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 +1419,42 @@ 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)
+            && let Some(brace_offset) = {
+                // find the first `{`
+                let chars = next_source.chars().collect::<Vec<_>>();
+                if chars.len() < 2 || chars[0] != '!' {
+                    None
+                } else {
+                    let mut idx = 1;
+                    while idx < chars.len() && chars[idx].is_ascii_whitespace() {
+                        idx += 1;
+                    }
+                    (idx < chars.len() && chars[idx] == '{').then_some(idx)
+                }
+            }
+        {
+            let bang_pos = ident.span.hi().0;
+            let lo = bang_pos + 1; // the pos after `!`
+            let hi = bang_pos + brace_offset as u32; // the pos of '{'
+            let span = ident.span.with_hi(BytePos(hi)).with_lo(BytePos(lo));
+            err.subdiagnostic(MaybeMissingMacroRulesName { 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 +1,24 @@
+macro_rules! {}
+//~^ ERROR cannot find macro `macro_rules` in this scope
+//~| NOTE maybe you have forgotten to define a name for this `macro_rules`
+
+macro_rules!{}
+//~^ ERROR cannot find macro `macro_rules` in this scope
+//~| NOTE maybe you have forgotten to define a name for this `macro_rules`
+
+macro_rules!  {}
+//~^ ERROR cannot find macro `macro_rules` in this scope
+//~| NOTE maybe you have forgotten to define a name for this `macro_rules`
+
+macro_rules! {
+
+}
+//~^^^ ERROR cannot find macro `macro_rules` in this scope
+//~| NOTE maybe you have forgotten to define a name for this `macro_rules`
+
+macro_rules![];
+//~^ ERROR cannot find macro `macro_rules` in this scope
+macro_rules!();
+//~^ ERROR cannot find macro `macro_rules` in this scope
+
+fn main() {}

tests/ui/resolve/issue-118295.stderr

@@ -0,0 +1,62 @@
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:21:1
+   |
+LL | macro_rules!();
+   | ^^^^^^^^^^^
+
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:19:1
+   |
+LL | macro_rules![];
+   | ^^^^^^^^^^^
+
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:13:1
+   |
+LL | macro_rules! {
+   | ^^^^^^^^^^^
+   |
+note: maybe you have forgotten to define a name for this `macro_rules`
+  --> $DIR/issue-118295.rs:13:13
+   |
+LL | macro_rules! {
+   |             ^
+
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:9:1
+   |
+LL | macro_rules!  {}
+   | ^^^^^^^^^^^
+   |
+note: maybe you have forgotten to define a name for this `macro_rules`
+  --> $DIR/issue-118295.rs:9:13
+   |
+LL | macro_rules!  {}
+   |             ^^
+
+error: cannot find macro `macro_rules` in this scope
+  --> $DIR/issue-118295.rs:5:1
+   |
+LL | macro_rules!{}
+   | ^^^^^^^^^^^
+   |
+note: maybe you have forgotten to define a name for this `macro_rules`
+  --> $DIR/issue-118295.rs:5:13
+   |
+LL | macro_rules!{}
+   |             ^
+
+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:1:13
+   |
+LL | macro_rules! {}
+   |             ^
+
+error: aborting due to 6 previous errors
+