add regression test

Author: RalfJung

tests/fail/rustc-error2.rs

@@ -0,0 +1,16 @@
+// Regression test for https://github.com/rust-lang/rust/issues/121508.
+struct Struct<T>(T);
+
+impl<T> std::ops::Deref for Struct<T> {
+    type Target = dyn Fn(T);
+    fn deref(&self) -> &assert_mem_uninitialized_valid::Target {
+        //~^ERROR: undeclared crate or module
+        unimplemented!()
+    }
+}
+
+fn main() {
+    let f = Struct(Default::default());
+    f(0);
+    f(0);
+}

tests/fail/rustc-error2.stderr

@@ -0,0 +1,9 @@
+error[E0433]: failed to resolve: use of undeclared crate or module `assert_mem_uninitialized_valid`
+  --> $DIR/rustc-error2.rs:LL:CC
+   |
+LL |     fn deref(&self) -> &assert_mem_uninitialized_valid::Target {
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `assert_mem_uninitialized_valid`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0433`.