std: change BufWriter to return ShortWrite/EndOfFile

[erickt]

Previously, BufWriter::write would just return an std::io::OtherIoError if someone attempted to write past the end of the wrapped buffer. This pull request changes the error to support partial writes and return a std::io::ShortWrite, or an io::io::EndOfFile if it's been fully exhausted.

I've also optimized away a bounds check inside BufWriter::write, which should help shave off some nanoseconds in an inner loops.

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[sfackler]

Should this be pub?

[erickt]

Good catch! Updated.

[alexcrichton]

Could this leave out the eof function for now? It would be nice to avoid expanding the public API for now. Additionally, do you have benchmark numbers for the before/after to justify the unsafe? I would expect writers with a need for speed to use the Writer implementation on Vec<u8>, not on a MemWriter (which will probably slow down with a Seek implementation in the future.

[erickt]

@alexcrichton: I removed eof and the unsafe code for now. My benchmarking showed that the safe and unsafe code performed nearly the same, but with a slight edge for unsafe code, which probably isn't that important.

Regarding speed, this is working on BufWriter, not MemWriter :) The neat thing is that even this safe version is approximately as fast as just using ptr::copy_nonoverlapping_memory. Vec<u8>/MemWriter for some reason are at least twice as slow as &[u8]/BufWriter, even if we aren't growing the underlying vector. Not sure why yet.