Rollup merge of rust-lang#31818 - GuillaumeGomez:error_display, r=brson

Fixes rust-lang#31788

Author: Manishearth

src/librustc/middle/lang_items.rs

@@ -184,10 +184,13 @@ impl<'a, 'tcx> LanguageItemCollector<'a, 'tcx> {
         // Check for duplicates.
         match self.items.items[item_index] {
             Some(original_def_id) if original_def_id != item_def_id => {
+                let cstore = &self.session.cstore;
                 span_err!(self.session, span, E0152,
-                    "duplicate entry for `{}`", LanguageItems::item_name(item_index));
+                          "duplicate entry for `{}`, first definition found in `{}`",
+                          LanguageItems::item_name(item_index),
+                          cstore.crate_name(item_def_id.krate));
             }
-            Some(_) | None => {
+            _ => {
                 // OK.
             }
         }

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() {}