Add other variants to std::io::ErrorKind enum

[Alphare]

I was trying to rewrite Python code in Rust and I need to match against what could be NotADirectory (ENOTDIR) and saw that ErrorKind's variants do not include many errnos present in libc - to speak only of the Unix platform - including the one I need.

Is there a reason why those variants were not implemented? Could this be my first contribution to Rust?

[Mark-Simulacrum]

The docs do say "This list is intended to grow over time and it is not recommended to exhaustively match against it." currently (and we enforce non exhaustive matching via an attribute); I think our hands technically aren't tied here but we'll want to be very cautious.

@rust-lang/libs -- does anyone have thoughts on growing io::ErrorKind to have more variants? I don't recall us doing so in the last several years...

To address your specific concern, you can use raw_os_error() on io::Error and compare with libc's ENOTDIR as needed.

[alexcrichton]

I agree that it's been so long since we've added one that we should probably do so carefully (aka use crater) but I think in general it's fine to add more.

[SimonSapin]

The enum has #[non_exhaustive] so I’m not too worried a priori.

[Alphare]

Is this something I can reasonably tackle as a first contribution?

[SimonSapin]

Add variants sounds fine, but when would NotADirectory be returned? If it’s in existing APIs, what do they return today?

[Alphare]

@SimonSapin I think one case where it can happen is when you try a filesystem operation on a/b where a is not a directory. As of now it falls into the Other variant, but the match clause should translate this constant into NotADirectory.