> But it is not 'a memory usage pattern not supported by the borrow checker'. You can absolutely write a crate with an API identical to slotmap without using unsafe.

I think that might actually be worse though, performance aside. You're performing memory / object lifetime management but the Rust borrow checker still would have no idea what's going on because now you've tricked it by using indices or an opaque handle instead of references. The program may compile just fine but could have use-after-free bugs.

At least with unsafe there's an explicit acknowledgement that the borrow checker is turned off.

Yes, using slotmap you can get "use after free"-style bugs that you would not encounter if you strictly stayed with the borrow checker. So if the borrow checker fits your purpose, by all means, go ahead.

But the borrow checker can't represent circular/self-referential structures you see very often in graphs. Nor is it convenient in some cases as it has a strict separation between references that can mutate, and those that can't, which doesn't fit all problems either because the mutable references are by necessity unique.

Note that a "use after free" in slotmap results in a None value, or a panic (exception for the C++ people), depending on which API you use. In other words, it is detected and you can handle it. It does not trigger undefined behavior, you don't get ABA-style spurious references, there are no security issues. It is not the same as the issues pointers have at all.