doesn't using these sorts of data structures turn the security safety properties of rust into silent logic errors?
replies(1):
1. All data allocated on the JavaScript heap is placed into a type-specific vector. Numbers go into the numbers vector, strings into the strings vector, and so on.
2. All heap references are type-discriminated indexes: A heap number is identified by its discriminant value and the index to which it points to in the numbers vector.
3. Objects are also split up into object kind -specific vectors. Ordinary objects go into one vector, Arrays go into another, DataViews into yet another, and so on.
4. Unordinary objects' heap data does not contain ordinary object data but instead they contain an optional index to the ordinary objects vector.
5. Objects are aggressively split into parts to avoid common use-cases having to reading parts that are known to be unused.
If this sounds interesting, I've written a few blog posts on the internals of Nova over in our blog, you can jump into that here: https://trynova.dev/blog/what-is-the-nova-javascript-engine
So, safery properties are not being silenced: The indexes definitely _are_ Rust wise unreliable where a pointer wouldn't be so bounds checks need to be done. But memory safety is not under threat here.
This does mean that we have to take care of garbage collection ourselves, Rust will not do that for us, but that was the case anyhow since Rust doesn't have a garbage collector we could use (thank heavens). If we make mistakes here, it will lead to the JavaScript heap being corrupted from the JS code point of view but from the engine point of view the memory is still fully safe: The worst thing that can happen is a panic from out of bounds vector indexing.
Note I did not say memory safety. I said security safety.
If you refer to referential safety, so that your reference to object X still refers to X later on, then that is indeed something we "lose" because we need to implement GC ourselves. But that wouldn't actually really meaningfully change with using pointers either, as updating pointers after a move would need to be done manually as well.
Using references is right out because we cannot explain the JavaScript memory ownership model to Rust: The two are simply not compatible. There are of course safe GC crates that give you reference APIs but they do the pointer updating manually on the inside (if they have moving GC anyway), so the situation doesn't meaningfully change.