Improve code suggestion for incorrect macro_rules! usage

compiler/rustc_parse/src/parser/item.rs

@@ -534,7 +534,7 @@ impl<'a> Parser<'a> {
         match self.parse_delim_args() {
             // `( .. )` or `[ .. ]` (followed by `;`), or `{ .. }`.
             Ok(args) => {
-                self.eat_semi_for_macro_if_needed(&args);
+                self.eat_semi_for_macro_if_needed(&args, Some(&path));
                 self.complain_if_pub_macro(vis, false);
                 Ok(MacCall { path, args })
             }
@@ -2392,7 +2392,7 @@ impl<'a> Parser<'a> {
         }
 
         let body = self.parse_delim_args()?;
-        self.eat_semi_for_macro_if_needed(&body);
+        self.eat_semi_for_macro_if_needed(&body, None);
         self.complain_if_pub_macro(vis, true);
 
         Ok(ItemKind::MacroDef(
@@ -2417,13 +2417,13 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn eat_semi_for_macro_if_needed(&mut self, args: &DelimArgs) {
+    fn eat_semi_for_macro_if_needed(&mut self, args: &DelimArgs, path: Option<&Path>) {
         if args.need_semicolon() && !self.eat(exp!(Semi)) {
-            self.report_invalid_macro_expansion_item(args);
+            self.report_invalid_macro_expansion_item(args, path);
         }
     }
 
-    fn report_invalid_macro_expansion_item(&self, args: &DelimArgs) {
+    fn report_invalid_macro_expansion_item(&self, args: &DelimArgs, path: Option<&Path>) {
         let span = args.dspan.entire();
         let mut err = self.dcx().struct_span_err(
             span,
@@ -2433,17 +2433,32 @@ impl<'a> Parser<'a> {
         // macros within the same crate (that we can fix), which is sad.
         if !span.from_expansion() {
             let DelimSpan { open, close } = args.dspan;
-            err.multipart_suggestion(
-                "change the delimiters to curly braces",
-                vec![(open, "{".to_string()), (close, '}'.to_string())],
-                Applicability::MaybeIncorrect,
-            );
-            err.span_suggestion(
-                span.with_neighbor(self.token.span).shrink_to_hi(),
-                "add a semicolon",
-                ';',
-                Applicability::MaybeIncorrect,
-            );
+            // Check if this looks like `macro_rules!(name) { ... }`
+            // a common mistake when trying to define a macro.
+            if let Some(path) = path
+                && path.segments.first().is_some_and(|seg| seg.ident.name == sym::macro_rules)
+                && args.delim == Delimiter::Parenthesis
+            {
+                let replace =
+                    if path.span.hi() + rustc_span::BytePos(1) < open.lo() { "" } else { " " };
+                err.multipart_suggestion(
+                    "to define a macro, remove the parentheses around the macro name",
+                    vec![(open, replace.to_string()), (close, String::new())],
+                    Applicability::MachineApplicable,
+                );
+            } else {
+                err.multipart_suggestion(
+                    "change the delimiters to curly braces",
+                    vec![(open, "{".to_string()), (close, '}'.to_string())],
+                    Applicability::MaybeIncorrect,
+                );
+                err.span_suggestion(
+                    span.with_neighbor(self.token.span).shrink_to_hi(),
+                    "add a semicolon",
+                    ';',
+                    Applicability::MaybeIncorrect,
+                );
+            }
         }
         err.emit();
     }

tests/ui/macros/issue-118786.fixed

@@ -0,0 +1,20 @@
+#![allow(unused_macros)]
+//@ compile-flags: --crate-type lib
+//@ dont-require-annotations: NOTE
+//@ run-rustfix
+
+// Regression test for issue 118786
+
+macro_rules! make_macro {
+    ($macro_name:tt) => {
+        macro_rules! $macro_name {
+        //~^ ERROR macro expansion ignores `{` and any tokens following
+        //~| ERROR cannot find macro `macro_rules` in this scope
+        //~| NOTE put a macro name here
+            () => {}
+        }
+    }
+}
+
+make_macro!(meow);
+//~^ ERROR macros that expand to items must be delimited with braces or followed by a semicolon

tests/ui/macros/issue-118786.rs

@@ -1,5 +1,7 @@
-//@ compile-flags: --crate-type lib -O -C debug-assertions=yes
+#![allow(unused_macros)]
+//@ compile-flags: --crate-type lib
 //@ dont-require-annotations: NOTE
+//@ run-rustfix
 
 // Regression test for issue 118786
 

tests/ui/macros/issue-118786.stderr

@@ -1,21 +1,17 @@
 error: macros that expand to items must be delimited with braces or followed by a semicolon
-  --> $DIR/issue-118786.rs:17:13
+  --> $DIR/issue-118786.rs:19:13
    |
 LL | make_macro!((meow));
    |             ^^^^^^
    |
-help: change the delimiters to curly braces
+help: to define a macro, remove the parentheses around the macro name
    |
 LL - make_macro!((meow));
-LL + make_macro!({meow});
+LL + make_macro!(meow);
    |
-help: add a semicolon
-   |
-LL |         macro_rules! $macro_name; {
-   |                                 +
 
 error: macro expansion ignores `{` and any tokens following
-  --> $DIR/issue-118786.rs:8:34
+  --> $DIR/issue-118786.rs:10:34
    |
 LL |         macro_rules! $macro_name {
    |                                  ^
@@ -26,7 +22,7 @@ LL | make_macro!((meow));
    = note: the usage of `make_macro!` is likely invalid in item context
 
 error: cannot find macro `macro_rules` in this scope
-  --> $DIR/issue-118786.rs:8:9
+  --> $DIR/issue-118786.rs:10:9
    |
 LL |         macro_rules! $macro_name {
    |         ^^^^^^^^^^^
@@ -35,7 +31,7 @@ LL | make_macro!((meow));
    | ------------------- in this macro invocation
    |
 note: maybe you have forgotten to define a name for this `macro_rules!`
-  --> $DIR/issue-118786.rs:8:20
+  --> $DIR/issue-118786.rs:10:20
    |
 LL |         macro_rules! $macro_name {
    |                    ^ put a macro name here

tests/ui/parser/macro-rules-paren-name-issue-150899.rs

@@ -0,0 +1,7 @@
+macro_rules!(i_think_the_name_should_go_here) {
+    //~^ ERROR macros that expand to items must be delimited with braces or followed by a semicolon
+    //~| ERROR expected item, found `{`
+    () => {}
+}
+
+fn main() {}

tests/ui/parser/macro-rules-paren-name-issue-150899.stderr

@@ -0,0 +1,22 @@
+error: macros that expand to items must be delimited with braces or followed by a semicolon
+  --> $DIR/macro-rules-paren-name-issue-150899.rs:1:13
+   |
+LL | macro_rules!(i_think_the_name_should_go_here) {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to define a macro, remove the parentheses around the macro name
+   |
+LL - macro_rules!(i_think_the_name_should_go_here) {
+LL + macro_rules! i_think_the_name_should_go_here {
+   |
+
+error: expected item, found `{`
+  --> $DIR/macro-rules-paren-name-issue-150899.rs:1:47
+   |
+LL | macro_rules!(i_think_the_name_should_go_here) {
+   |                                               ^ expected item
+   |
+   = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
+
+error: aborting due to 2 previous errors
+