Add type alias for raw OS errors

[joboet]

Proposal

Problem statement

io::Error assumes OS error values always fit into an i32, but that is not true for all platforms.

Motivation, use-cases

rust-lang/rust#105861 introduces std support for UEFI applications. There is however a problem with the way native OS errors are handled in std, as the current API expects error values to fit into an i32. While this the case for all currently supported platforms, UEFI diverges from the pattern by using an usize¹ as error type. This forces code supporting UEFI to convert the native errors to custom io::Errors, which does not fit well with the current API and looses information.

Solution sketches

Add a new type alias to std::io (naming open to bikeshedding):

pub type RawOsError = /* platform error type, `i32` most of the time */;

and use it in place of i32 in io::Error::from_raw_os_error and io::Error::raw_os_error. This is a backwards-compatible change on all current platforms, but new code wishing to support platforms like UEFI will need to use the new error type to ensure portability.

CC @Ayush1325

Footnotes

1. At least according to the specification. However, error values always have the highest bit set, so isize would be more convenient... ↩

[Ayush1325]

This is a perfect suggestion.

Also, the current limitation of using i32 does not seem completely consistent inside std. The rust lang_start returns isize and not i32.

Finally, I have already merged rust-lang/rust#104001 which makes it easier to use custom entry points for custom targets and thus it would be great to have a more generic os errors.

[Amanieu]

I agree with this change, it does seem like we want a more flexible definition of a raw OS error.

@rustbot label +initial-comment-period

[joboet]

Closing in favour of the tracking issue: rust-lang/rust#107792