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.

Author: folkertdev

tests/ui/cmse-nonsecure/cmse-nonsecure-call/return-via-stack.rs

@@ -42,19 +42,25 @@ 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,
 }
 
 #[no_mangle]
 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>,
 ) {
 }

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>) {}