syntax: Add #[allow(unstable)] to --test expansion

This will temporarily prevent warnings generated from expanding to code that the
test harness itself uses. This solution will require tweaking around the beta
cycle, but it will prevent spurious warnings for now.

Closes #20823

src/libsyntax/test.rs

@@ -439,13 +439,19 @@ fn mk_test_module(cx: &mut TestCtxt) -> (P<ast::Item>, Option<ast::ViewItem>) {
     let item_ = ast::ItemMod(testmod);
 
     let mod_ident = token::gensym_ident("__test");
+    let allow_unstable = {
+        let unstable = P(nospan(ast::MetaWord(InternedString::new("unstable"))));
+        let allow = P(nospan(ast::MetaList(InternedString::new("allow"),
+                                           vec![unstable])));
+        attr::mk_attr_inner(attr::mk_attr_id(), allow)
+    };
     let item = ast::Item {
         ident: mod_ident,
-        attrs: Vec::new(),
         id: ast::DUMMY_NODE_ID,
         node: item_,
         vis: ast::Public,
         span: DUMMY_SP,
+        attrs: vec![allow_unstable],
     };
     let reexport = cx.reexport_test_harness_main.as_ref().map(|s| {
         // building `use <ident> = __test::main`