Give better error for macro_rules! name!

Author: aDotInTheVoid

compiler/rustc_parse/src/parser/item.rs

@@ -1547,6 +1547,20 @@ impl<'a> Parser<'a> {
         self.expect(&token::Not)?; // `!`
 
         let ident = self.parse_ident()?;
+
+        if self.eat(&token::Not) {
+            // Handle macro_rules! foo!
+            let span = self.prev_token.span;
+            self.struct_span_err(span, "macro names aren't followed by a `!`")
+                .span_suggestion(
+                    span,
+                    "remove the `!`",
+                    "".to_owned(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+
         let body = self.parse_mac_args()?;
         self.eat_semi_for_macro_if_needed(&body);
         self.complain_if_pub_macro(vis, true);

src/test/ui/macros/bang-after-name.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+#[allow(unused_macros)]
+
+macro_rules! foo { //~ ERROR macro names aren't followed by a `!`
+    () => {};
+}
+
+fn main() {}

src/test/ui/macros/bang-after-name.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+#[allow(unused_macros)]
+
+macro_rules! foo! { //~ ERROR macro names aren't followed by a `!`
+    () => {};
+}
+
+fn main() {}

src/test/ui/macros/bang-after-name.stderr

@@ -0,0 +1,8 @@
+error: macro names aren't followed by a `!`
+  --> $DIR/bang-after-name.rs:4:17
+   |
+LL | macro_rules! foo! {
+   |                 ^ help: remove the `!`
+
+error: aborting due to previous error
+