fix missing_doc lint on private traits

Fixes #10069.

src/librustc/middle/lint.rs

@@ -942,10 +942,9 @@ fn visit_item(&mut self, it: @ast::item, _: ()) {
             // trait (which was already linted)
             ast::item_impl(_, Some(*), _, _) => return,
 
-            ast::item_trait(*) if it.vis == ast::public => {
-                self.check_attrs(it.attrs, it.id, it.span,
-                                 ~"missing documentation for a trait");
-            }
+            ast::item_trait(*) if it.vis != ast::public => return,
+            ast::item_trait(*) => self.check_attrs(it.attrs, it.id, it.span,
+                                                   ~"missing documentation for a trait"),
 
             ast::item_fn(*) if it.vis == ast::public => {
                 self.check_attrs(it.attrs, it.id, it.span,

src/test/compile-fail/lint-missing-doc.rs

@@ -45,10 +45,9 @@ pub trait C {} //~ ERROR: missing documentation
 #[allow(missing_doc)] pub trait D {}
 
 trait Bar {
-    /// dox
     fn foo();
-    fn foo2(); //~ ERROR: missing documentation
-    fn foo3(); //~ ERROR: missing documentation
+    fn foo_with_impl() {
+    }
 }
 
 impl Foo {