API to split owned Vec into owned Vec chunks

[NeoLegends]

I just came across the situation where it would be very useful to be able to split a Vec into several owned chunks just like std::slice::Chunks without an additional allocation. It seems to me like one would only have to update some pointers and lengths. split_off (unfortunately) allocates, so it cannot really be used for performance-critical paths.

How are your thoughts on such an API? Is this even possible with most allocators?

[sfackler]

I don't believe this is possible with any allocator Rust currently supports.

[codyps]

It might be possible without breaking Vec's current public API, but consider that we'd need to ref-count the memory allocation backing the Vec (as multiple Vec's now can refer to (different pieces of) the same memory allocation). And further accounting about what is still in use would need to be kept for more efficient memory utilization (ie: to prevent us from having 1 ref left keeping a massive allocation alive).

There may be a place for a data structure that supports that (perhaps as simple as {offs: usize, len: usize, data: Arc<Vec<T>>}), but it doesn't seem like it would be Vec itself.

[NeoLegends]

Ah, so it's not possible to yield (nomoverlapping) fragments of a previous allocation back to the allocator?

[sfackler]

Yep. The only pointers you can pass to free are those returned from malloc.

[NeoLegends]

Thanks for the clarification! Do you wanna close this or keep it in the backlog?

[steveklabnik]

We don't usually let minor feature requests like this stay open, as adding them is "send in a PR please!" and that's not possible. So I'm gonna give it a close. Someday maybe though! 😄

[NeoLegends]

Yeah I was just wondering because you moved some labels around. I might come back to this once its possible and once I am in need of something like this again. ;) In the meantime, I think I'll stick to bytes instead.