tip for define macro name after macro_rules!

Author: bvanjoi

compiler/rustc_resolve/messages.ftl

@@ -181,6 +181,8 @@ resolve_method_not_member_of_trait =
     method `{$method}` is not a member of trait `{$trait_}`
     .label = not a member of trait `{$trait_}`
 
+resolve_missing_macro_rules_name = maybe you have forgotten to define a name for this `macro_rules!`
+
 resolve_module_only =
     visibility must resolve to a module
 

compiler/rustc_resolve/src/diagnostics.rs

@@ -27,10 +27,8 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Span, SyntaxContext};
 use thin_vec::{thin_vec, ThinVec};
 
-use crate::errors::{
-    AddedMacroUse, ChangeImportBinding, ChangeImportBindingSuggestion, ConsiderAddingADerive,
-    ExplicitUnsafeTraits,
-};
+use crate::errors::{AddedMacroUse, ChangeImportBinding, ChangeImportBindingSuggestion};
+use crate::errors::{ConsiderAddingADerive, ExplicitUnsafeTraits, MaybeMissingMacroRulesName};
 use crate::imports::{Import, ImportKind};
 use crate::late::{PatternSource, Rib};
 use crate::path_names_to_string;
@@ -1421,14 +1419,26 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             "",
         );
 
+        if macro_kind == MacroKind::Bang
+            && ident.name == sym::macro_rules
+            && let source_map = self.tcx.sess.source_map()
+            && let Ok(next_source) = source_map.span_to_next_source(ident.span)
+            && next_source.starts_with("!")
+        {
+            err.subdiagnostic(MaybeMissingMacroRulesName { span: ident.span });
+            return;
+        }
+
         if macro_kind == MacroKind::Derive && (ident.name == sym::Send || ident.name == sym::Sync) {
             err.subdiagnostic(ExplicitUnsafeTraits { span: ident.span, ident });
             return;
         }
+
         if self.macro_names.contains(&ident.normalize_to_macros_2_0()) {
             err.subdiagnostic(AddedMacroUse);
             return;
         }
+
         if ident.name == kw::Default
             && let ModuleKind::Def(DefKind::Enum, def_id, _) = parent_scope.module.kind
         {

compiler/rustc_resolve/src/errors.rs

@@ -665,6 +665,13 @@ pub(crate) struct ExplicitUnsafeTraits {
     pub(crate) ident: Ident,
 }
 
+#[derive(Subdiagnostic)]
+#[note(resolve_missing_macro_rules_name)]
+pub(crate) struct MaybeMissingMacroRulesName {
+    #[primary_span]
+    pub(crate) span: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[help(resolve_added_macro_use)]
 pub(crate) struct AddedMacroUse;

tests/ui/resolve/issue-118295.rs

@@ -0,0 +1,5 @@
+macro_rules! {}
+//~^ ERROR cannot find macro `macro_rules` in this scope
+//~| NOTE maybe you have forgotten to define a name for this `macro_rules!`
+
+fn main() {}

tests/ui/resolve/issue-118295.stderr

@@ -0,0 +1,14 @@
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:1:1
+   |
+LL | macro_rules! {}
+   | ^^^^^^^^^^^
+   |
+note: maybe you have forgotten to define a name for this `macro_rules!`
+  --> $DIR/issue-118295.rs:1:1
+   |
+LL | macro_rules! {}
+   | ^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+