[berkut]

> or intentionally don't write const because writing it everywhere clutters up the code

I don't often like being judgemental (at least publicly!), but I'd argue that's just people being very bad developers...

You could argue having to add '&mut' at call sites everywhere (i.e. opposite to the way C++ does const in terms of call site vs target site) also clutters up the code in terms of how verbose it is, but it's still largely a good thing.