Use allow_internal_unstable in rustc itself

Author: oli-obk

src/librustc_data_structures/macros.rs

@@ -1,7 +1,8 @@
 /// A simple static assertion macro. The first argument should be a unique
 /// ALL_CAPS identifier that describes the condition.
 #[macro_export]
-#[allow_internal_unstable]
+#[cfg_attr(stage0, allow_internal_unstable)]
+#[cfg_attr(not(stage0), allow_internal_unstable(type_ascription))]
 macro_rules! static_assert {
     ($name:ident: $test:expr) => {
         // Use the bool to access an array such that if the bool is false, the access