Fix small FixMe task in rust macro builtins

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc: Change BuiltinMacro in
	builtin_macro_from_string to tl::optional<>
	* expand/rust-macro-builtins.h (enum class): Change BuiltinMacro
	in builtin_macro_from_string to tl::optional<>
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit):
	Resolved wrong type dependency of builtin_macro_from_string

Signed-off-by: jjasmine <tanghocle456@gmail.com>

Author: badumbatish

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);

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);
 
 /**

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 ();