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If your use case can be split into phases you can just allocate memory from an arena, copy out whatever needs to survive the phase at the end and free all the memory at once. That takes care of 90%+ of all allocations I ever need to do in my work.

For the rest you need more granular manual memory management, and defer is just a convenience in that case compared to C.

I can have graphs with pointers all over the place during the phase, I don't have to explain anything to a borrow checker, and it's safe as long as you are careful at the phase boundaries.

Note that I almost never have things that need to survive a phase boundary, so in practice the borrow checker is just a nuissance in my work.

There other use cases where this doesn't apply, so I'm not "anti borrow checker", but it's a tool, and I don't need it most of the time.

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Rusky 5 days ago | parent [-]

You can explain this sort of pattern to the borrow checker quite trivially: slap a single `'arena` lifetime on all the references that point to something in that arena. This pattern is used all over the place, including rustc itself.

(To be clear I agree that this is an easy pattern to write correctly without a borrow checker as well. It's just not a good example of something that's any harder to do in Rust, either.)

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francasso 5 days ago | parent [-]

I remember having multiple issues doing this in rust, but can't recall the details. Are you sure I would just be able to have whatever refs I want and use them without the borrow checker complaining about things that are actually perfectly safe? I don't remember that being the case.

Edit: reading wavemode comment above "Namely, in Rust it is undefined behavior for multiple mutable references to the same data to exist, ever. And it is also not enough for your program to not create multiple mut - the compiler also has to be able to prove that it can't." that I think was at least one of the problems I had.

	▲	steveklabnik 5 days ago \| parent \| next [-]
		The main issue with using arenas in Rust right now is that the standard library collections use the still-unstable allocator API, so you cannot use those with them. However, this is a systems language, so you can use whatever you want for your own data structures. > reading wavemode comment above This is true for `&mut T` but that isn't directly related to arenas. Furthermore, you can have multiple mutable aliased references, but you need to not use `&mut T` while doing so: you can take advantage of some form of internal mutability and use `&T`, for example. What is needed depends on the circumstances.
	▲	Rusky 5 days ago \| parent \| prev [-]
		wavemode's comment only applies to `&mut T`. You do not have to use `&mut T` to form the reference graph in your arena, which indeed would be unlikely to work out.