This is a very tempting and commonly used strategy in Rust to bypass the borrow checker. I've used it to implement tries/DFAs with great success (though I can't find the code anymore)

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Animats 5 days ago | parent | next [-]

The trouble is, you've just replicated the problems of raw pointers. You can have dangling indices if the underlying object is reused or cleared. You can have aliasing, with two indices to the same object.

It's a problem in practice. Of the three times I've ever had to use a debugger on Rust code, two came from code someone had written to do their own index allocation. They'd created a race condition that Rust would ordinary prevent.

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IshKebab 5 days ago | parent | next [-]

You don't replicate all the problems of raw pointers. You can't have type confusion or undefined behaviour. It's totally memory safe. That's a pretty huge difference.

But I agree, it does give up some of the benefits of using native references.

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Animats 5 days ago | parent [-]

True. In some ways, that's worse. Instead of crashes, you get something working on the wrong data.

I had one bug in a renderer where I'd see object shadows moving across a 3D scene, but not the object casting the shadow. Didn't crash. That was hard to find. A dead index was still in use, and it pointed to something valid enough to be drawn.

	▲	10000truths 4 days ago \| parent \| next [-]
		Use-after-frees and ABA problems can be addressed by using generation counts with your object pool. Rust's slotmap crate handles this out of the box, AFAIK.
	▲	4 days ago \| parent \| prev [-]
		[deleted]

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rstuart4133 5 days ago | parent | prev | next [-]

> You can have dangling indices if the underlying object is reused or cleared.

You can just wrap your pointers, and then say the underlying collection they point to must outlive the pointers returned. Granted, that doesn't work so well if the collection allows removal. But if your collection is append only you've hit the sweet spot for this pattern, because with the lifetime constrained pointers and no deletions literally nothing can go wrong.

He says he's only seen it in Zig. I guess that means he hasn't done much Rust, because the Rust nudges you in that direction. Wrapped pointers are common in the language and it's libraries. They aren't used to save space. They are used to get around lifetime restrictions.

When I've done it in Rust, it has been to save space. A u32 index occupies 1/4 of the space of an Rc on 64 bit. If you're handling millions of them, it's hard to ignore. Ditto using u16 offsets to describe slices into strings I know won't be more than 64k. Again, it's a 4 fold saving on 64 bit.

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4 days ago | parent | prev [-]

[deleted]

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skeezyboy 5 days ago | parent | prev | next [-]

> This is a very tempting and commonly used strategy in Rust to bypass the borrow checker.

Are you even allowed to publicly disparage the borrow checker like that?

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bombela 5 days ago | parent | prev [-]

You don't bypass the borrow checker. Instead you use it the way it wants to be used.

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lmm 5 days ago | parent [-]

Often you are bypassing it. E.g. if you rebalance a tree then references into that tree may now be invalid, so the borrow checker will prevent you from doing that while you have live references. But you may also invalidate indices into the tree, and the borrow checker can't help you with that.

	▲	skeezyboy 5 days ago \| parent [-]
		> and the borrow checker can't help you with that. good thing youve got a brain in your nut then