Warn on pub extern crate.

Temporary 'fix' for #26775

Author: nrc

src/libsyntax/parse/parser.rs

@@ -5116,12 +5116,19 @@ impl<'a> Parser<'a> {
         try!(self.expect(&token::Semi));
 
         let last_span = self.last_span;
+
+        if visibility == ast::Public {
+            self.span_warn(mk_sp(lo, last_span.hi),
+                           "`pub extern crate` does not work as expected and should not be used. \
+                            Likely to become an error. Prefer `extern crate` and `pub use`.");
+        }
+
         Ok(self.mk_item(lo,
-                     last_span.hi,
-                     ident,
-                     ItemExternCrate(maybe_path),
-                     visibility,
-                     attrs))
+                        last_span.hi,
+                        ident,
+                        ItemExternCrate(maybe_path),
+                        visibility,
+                        attrs))
     }
 
     /// Parse `extern` for foreign ABIs

src/test/compile-fail/warn-pub-extern-crate.rs

@@ -0,0 +1,18 @@
+// 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.
+
+// Check that `pub extern crate` gives a warning.
+
+
+pub extern crate core; //~WARN `pub extern crate` does not work
+//~^ ERROR core
+
+fn main() {
+}