In the process you avoid use-after-free, double-free, and accessing unallocated memory because the borrowers always live shorter lives than the owners, and can only access borrowed and thus allocated memory that is deallocated once the owner dies, and there is nobody else who can use it or free it once more.
You don't really avoid those things, though. In the process you end up with index-use-after-index-free, double-index-free, and accessing index-unallocated array entries.
These are the exact same bugs the borrow checker prevents in main memory, just hidden from the borrow checker by adding a layer of abstraction.
These are memory-unsafety bugs. You still get the same wrong answers caused by coding errors. Random junk values and behaviours, when dereferencing use-after-free invalid pointers that point to memory reused for a new object. It's just that pointers are now called indexes, and the borrow checker doesn't check these pointers.
It's like turning the borrow checker off for this set of objects. Those long-lived entities you mentioned act as a mechanism to enable that. That's useful to do, but nobody should be under the impression use-after-free, pointers to the wrong objects and other memory-unsafety bugs don't happen in the array index model.
A compiler can only prevent bad things from happening within a system, typically just one process. The world around it doesn’t work that way. A system is often a cache, not an owner, and caches go out of date because the world changed without notice. You want to update on what notices you get that the world changed. You can’t prevent inconsistency, only detect it and react by updating or removing outdated information.
This probably isn’t relevant within a batch compiler, but it would be for a language service in an IDE.
Games often simulate worlds where references shouldn’t own things. It would be a weird form of power to remotely prevent something from getting destroyed because you remember it. Even though both objects are within the same process, they’re modeled as independent systems where pointer ownership doesn’t happen.
It also checks that the slot you are trying to get is occupied, and if it's not it won't let you access it. It actually is forced to check that the slot is occupied because the slot is an enum with vacant and occupied variants, and the vacant variant doesn't have a value to get in the first place. In rust you can't read a data out of an enum without checking the variant first.
With these two restrictions, you can't use stale keys, you can't UAF, and you can't read junk data at all, it's enforced by the type system and the runtime checks.
You can still have keys that are invalid, but the slotmap's "get" method returns an option type, and invalid keys cause it to return "None".
So in practice, you will end up panicking if your keys become invalid (the index operator panics when the return value is "None"), or you will explicitly handle the "None" case and do whatever you want to do.
All of this only applies to this specific data structure though, and the checks do have some performance cost.
It seems that these are memory-unsafety bugs with the caveat that they have nothing to do with the memory allocator. Maybe a sort of sandboxed memory unsafety? I don’t know.
It's an application-specific memory allocator.
AFAICT, a language would need something like higher RAII [0] or linear types [1] for that. I'd love to see Rust adopt these features too one day, though it may be difficult to do backwards compatibly.
A reference is a new object that references an existing memory value. You can not store a reference unless the borrow-checker can prove that the object that stores it has a shorter lifetime than the referenced object.
That is also why you can't just pass that reference around willy-nilly, because the reference is consumed due to affine types.
I may be misunderstanding something though so feel free to correct me.