Add Reader impl for &[u8] and Writer impl for &mut [u8]

[erickt]

This continues the work @thestinger started in #18885 (which hasn't landed yet, so wait for that to land before landing this one). Instead of adding more methods to BufReader, this just allows a &[u8] to be used directly as a Reader. It also adds an impl of Writer for &mut [u8].

[tomjakubowski]

Needs a rebase.

[erickt]

Rebased, and ready for a re-review.

[reem]

couldn't this be a ShortWrite?

[erickt]

How does an EndOfFile if self.len() is 0, and ShortWrite if it was able to partially right the file sound?

[reem]

Sounds great :)

[erickt]

Should writing a &[] to a &[] be an EndOfFile, or be okay?

[sfackler]

The &mut [u8] impl will cause some weird interactions with write!:

let mut buf = vec![];

write!(&mut buf, "hello world"); // Calls Vec's impl
write(buf, "hello world"); // Calls &mut [u8]'s impl

[erickt]

@sfackler: That is... weird. I would have expected write!(buf, "hello world") would have used the Vec impl. I wonder if this is related to #19147? cc @nikomatsakis.

[sfackler]

@erickt it's due to this line, which was apparently added to force people to pass references in like you would a function: https://github.com/rust-lang/rust/blob/master/src/libstd/macros.rs#L266

[alexcrichton]

While it's possible to implement Writer on &mut [u8], it may be going a little too far. I think @sfackler's example puts me over the edge in thinking that it's not worth implementing. I personally find it a little odd that memory buffers are I/O objects (albeit making sense technically), and I wouldn't expect a write into a &mut [u8] to be too super common anyway.

Other than that this looks great though, thanks @erickt!

[erickt]

I've updated my PR to factor out the Writer impl for &mut [u8] since everything else seems non-confrontational.

[alexcrichton]

Shouldn't this be let mut buffer = b"...";

[erickt]

Nice catch.

[alexcrichton]

Looks great, thanks @erickt! I think there may be a waiting failure in the doctest, but otherwise r=me

[sfackler]

This needs a .as_bytes().