Fail when using safe/unsafe items inside unadorned extern blocks

compiler/rustc_ast_passes/src/errors.rs

@@ -216,6 +216,15 @@ pub enum ExternBlockSuggestion {
     },
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_extern_invalid_safety)]
+pub struct InvalidSafetyOnExtern {
+    #[primary_span]
+    pub item_span: Span,
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub block: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_bound_in_context)]
 pub struct BoundInContext<'a> {