mbe: Emit an error if a macro call has no function-like rules

Add a FIXME for moving this error earlier.

Author: joshtriplett

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -7,7 +7,7 @@ use rustc_macros::Subdiagnostic;
 use rustc_parse::parser::{Parser, Recovery, token_descr};
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::SourceMap;
-use rustc_span::{ErrorGuaranteed, Ident, Span};
+use rustc_span::{DUMMY_SP, ErrorGuaranteed, Ident, Span};
 use tracing::debug;
 
 use super::macro_rules::{MacroRule, NoopTracker, parser_from_cx};
@@ -25,6 +25,12 @@ pub(super) fn failed_to_match_macro(
  rules: &[MacroRule],
 ) -> (Span, ErrorGuaranteed) {
  debug!("failed to match macro");
+ let def_head_span = if !def_span.is_dummy() && !psess.source_map().is_imported(def_span) {
+ psess.source_map().guess_head_span(def_span)
+ } else {
+ DUMMY_SP
+ };
+
  // An error occurred, try the expansion again, tracking the expansion closely for better
  // diagnostics.
  let mut tracker = CollectTrackerAndEmitter::new(psess.dcx(), sp);
@@ -47,15 +53,27 @@ 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 !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);
+ if !def_head_span.is_dummy() {
+ let msg = "this macro has no rules for function-like invocation";
+ err.span_label(def_head_span, msg);
+ }
+ return (sp, err.emit());
+ }
  return (sp, psess.dcx().span_delayed_bug(sp, "failed to match a macro"));
  };
 
  let span = token.span.substitute_dummy(sp);
 
  let mut err = psess.dcx().struct_span_err(span, parse_failure_msg(&token, None));
  err.span_label(span, label);
- if !def_span.is_dummy() && !psess.source_map().is_imported(def_span) {
- err.span_label(psess.source_map().guess_head_span(def_span), "when calling this macro");
+ if !def_head_span.is_dummy() {
+ err.span_label(def_head_span, "when calling this macro");
  }
 
  annotate_doc_comment(&mut err, psess.source_map(), span);