Lint request: unbuffered I/O usage

[olson-dan]

I and several other beginners have run into the surprising issue that Rust's default file reading and writing methods are unbuffered and buffered IO is purely opt-in with this language. It may be useful to add a lint to warn users about this, as it seems to come up from time to time when comparing programs to other languages such as Python.

[clarfonthey]

This shouldn't apply when methods like read_to_end or read_to_string are used, because no intermediate buffering is necessary in that case.

[olson-dan]

Yes, I agree, the lint would need to indicate unbuffered IO as an issue only when there is a likely performance issue.

[llogiq]

The lint would trigger on all eligible Read/Write operations where self is a File/Stdin/Stdout/StdinLock/StdoutLock/TcpStream/UnixStream. Suggest wrapping self in a BufReader/BufWriter to improve performance.

Note that there will be false positives where a) the direct IO is really intentional or b) a custom buffering solution is taken. Those are unlikely to be hit by new users, and one can deactivate this lint on a case-by-case basis. The docs should reflect this.

[pickfire]

Note that StdinLock is buffered. I will open another issue for this and it could be specifically for BufReader::new(io::stdin()).