Fix rustdoc::private_doc_tests lint for public re-exported items

This involves changing the lint to check the access level is exported,
rather than public. The exported access level accounts for public items
and items accessible to other crates with the help of `pub use`
re-exports. The pattern of re-exporting public items from a private
module is usage seen in a number of popular crates.

Author: avitex

src/librustdoc/passes/check_doc_test_visibility.rs

@@ -131,7 +131,7 @@ crate fn look_for_tests<'tcx>(cx: &DocContext<'tcx>, dox: &str, item: &Item) {
             );
         }
     } else if tests.found_tests > 0
-        && !cx.cache.access_levels.is_public(item.def_id.expect_def_id())
+        && !cx.cache.access_levels.is_exported(item.def_id.expect_def_id())
     {
         cx.tcx.struct_span_lint_hir(
             crate::lint::PRIVATE_DOC_TESTS,

src/test/rustdoc-ui/public-reexported-item-doc-test.rs

@@ -0,0 +1,14 @@
+// check-pass
+
+#![deny(rustdoc::private_doc_tests)]
+
+mod foo {
+    /// re-exported doc test
+    ///
+    /// ```
+    /// assert!(true);
+    /// ```
+    pub fn bar() {}
+}
+
+pub use foo::bar;