Disallow Readers that return Ok(0) from read

[mahkoh]

so that clients can assume that read returns a positive number of bytes read or an error. Here, error includes EOF (EndOfFile) and EAGAIN/EWOULDBLOCK (ResourceUnavailable).

The current docs already state that Reader implementation should never return Ok(0) if they can avoid it. This should be changed to a clear statement that Readers are never allowed to return Ok(0).

It's probably not possible to enforce this at compile- or runtime right now. If Rust gets types that can contain certain ranges of integers, the return type of read can be changed to IoResult<[1..UINT_MAX]>.

[nodakai]

Regarding to the last paragraph:

@nick29581 has written a simplistic plugin for DbC, thougth I haven't played with it yet.

• http://featherweightmusings.blogspot.com/2014/07/libhoare-pre-and-postconditions-in-rust.html

There was a discussion on the use of unsigned integers:

• integer type style guidelines rust-guidelines#24 (comment)

Google C++ style guide discouraging the use of unsigned drew my attention.

Also note that 32 bit Windows with 3G/1G split (4GT) is probably the only OS whose read(2)-like syscall can process more than ssize_max bytes at once.

[alexcrichton]

This was fixed with the new std::io design.