Allow reclaiming the current allocation

[shahn]

This is based on #680, where it was noted that it is hard to use BytesMut without additional allocations in some circumstances.

[shahn]

This is a draft to get some comments on whether such an API addition would be acceptable

[braddunbar]

This looks a lot like BytesMut::reserve with the allocating parts cut out, which sounds a lot like a special case of BytesMut::try_reserve(ORIGINAL_CAPACITY). What would y'all think of a more general implementation in that vein?

[shahn]

I think it is hard to know what the original capacity is, generally, as it could've been influenced by lots of hidden operations before.
In addition, try_reclaim also doesn't move any bytes internally, with the goal of being very cheap to call. To me, the allocation behaviour of the entire crate is sometimes a bit hard to predict, so a separate function for it seems sensible

[braddunbar]

I think it is hard to know what the original capacity is, generally, as it could've been influenced by lots of hidden operations before.

That's true, but presumably you know how much data you'll need to reserve regardless (otherwise dynamic allocation seems like the best strategy). So it doesn't particularly matter if you can reclaim the entire buffer, only if it can fit the data you're receiving. Or am I missing the point here?

In addition, try_reclaim also doesn't move any bytes internally, with the goal of being very cheap to call.

Very cheap relative to what though? Based on your description in #680, your only other alternative would be to allocate anyway, right? And that's definitely expensive relative to moving bytes. If that's not the alternative, what is?

[shahn]

My usecase is that I have a couple of BytesMuts and I would only allocate more memory for the current BytesMut if I couldn't reclaim the other. Basically, switch between the buffers, filling them up, while the data is processed on another thread. Once all the handles have been dropped I could reclaim the allocation and start filling the other buffer, etc.