Add new error code for trait privacy error

Author: GuillaumeGomez

src/librustc_privacy/diagnostics.rs

@@ -0,0 +1,38 @@
+// Copyright 2014 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.
+
+#![allow(non_snake_case)]
+
+register_long_diagnostics! {
+
+E0445: r##"
+A private trait was used on a "public" type. Erroneous code example:
+
+```
+trait Foo {
+    fn dummy(&self) { }
+}
+
+pub trait Bar : Foo {} // error: private trait in exported type parameter bound
+```
+
+To solve this error, please ensure the trait is accessible at the same level of
+the type(s) on which it's implemented. Example:
+
+```
+pub trait Foo { // we set the Foo trait public
+    fn dummy(&self) { }
+}
+
+pub trait Bar : Foo {} // ok!
+```
+"##,
+
+}

src/librustc_privacy/lib.rs

@@ -1193,8 +1193,8 @@ impl<'a, 'tcx> VisiblePrivateTypesVisitor<'a, 'tcx> {
             if !self.tcx.sess.features.borrow().visible_private_types &&
                 self.path_is_private_type(trait_ref.trait_ref.ref_id) {
                     let span = trait_ref.trait_ref.path.span;
-                    self.tcx.sess.span_err(span, "private trait in exported type \
-                                                  parameter bound");
+                    span_err!(self.tcx.sess, span, E0445,
+                              "private trait in exported type parameter bound");
             }
         }
     }