Add test for #43398

Closes #43398

src/test/ui/issues/issue-43398.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+#![feature(core_intrinsics)]
+#![feature(repr128)]
+
+#[repr(i128)]
+enum Big { A, B }
+
+fn main() {
+    unsafe {
+        println!("{} {:?}",
+            std::intrinsics::discriminant_value(&Big::A),
+            std::mem::discriminant(&Big::B));
+    }
+}