mbe: Add FIXME for moving macro kind error earlier

Author: joshtriplett

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -58,6 +58,9 @@ pub(super) fn failed_to_match_macro(
 
     let Some(BestFailure { token, msg: label, remaining_matcher, .. }) = tracker.best_failure
     else {
+        // FIXME: we should report this at macro resolution time, as we do for
+        // `resolve_macro_cannot_use_as_attr`. We can do that once we track multiple macro kinds for a
+        // Def.
         if attr_args.is_none() && !rules.iter().any(|rule| matches!(rule, MacroRule::Func { .. })) {
             let msg = format!("macro has no rules for function-like invocation `{name}!`");
             let mut err = psess.dcx().struct_span_err(sp, msg);