Add FXIME test

Author: JohnTitor

src/test/ui/rfcs/rfc-2574-simd-ffi/simd-ffi-incorrect-type-use.rs

@@ -0,0 +1,20 @@
+// only-x86_64
+// check-pass <- FIXME, see the below comment.
+
+// The current stable rustc accepts this but it should fail.
+
+#![allow(improper_ctypes, improper_ctypes_definitions)]
+
+use std::arch::x86_64::__m128;
+
+extern "C" {
+    pub fn a(x: A);
+    pub fn b(x: B);
+}
+
+#[repr(transparent)] pub struct A(__m128);
+#[repr(C)] pub struct B(__m128);
+
+pub extern "C" fn foo(x: __m128) -> __m128 { x }
+
+fn main() {}