diff --git a/gcc/rust/expand/rust-macro-builtins.cc b/gcc/rust/expand/rust-macro-builtins.cc
index 2af05a5e3777..f103759acdd7 100644
--- a/gcc/rust/expand/rust-macro-builtins.cc
+++ b/gcc/rust/expand/rust-macro-builtins.cc
@@ -126,8 +126,7 @@ std::unordered_map<std::string, AST::MacroTranscriberFunc>
     {"Hash", MacroBuiltin::proc_macro_builtin},
 };
 
-// FIXME: This should return an tl::optional
-BuiltinMacro
+tl::optional<BuiltinMacro>
 builtin_macro_from_string (const std::string &identifier)
 {
   auto macro = MacroBuiltin::builtins.lookup (identifier);
diff --git a/gcc/rust/expand/rust-macro-builtins.h b/gcc/rust/expand/rust-macro-builtins.h
index f9ab3fc3698e..1d6b30b59332 100644
--- a/gcc/rust/expand/rust-macro-builtins.h
+++ b/gcc/rust/expand/rust-macro-builtins.h
@@ -75,7 +75,7 @@ enum class BuiltinMacro
   Hash,
 };
 
-BuiltinMacro
+tl::optional<BuiltinMacro>
 builtin_macro_from_string (const std::string &identifier);
 
 /**
diff --git a/gcc/rust/resolve/rust-early-name-resolver.cc b/gcc/rust/resolve/rust-early-name-resolver.cc
index 422dd92e4620..d70f9ca9806e 100644
--- a/gcc/rust/resolve/rust-early-name-resolver.cc
+++ b/gcc/rust/resolve/rust-early-name-resolver.cc
@@ -497,7 +497,7 @@ EarlyNameResolver::visit (AST::MacroInvocation &invoc)
     {
       auto builtin_kind
 	= builtin_macro_from_string (rules_def->get_rule_name ().as_string ());
-      invoc.map_to_builtin (builtin_kind);
+      invoc.map_to_builtin (builtin_kind.value ());
     }
 
   auto attributes = rules_def->get_outer_attrs ();