std: Implement io traits for io::Cursor of Vec references

[ruuda]

For example, io::Cursor<&Vec<u8>> did not implement io::Read, whereas io::Cursor<Vec<u8>> did. There might be a good reason for this that I missed, but it is not impossible as this PR demonstrates.

Constructing a cursor from a reference is useful when you already have the vector. You can temporarily wrap it in a cursor to read from it or write to it, and continue using the vector when the cursor goes out of scope. There is no need to call into_inner.

I don’t know what the procedure is for stability attributes, so I did not add them yet.

r? @aturon

[bluss]

You could use Cursor<&[u8]> and similar, right? &Vec<T> is almost always replaceable with &[T]

[ruuda]

For reading, yes, but for writing, no, because &mut [u8] cannot grow, and &mut Vec<u8> can.

[bluss]

Ah but &mut Vec doesn't need a cursor (its length is the cursor).

[ruuda]

It does if you want it to be Seek + Write.

[alexcrichton]

Due to the way stability of trait implementations works, we'd be forced to mark all these with #[stable(feature = "...", since = "1.1.0")] (in terms of stability attributes), and I agree with @bluss that &Vec<u8> is probably too similar to &[u8] to add for now.

[Gankro]

@alexcrichton Should this be closed?

Edit: It occurs to me that doing lazy-sunday triage when we're doing hardcore triage tomorrow is kinda pointless.

[alexcrichton]

@Gankro ah yes this has gone a little stale, so I think I'll close this for now. We can always continue to explore these options in the future though!