Refactor AST trait bound modifiers

compiler/rustc_ast_pretty/src/lib.rs

@@ -4,6 +4,7 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(box_patterns)]
+#![feature(let_chains)]
 #![recursion_limit = "256"]
 
 mod helpers;