test valid cases of c-variadic function definitions

Author: folkertdev

tests/ui/c-variadic/valid.rs

@@ -0,0 +1,29 @@
+//@ run-pass
+#![feature(c_variadic)]
+
+// In rust (and C23 and above) `...` can be the only argument.
+unsafe extern "C" fn only_dot_dot_dot(mut ap: ...) -> i32 {
+    unsafe { ap.arg() }
+}
+
+unsafe extern "C-unwind" fn abi_c_unwind(mut ap: ...) -> i32 {
+    unsafe { ap.arg() }
+}
+
+#[allow(improper_ctypes_definitions)]
+unsafe extern "C" fn mix_int_float(mut ap: ...) -> (i64, f64, *const i32, f64) {
+    (ap.arg(), ap.arg(), ap.arg(), ap.arg())
+}
+
+fn main() {
+    unsafe {
+        assert_eq!(only_dot_dot_dot(32), 32);
+        assert_eq!(abi_c_unwind(32), 32);
+
+        // Passing more arguments than expected is allowed.
+        assert_eq!(only_dot_dot_dot(32, 1i64, core::ptr::null::<i32>(), 3.14f64), 32);
+
+        let ptr = &14i32 as *const i32;
+        assert_eq!(mix_int_float(12i64, 13.0f64, ptr, 15.0f64), (12, 13.0, ptr, 15.0));
+    }
+}