Add test for E0152 error message improvement

src/test/compile-fail/duplicate_entry_error.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 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.
+
+// Test for issue #31788
+
+// error-pattern: duplicate entry for `panic_fmt`, first definition found in `std`
+
+#![feature(lang_items)]
+
+#[lang = "panic_fmt"]
+fn panic_fmt() -> ! {
+    loop {}
+}
+
+fn main() {}