5 files changed, 26 insertions, 10 deletions
diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
index 3cabf24492e..324d698eaeb 100644
--- a/library/std/src/io/error.rs
+++ b/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,
diff --git a/library/std/src/io/error/repr_bitpacked.rs b/library/std/src/io/error/repr_bitpacked.rs
index 3581484050d..f94f88bac41 100644
--- a/library/std/src/io/error/repr_bitpacked.rs
+++ b/library/std/src/io/error/repr_bitpacked.rs
@@ -102,7 +102,7 @@
 //! to use a pointer type to store something that may hold an integer, some of
 //! the time.
 
-use super::{Custom, ErrorData, ErrorKind, SimpleMessage};
+use super::{Custom, ErrorData, ErrorKind, RawOsError, SimpleMessage};
 use alloc::boxed::Box;
 use core::marker::PhantomData;
 use core::mem::{align_of, size_of};
@@ -172,7 +172,7 @@ impl Repr {
     }
 
     #[inline]
-    pub(super) fn new_os(code: i32) -> Self {
+    pub(super) fn new_os(code: RawOsError) -> Self {
         let utagged = ((code as usize) << 32) | TAG_OS;
         // Safety: `TAG_OS` is not zero, so the result of the `|` is not 0.
         let res = Self(unsafe { NonNull::new_unchecked(ptr::invalid_mut(utagged)) }, PhantomData);
@@ -250,7 +250,7 @@ where
     let bits = ptr.as_ptr().addr();
     match bits & TAG_MASK {
         TAG_OS => {
-            let code = ((bits as i64) >> 32) as i32;
+            let code = ((bits as i64) >> 32) as RawOsError;
             ErrorData::Os(code)
         }
         TAG_SIMPLE => {
@@ -374,6 +374,9 @@ static_assert!((TAG_MASK + 1).is_power_of_two());
 static_assert!(align_of::<SimpleMessage>() >= TAG_MASK + 1);
 static_assert!(align_of::<Custom>() >= TAG_MASK + 1);
 
+// `RawOsError` must be an alias for `i32`.
+const _: fn(RawOsError) -> i32 = |os| os;
+
 static_assert!(@usize_eq: TAG_MASK & TAG_SIMPLE_MESSAGE, TAG_SIMPLE_MESSAGE);
 static_assert!(@usize_eq: TAG_MASK & TAG_CUSTOM, TAG_CUSTOM);
 static_assert!(@usize_eq: TAG_MASK & TAG_OS, TAG_OS);
diff --git a/library/std/src/io/error/repr_unpacked.rs b/library/std/src/io/error/repr_unpacked.rs
index d6ad55b99f5..093fde33757 100644
--- a/library/std/src/io/error/repr_unpacked.rs
+++ b/library/std/src/io/error/repr_unpacked.rs
@@ -2,7 +2,7 @@
 //! non-64bit targets, where the packed 64 bit representation wouldn't work, and
 //! would have no benefit.
 
-use super::{Custom, ErrorData, ErrorKind, SimpleMessage};
+use super::{Custom, ErrorData, ErrorKind, RawOsError, SimpleMessage};
 use alloc::boxed::Box;
 
 type Inner = ErrorData<Box<Custom>>;
@@ -18,7 +18,7 @@ impl Repr {
         Self(Inner::Custom(b))
     }
     #[inline]
-    pub(super) fn new_os(code: i32) -> Self {
+    pub(super) fn new_os(code: RawOsError) -> Self {
         Self(Inner::Os(code))
     }
     #[inline]
diff --git a/library/std/src/io/error/tests.rs b/library/std/src/io/error/tests.rs
index 9aea62a5b94..36d52aef03c 100644
--- a/library/std/src/io/error/tests.rs
+++ b/library/std/src/io/error/tests.rs
@@ -71,7 +71,7 @@ fn test_const() {
 
 #[test]
 fn test_os_packing() {
-    for code in -20i32..20i32 {
+    for code in -20..20 {
         let e = Error::from_raw_os_error(code);
         assert_eq!(e.raw_os_error(), Some(code));
         assert_matches!(
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index de528e85368..5907ba5d5fb 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -262,6 +262,8 @@ use crate::sys_common::memchr;
 
 #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
 pub use self::buffered::WriterPanicked;
+#[unstable(feature = "raw_os_error_ty", issue = "none")]
+pub use self::error::RawOsError;
 pub(crate) use self::stdio::attempt_print_to_stderr;
 #[unstable(feature = "internal_output_capture", issue = "none")]
 #[doc(no_inline, hidden)]