Don't allow unsafe statics outside of extern blocks

compiler/rustc_ast_passes/src/errors.rs

@@ -224,6 +224,13 @@ pub struct InvalidSafetyOnBareFn {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_unsafe_static)]
+pub struct UnsafeStatic {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_bound_in_context)]
 pub struct BoundInContext<'a> {