Box the biggest ast::ItemKind variants

compiler/rustc_resolve/src/lib.rs

@@ -9,6 +9,7 @@
 //! Type-relative name resolution (methods, fields, associated items) happens in `librustc_typeck`.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(box_patterns)]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(format_args_capture)]