That's not an array, that's a Vec. Notice that you need an exclusive mutable reference to the Vec to call this method, so if somebody else gave you a slice reference backed by that Vec, they can't do this to the underlying Vec as they no longer have exclusive access to it while you have that borrowed reference.

If you've decided to treat indexes into a Vec as handles, then yeah, you mustn't use Vec::remove because it's nonsense in your context. You also mustn't Vec::retain and numerous other "I don't care about ordering" operations, because you do care about ordering.

I'd argue it's probably sensible to make your own custom container which cares about ordering for this work since that preserves the invariant you've decided upon, rather than trust yourself not to make mistakes. You can build one out of a Vec pretty easily.

And sure enough that's exactly what the project you're referencing seems to actually do, they've got themselves a custom container which preserves their intended semantics.