Start implement a regression test for rust#51798.

Derived from example in jeremyBanks/0dmg@236d70c

Author: Jeremy Banks

src/test/compile-fail/auxiliary/issue-51798.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 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.
+
+#![crate_name="rust_issue_51798_example_child"]
+#![crate_type = "lib"]
+
+pub fn vec() -> Vec<u8> { vec![] }

src/test/compile-fail/issue-51798.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+// compile-flags:--edition=2018
+// aux-build:issue-51798.rs
+#![crate_type = "lib"]
+
+use rust_issue_51798_example_child;
+
+mod server {
+    use crate::rust_issue_51798_example_child;
+
+    fn f() {
+        let mut v = rust_issue_51798_example_child::vec();
+        v.clear();
+    }
+}