Implement mut_chunks() method for MutableVector trait.

[DaGenix]

mut_chunks() returns an iterator that produces mutable slices. This is the mutable version of the existing chunks() method on the ImmutableVector trait.

EDIT: This uses only safe code now.

PREVIOUSLY:
I tried to get this working with safe code only, but I couldn't figure out how to make that work. Before #8624, the exact same code worked without the need for the transmute() call. With that fix and without the transmute() call, the compiler complains about the call to mut_slice(). I think the issue is that the mutable slice that is returned will live longer than the self parameter since the self parameter doesn't have an explicit lifetime. However, that is the way that the Iterator trait defines the next() method. I'm sure there is a good reason for that, although I don't quite understand why. Anyway, I think the interface is safe, since the MutChunkIter will only hand out non-overlapping pointers and there is no way to get it to hand out the same pointer twice.

[TeXitoi]

I manage to do that with safe code in my future mut_split() (that I'll submit later). The idea is to keep a mut_slice in the iterator, and split it. Here, that give me something like that:

            Some(idx) => {
                let mut tmp = &mut [];
                std::util::swap(&mut tmp, &mut self.v);
                let (h, t) = tmp.mut_split(idx);
                self.v = t.mut_slice_from(1);
                Some(h)
            }

[DaGenix]

@TeXitoi Cool, that does work. Thanks! I rebased to get rid of the unsafe code.

I'm still keeping a separate variable around with the remaining length of the vector which feels like a bit of duplication since the slice already knows it length. However, the size hint method is called on a &self and any attempt I make to call self.v.len() results in the error: cannot borrow an &mutin a&pointer; try an&mut instead. I feel like there might be some way around doing that as well, but, I'm not sure how.

[DaGenix]

@alexcrichton Cool, I made that update. Thanks!

EDIT: When I rebased, it looks like I knocked out the comment I was replying to. The update I made was to do: let tmp = util::replace(&mut self.v, &mut []); instead of using util::swap.

[huonw]

Is it possible to make this a double ended iterator like ChunkIter?

[DaGenix]

I can. Should I push something here or open a new pull request?

[huonw]

Pushing it here is fine. (You will need a new r+ though.)

[DaGenix]

Ok, done. Thanks!

[nikomatsakis]

On Sat, Nov 30, 2013 at 03:53:22PM -0800, DaGenix wrote:

I'm still keeping a separate variable around with the remaining length of the vector which feels like a bit of duplication since the slice already knows it length. However, the size hint method is called on a &self and any attempt I make to call self.v.len() results in the error: "cannot borrow an &mut in a & pointer; try an &mut instead". I feel like there might be some way around doing that as well, but, I'm not sure how.

See issue #9629 (there is a pending PR to fix it)