add foregin type tests

add tests/ui/rfcs/rfc-1861-extern-types/comparison.rs

tests/ui/rfcs/rfc-1861-extern-types/comparison.rs

@@ -0,0 +1,35 @@
+// Foreign type tests not covering all operations
+//@ only-nightly
+//@ build-pass
+
+#![feature(extern_types)]
+
+#![allow(ambiguous_wide_pointer_comparisons)]
+
+extern "C" {
+    type ForeignType;
+}
+
+#[repr(C)]
+struct Example {
+    field: ForeignType,
+}
+
+fn main() {
+    // pointer comparison
+    let a = std::ptr::null::<ForeignType>();
+    let b = std::ptr::null::<ForeignType>();
+
+    assert!(a == b);
+
+    // field address computation
+    let p = std::ptr::null::<Example>();
+    unsafe {
+        let _ = &(*p).field;
+    }
+
+    // pointer casts involving extern types
+    let raw = std::ptr::null::<()>();
+    let ext = raw as *const ForeignType;
+    let _ = ext as *const ();
+}