Add error code for privacy error on exported signature

Author: GuillaumeGomez

src/librustc_privacy/diagnostics.rs

@@ -35,4 +35,31 @@ pub trait Bar : Foo {} // ok!
 ```
 "##,
 
+E0446: r##"
+A private type was used in an exported type signature. Erroneous code example:
+
+```
+mod Foo {
+    struct Bar(u32);
+
+    pub fn bar() -> Bar { // error: private type in exported type signature
+        Bar(0)
+    }
+}
+```
+
+To solve this error, please ensure the type is accessible at the same level of
+the exported type signature. Example:
+
+```
+mod Foo {
+    pub struct Bar(u32); // we set the Bar type public
+
+    pub fn bar() -> Bar { // ok!
+        Bar(0)
+    }
+}
+```
+"##,
+
 }

src/librustc_privacy/lib.rs

@@ -1435,7 +1435,8 @@ impl<'a, 'tcx, 'v> Visitor<'v> for VisiblePrivateTypesVisitor<'a, 'tcx> {
         if let hir::TyPath(_, ref p) = t.node {
             if !self.tcx.sess.features.borrow().visible_private_types &&
                 self.path_is_private_type(t.id) {
-                self.tcx.sess.span_err(p.span, "private type in exported type signature");
+                span_err!(self.tcx.sess, p.span, E0446,
+                          "private type in exported type signature");
             }
         }
         visit::walk_ty(self, t)