Merge attrs, better validation

compiler/rustc_span/src/symbol.rs

@@ -817,6 +817,7 @@ symbols! {
         impl_trait_in_bindings,
         impl_trait_in_fn_trait_return,
         impl_trait_projections,
+        implement_via_object,
         implied_by,
         import,
         import_name_type,
@@ -1270,7 +1271,6 @@ symbols! {
         rustc_diagnostic_macros,
         rustc_dirty,
         rustc_do_not_const_check,
-        rustc_do_not_implement_via_object,
         rustc_doc_primitive,
         rustc_dummy,
         rustc_dump_env_program_clauses,