Add error code for unnecessary visibility qualifier

Author: GuillaumeGomez

src/librustc_privacy/diagnostics.rs

@@ -103,4 +103,26 @@ pub enum Foo {
 ```
 "##,
 
+E0449: r##"
+A visibility qualifier was used when it was unnecessary. Erroneous code
+examples:
+
+```
+struct Bar;
+
+trait Foo {
+    fn foo();
+}
+
+pub impl Bar {} // error: unnecessary visibility qualifier
+
+pub impl Foo for Bar { // error: unnecessary visibility qualifier
+    pub fn foo() {} // error: unnecessary visibility qualifier
+}
+```
+
+To fix this error, please remove the visibility qualifier when it is not
+required.
+"##,
+
 }

src/librustc_privacy/lib.rs

@@ -1045,7 +1045,8 @@ impl<'a, 'tcx> SanePrivacyVisitor<'a, 'tcx> {
         let tcx = self.tcx;
         let check_inherited = |sp: Span, vis: hir::Visibility, note: &str| {
             if vis != hir::Inherited {
-                tcx.sess.span_err(sp, "unnecessary visibility qualifier");
+                span_err!(tcx.sess, sp, E0449,
+                          "unnecessary visibility qualifier");
                 if !note.is_empty() {
                     tcx.sess.span_note(sp, note);
                 }