Complete rewrites have been done before. They're not impossible tasks. C to C++ (like with fish), C++ to Rust (fish again, Tor, etc), Java to C# (something I've seen), C# to C++ (evernote), Blender Game Engine to Godot (many small cases), Godot to Lumberyard (something I've seen), Unity to Godot (many small cases), etc. And there are all the myriad rewrites within the same language, or across major breaking changes (like Python 2 to Python 3).

I think "you should never do a full rewrite" is something of a reactionary response to the juvenile idea new devs and interns get every now and then. But sometimes, a rewrite really is a good idea.

But in this case, per the announcement ( https://blog.torproject.org/announcing-arti/ ), the reasons are pretty convincing. The major reasons are, unsurprisingly, all around memory safety.

Comelete rewrite are not always bad, they are _just_ very costly. You just need to do cost benefit analsys and compare it with the cost and benefit of other actions.

Fish did a complete rewrite in Rust, but they did it in the right way.

1. Rewrite each C file in Rust as a 1:1 mapping.

2. Rewrite the Rust to make it idiomatic.

3. Start making structural changes.

Well, an underrated aspect of the Rust rewrites, is that it's easy to publish and share official libraries from the projects that the community can use, something that is too hard in C land.

Nobody ever said rewrites are always a bad idea. You just made that up. It has been done successfully many times, with many more happening (e.g. TypeScript compiler rewrite).

What matters is how the project is planned and implemented.

Typically, complete rewrites that halt all feature development and fail to achieve feature parity is a recipe for disaster. On the other hand, balancing the work for feature/regular maintenance and rewrite and gradually achieving feature parity before rolling things out has worked well.

Opinions with “always” in them are always a bad idea.

I worked at a startup where the original code base was outsourced to cheap developers and was riddled with security vulnerabilities.

A complete rewrite made sense.