Add internal diagnistics lints

Author: hampuslidin

compiler/rustc_attr/src/lib.rs

@@ -5,6 +5,8 @@
 //! to this crate.
 
 #![feature(let_else)]
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 
 #[macro_use]
 extern crate rustc_macros;