Repository: rust
Issue: Tracking Issue for `RawOsError`
Labels: T-libs-api,C-tracking-issue

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Feature gate: `#![feature(raw_os_error_ty)]`

This is a tracking issue for the raw OS error type alias, used in [`io::Error::raw_os_error`](https://doc.rust-lang.org/nightly/std/io/struct.Error.html#method.raw_os_error) and [`io::Error::from_raw_os_error`](https://doc.rust-lang.org/nightly/std/io/struct.Error.html#method.from_raw_os_error) (both are stable APIs).

`std` used to assume that the OS error type (return by `errno` and such) is an `i32`. This is however not the case on platforms like UEFI (which uses `usize`). To ensure idiomatic and convenient handling of native errors without loosing information, the `RawOsError` type is introduced to allow future ports of the standard library to use the correct error type for their respective platform. As this does not change the error type on already supported targets, this is a backwards-compatible change. Current software can still use `i32` directly while the `RawOsError` type alias is unstable. However, this feature should probably be stabilized before ports using a different type are merged, to allow software to be more easily ported.

### Public API

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```rust
// std::io

pub type RawOsError = /* platform error type, `i32` most of the time */;
```

### Steps / History

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- [x] API change proposal: rust-lang/libs-team#173
- [x] Implementation: #107519
- [ ] Final comment period (FCP)[^1]
- [ ] Stabilization PR

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### Unresolved Questions

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- None yet.

[^1]: https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html