Ensure that the static_assert! argument is a bool

src/librustc_data_structures/macros.rs

@@ -11,11 +11,12 @@
 /// A simple static assertion macro. The first argument should be a unique
 /// ALL_CAPS identifier that describes the condition.
 #[macro_export]
+#[allow_internal_unstable]
 macro_rules! static_assert {
     ($name:ident: $test:expr) => {
         // Use the bool to access an array such that if the bool is false, the access
         // is out-of-bounds.
         #[allow(dead_code)]
-        static $name: () = [()][!$test as usize];
+        static $name: () = [()][!($test: bool) as usize];
     }
 }