Lots of (most?) rust libraries are "not thread safe" in the sense that those data structures aren't though. The reason this is ok is that to actually share a piece of data across threads it needs to implement Sync (which requires unsafe).

Also, as a sibling comment points out, the "thread safety" guarantee is relatively narrow -- it won't guarantee a general lack of concurrency bugs any more than a gc will guarentee reasonable memory usage.

I haven't thought through the details, but I suspect it's possible to adjust the semantics of rust in a way that allows multiple mutable references without sacrificing memory safety.

What you would hit is that the compiler would be hindered when doing optimizations in the same way that c and c++ compilers are, because of pointer aliasing.

There's a trade off there, and I think you can make the argument either way.