fix Const generics are handled incorrectly

Author: rchaser53

src/comment.rs

@@ -880,6 +880,12 @@ pub(crate) fn recover_missing_comment_in_span(
         Some(String::new())
     } else {
         let missing_snippet = context.snippet(span);
+        let trimmed_missing_snippet = missing_snippet.trim();
+        if !trimmed_missing_snippet.starts_with("//") && !trimmed_missing_snippet.starts_with("/*")
+        {
+            return None;
+        };
+
         let pos = missing_snippet.find('/').unwrap_or(0);
         // 1 = ` `
         let total_width = missing_comment.len() + used_width + 1;

tests/target/issue-3554.rs

@@ -0,0 +1,4 @@
+#![feature(const_generics)]
+
+pub struct S<const N: usize>;
+impl S<{ 0 }> {}