Add ui test for unnamed receiver type error message
diff --git a/tests/ui/unnamed_receiver.stderr b/tests/ui/unnamed_receiver.stderr
new file mode 100644
index 0000000..e66d9e3
--- /dev/null
+++ b/tests/ui/unnamed_receiver.stderr
@@ -0,0 +1,11 @@
+error: unnamed receiver type is only allowed if the surrounding extern block contains exactly one extern type; use `self: &mut TheType`
+ --> $DIR/unnamed_receiver.rs:6:14
+ |
+6 | fn f(&mut self);
+ | ^^^^^^^^^
+
+error: unnamed receiver type is only allowed if the surrounding extern block contains exactly one extern type; use `self: &TheType`
+ --> $DIR/unnamed_receiver.rs:10:20
+ |
+10 | fn f(self: &Self);
+ | ^^^^^