[WIP] Allocators

[gereeter]

This includes a (mostly) ported version of Vec. Currently, there is no documentation and the doc tests on Vec fail.

[Gankra]

If you haven't before, consider checking out: https://github.com/Gankro/raw-rs/tree/master/src/alloc

[Gankra]

Also I'm wondering: why try to roll our own allocators before @pnkfelix is done their research into the matter?

[gereeter]

To the first point: Thanks! I hadn't seen raw-rs yet, though I did see your PR in the main rust repo with a similar allocator API. That PR is actually where I got the shrink/grow idea from, and my mem_descriptor module was an attempt to generalize the _array functions.

To the second point: It seemed to me that collect-rs was the perfect place to do research about allocators, complementing what @pnkfelix is doing. Collections are going to be a big use case for parameterizing on allocators, and actually writing experimental collections that use allocators is a good way to ensure that the APIs we end up with can be used cleanly and efficiently. Also, I thought that @pnkfelix was mostly working on integration with a GC via a typed allocation layer above something raw like this.

[Gankra]

You can just check if new size >= desired size here.

[Gankra]

Is there any feedback in particular you're interested in at this stage?

[Gankra]

Some quick notes:

• Composition of the array stuff is very cool!
• I don't think there's any reason not to handle zero-sized types automatically in a good Rust allocator API. Having to write that logic as a consumer of the API is tedious and error-prone. The fact that we do now is completely legacy as far as I know. Unfortunately that might not be compatible with your design as e.g. a zero-length array is equivalent to zero-sized type; maybe that's correct, though?
• I still think it's good to have "returns null" and "OOMs" versions of the API-- former for power-users, latter for most users.
• Toss Vec in proto maybe? I can understand why you put it where you did, though. Your call.
• Returning Results seems more appropriate in the shrink/grow methods, as they are legitimately failing ops.

[Gankra]

Feel free to re-open this if you find the time to flesh it out more.