From bbc212a14319e7a71f612eec4b1fd16989a62311 Mon Sep 17 00:00:00 2001
From: Zac Harrold <zac@harrold.com.au>
Date: Wed, 5 Aug 2026 19:55:18 +1000
Subject: [PATCH] Default `RawOsError` to `i16` on 16-bit targets

---
 library/core/src/io/error.rs        |  7 +++++--
 library/coretests/tests/io/error.rs | 11 +++++++++++
 library/coretests/tests/io/mod.rs   |  1 +
 library/coretests/tests/lib.rs      |  1 +
 4 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 library/coretests/tests/io/error.rs

diff --git a/library/core/src/io/error.rs b/library/core/src/io/error.rs
index 61abdb16d07da..502817c70bb3c 100644
--- a/library/core/src/io/error.rs
+++ b/library/core/src/io/error.rs
@@ -714,13 +714,16 @@ impl CustomOwner {
 ///
 /// 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.
+/// [`usize`] or [`i16`]. Use `as` or [`into`] conversions where applicable to
+/// ensure maximum portability.
 ///
 /// [`into`]: Into::into
 #[unstable(feature = "raw_os_error_ty", issue = "107792")]
 pub type RawOsError = cfg_select! {
     target_os = "uefi" => usize,
+    // For 16-bit AVR and MSP430, i16 is equivalent to c_int.
+    // Using i16 to be explicit.
+    target_pointer_width = "16" => i16,
     _ => i32,
 };
 
diff --git a/library/coretests/tests/io/error.rs b/library/coretests/tests/io/error.rs
new file mode 100644
index 0000000000000..62f6f6cace8b3
--- /dev/null
+++ b/library/coretests/tests/io/error.rs
@@ -0,0 +1,11 @@
+use core::io::RawOsError;
+
+/// On all targets to date, [`RawOsError`] is equivalent to a `c_int`, with the
+/// notable exception of UEFI, where it is instead defined as `usize`.
+#[test]
+fn raw_os_error_ffi_guarantees() {
+    let _: RawOsError = cfg_select! {
+        target_os = "uefi" => 0 as usize,
+        _ => 0 as core::ffi::c_int,
+    };
+}
diff --git a/library/coretests/tests/io/mod.rs b/library/coretests/tests/io/mod.rs
index 7683131a54034..9341f1d85550b 100644
--- a/library/coretests/tests/io/mod.rs
+++ b/library/coretests/tests/io/mod.rs
@@ -1,2 +1,3 @@
 mod borrowed_buf;
+mod error;
 mod io_slice;
diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index 06c5926ecc2f8..35327264ccd77 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -97,6 +97,7 @@
 #![feature(pointer_is_aligned_to)]
 #![feature(portable_simd)]
 #![feature(ptr_metadata)]
+#![feature(raw_os_error_ty)]
 #![feature(rustc_attrs)]
 #![feature(signed_bigint_helpers)]
 #![feature(slice_from_ptr_range)]