Box the biggest ast::ItemKind variants

compiler/rustc_builtin_macros/src/lib.rs

@@ -2,6 +2,8 @@
 //! injecting code into the crate before it is lowered to HIR.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(box_patterns)]
+#![feature(box_syntax)]
 #![feature(bool_to_option)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]