Switch impl_tag! from explicit tags to ${index()}

compiler/rustc_data_structures/src/lib.rs

@@ -31,6 +31,7 @@
 #![feature(unwrap_infallible)]
 #![feature(strict_provenance)]
 #![feature(ptr_alignment_type)]
+#![feature(macro_metavar_expr)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]