std: add type alias for raw OS errors

Implement rust-lang/libs-team#173.

library/std/src/io/error.rs

@@ -88,12 +88,23 @@ impl From<alloc::ffi::NulError> for Error {
 // doesn't accidentally get printed.
 #[cfg_attr(test, derive(Debug))]
 enum ErrorData<C> {
-    Os(i32),
+    Os(RawOsError),
     Simple(ErrorKind),
     SimpleMessage(&'static SimpleMessage),
     Custom(C),
 }
 
+/// The type of raw OS error codes returned by [`Error::raw_os_error`].
+///
+/// This is an [`i32`] on all currently supported platforms, but platforms
+/// added in the future (such as UEFI) may use a different primitive type like
+/// [`usize`]. Use `as`or [`into`] conversions where applicable to ensure maximum
+/// portability.
+///
+/// [`into`]: Into::into
+#[unstable(feature = "raw_os_error_ty", issue = "none")]
+pub type RawOsError = i32;
+
 // `#[repr(align(4))]` is probably redundant, it should have that value or
 // higher already. We include it just because repr_bitpacked.rs's encoding
 // requires an alignment >= 4 (note that `#[repr(align)]` will not reduce the
@@ -579,7 +590,7 @@ impl Error {
     #[must_use]
     #[inline]
     pub fn last_os_error() -> Error {
-        Error::from_raw_os_error(sys::os::errno() as i32)
+        Error::from_raw_os_error(sys::os::errno())
     }
 
     /// Creates a new instance of an [`Error`] from a particular OS error code.
@@ -610,7 +621,7 @@ impl Error {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    pub fn from_raw_os_error(code: i32) -> Error {
+    pub fn from_raw_os_error(code: RawOsError) -> Error {
         Error { repr: Repr::new_os(code) }
     }
 
@@ -646,7 +657,7 @@ impl Error {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
     #[inline]
-    pub fn raw_os_error(&self) -> Option<i32> {
+    pub fn raw_os_error(&self) -> Option<RawOsError> {
         match self.repr.data() {
             ErrorData::Os(i) => Some(i),
             ErrorData::Custom(..) => None,