Add test for #30079

src/test/compile-fail/issue-30079.rs

+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+#![allow(unused)]
+
+struct SemiPriv;
+
+mod m1 {
+    struct Priv;
+    impl ::SemiPriv {
+        pub fn f(_: Priv) {} //~ WARN private type in public interface
+        //~^ WARNING hard error
+    }
+
+    impl Priv {
+        pub fn f(_: Priv) {} // ok
+    }
+}
+
+mod m2 {
+    struct Priv;
+    impl ::std::ops::Deref for ::SemiPriv {
+        type Target = Priv; //~ WARN private type in public interface
+        //~^ WARNING hard error
+        fn deref(&self) -> &Self::Target { unimplemented!() }
+    }
+
+    impl ::std::ops::Deref for Priv {
+        type Target = Priv; // ok
+        fn deref(&self) -> &Self::Target { unimplemented!() }
+    }
+}
+
+trait SemiPrivTrait {
+    type Assoc;
+}
+
+mod m3 {
+    struct Priv;
+    impl ::SemiPrivTrait for () {
+        type Assoc = Priv; //~ WARN private type in public interface
+        //~^ WARNING hard error
+    }
+}
+
+#[rustc_error]
+fn main() {} //~ ERROR compilation successful