself from C++ into Rust, and the remaining work is the reader, screen handling, input and the pager. That is still about 6,500 lines of interactive code. The maintainers admit the Rust build could be about 20% slower in some cases. Rewrites always finish the easy 70% first.