errno: io::Error to Errno conversion sugar

[lucab]

I'd like to have some conversion sugar to get an nix::errno::Errno from a std::io::Error. That is, going in the opposite direction of #613.

Strawman would be:

/// Convert into an `Errno`.
pub trait AsErrno {
    /// Try to convert into an `Errno`.
    ///
    /// If the conversion is not semantically meaningful, `None` is returned.
    fn as_errno(&self) -> Option<Errno>;
}

impl AsErrno for std::io::Error {
    fn as_errno(&self) -> Option<Errno> {
        self.raw_os_error().map(Errno::from_i32)
    }
}

The use case is to improve ergonomics when pattern matching on stdlib results, like this:

use nix::errno::{AsErrno, Errno};
match std::io::xyz() {
        Err(ref err) if err.as_errno() == Some(Errno::EPERM) => foo(),
       _ => bar()
}

Would something like this be welcome in nix::errno?

[lucab]

Thanks @asomers! 💚