fix: prevent panic when whitespace separates macro_rules and ! (6990)

Co-authored-by: Yacin Tmimi <yacintmimi@gmail.com>

Author: tninesling

src/macros.rs

@@ -431,7 +431,7 @@ pub(crate) fn rewrite_macro_def(
     };
 
     let mut header = if def.macro_rules {
-        let pos = context.snippet_provider.span_after(span, "macro_rules!");
+        let pos = context.snippet_provider.span_after(span, "!");
         vec![HeaderPart::new("macro_rules!", span.with_hi(pos))]
     } else {
         let macro_lo = context.snippet_provider.span_before(span, "macro");

tests/source/issue-6985.rs

@@ -0,0 +1,21 @@
+macro_rules ! say_hello {
+    () => {
+        println!("Hello!")
+    };
+}
+
+macro_rules /* comment */ ! say_goodbye {
+    () => {
+        println!("Goodbye!")
+    };
+}
+
+macro_rules // comment
+! do_nothing {
+    () => {};
+}
+
+fn main() {
+    say_hello!();
+    say_goodbye!()
+}

tests/target/issue-6985.rs

@@ -0,0 +1,20 @@
+macro_rules! say_hello {
+    () => {
+        println!("Hello!")
+    };
+}
+
+macro_rules! say_goodbye {
+    () => {
+        println!("Goodbye!")
+    };
+}
+
+macro_rules! do_nothing {
+    () => {};
+}
+
+fn main() {
+    say_hello!();
+    say_goodbye!()
+}