From aa38b7fcacbc5443a3bc6b205cd9ef50feabc932 Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Wed, 8 Oct 2025 15:29:24 +0200
Subject: [PATCH 1/2] add `MaybeUninit` to `minicore`

---
 tests/auxiliary/minicore.rs | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/tests/auxiliary/minicore.rs b/tests/auxiliary/minicore.rs
index 24da2c4a1c4e..4830253e89d4 100644
--- a/tests/auxiliary/minicore.rs
+++ b/tests/auxiliary/minicore.rs
@@ -27,6 +27,7 @@
     decl_macro,
     f16,
     f128,
+    transparent_unions,
     asm_experimental_arch,
     unboxed_closures
 )]
@@ -127,6 +128,25 @@ pub struct ManuallyDrop<T: PointeeSized> {
 }
 impl<T: Copy + PointeeSized> Copy for ManuallyDrop<T> {}
 
+#[lang = "maybe_uninit"]
+#[repr(transparent)]
+pub union MaybeUninit<T> {
+    uninit: (),
+    value: ManuallyDrop<T>,
+}
+
+impl<T: Copy + PointeeSized> Copy for MaybeUninit<T> {}
+
+impl<T> MaybeUninit<T> {
+    pub const fn uninit() -> Self {
+        Self { uninit: () }
+    }
+
+    pub const fn new(value: T) -> Self {
+        Self { value: ManuallyDrop { value } }
+    }
+}
+
 #[repr(transparent)]
 #[rustc_nonnull_optimization_guaranteed]
 pub struct NonNull<T: ?Sized> {

From 640c4b4f1e96fe37675493ccd8ab2e3c2229fd39 Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Sun, 19 Apr 2026 18:30:12 +0200
Subject: [PATCH 2/2] cmse: test returning `MaybeUninit<T>`

The `MaybeUninit` type is `repr(transparent)`, so returning a `MaybeUninit<u64>` should work. The same is not true when using C or rust union types.
---
 .../cmse-nonsecure-call/return-via-stack.rs   | 14 +++++++----
 .../cmse-nonsecure-entry/params-via-stack.rs  | 23 +++++++++++++++++++
 2 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/tests/ui/cmse-nonsecure/cmse-nonsecure-call/return-via-stack.rs b/tests/ui/cmse-nonsecure/cmse-nonsecure-call/return-via-stack.rs
index 55160f7a0f00..a8c69216e204 100644
--- a/tests/ui/cmse-nonsecure/cmse-nonsecure-call/return-via-stack.rs
+++ b/tests/ui/cmse-nonsecure/cmse-nonsecure-call/return-via-stack.rs
@@ -42,13 +42,13 @@ struct Test {
     i128: extern "cmse-nonsecure-call" fn() -> i128, //~ ERROR [E0798]
 }
 
-#[repr(C)]
-pub union ReprCUnionU64 {
+#[repr(Rust)]
+pub union ReprRustUnionU64 {
     _unused: u64,
 }
 
-#[repr(Rust)]
-pub union ReprRustUnionU64 {
+#[repr(C)]
+pub union ReprCUnionU64 {
     _unused: u64,
 }
 
@@ -56,5 +56,11 @@ pub union ReprRustUnionU64 {
 pub fn test_union(
     f1: extern "cmse-nonsecure-call" fn() -> ReprRustUnionU64, //~ ERROR [E0798]
     f2: extern "cmse-nonsecure-call" fn() -> ReprCUnionU64,    //~ ERROR [E0798]
+
+    // MaybeUninit is a transparent union, and hence MaybeUninit<u64> is abi-compatible with u64,
+    // and thus allowed as a return type.
+    f3: extern "cmse-nonsecure-call" fn() -> MaybeUninit<u32>,
+    f4: extern "cmse-nonsecure-call" fn() -> MaybeUninit<u64>,
+    f5: extern "cmse-nonsecure-call" fn() -> MaybeUninit<f64>,
 ) {
 }
diff --git a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/params-via-stack.rs b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/params-via-stack.rs
index 2e73ef8c32f5..b46e73abad0e 100644
--- a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/params-via-stack.rs
+++ b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/params-via-stack.rs
@@ -45,3 +45,26 @@ pub extern "cmse-nonsecure-entry" fn four(_: Four) {}
 
 #[no_mangle]
 pub extern "cmse-nonsecure-entry" fn five(_: Five) {} //~ ERROR [E0798]
+
+#[repr(Rust)]
+pub union ReprRustUnionU64 {
+    _unused: u64,
+}
+
+#[repr(C)]
+pub union ReprCUnionU64 {
+    _unused: u64,
+}
+
+#[no_mangle]
+#[allow(improper_ctypes_definitions)]
+pub extern "cmse-nonsecure-entry" fn union_rust(_: ReprRustUnionU64) {}
+
+#[no_mangle]
+pub extern "cmse-nonsecure-entry" fn union_c(_: ReprCUnionU64) {}
+
+#[no_mangle]
+pub extern "cmse-nonsecure-entry" fn maybe_uninit_32bit(_: MaybeUninit<u32>) {}
+
+#[no_mangle]
+pub extern "cmse-nonsecure-entry" fn maybe_uninit_64bit(_: MaybeUninit<f64>) {}