address review comments and fix new clippy findings introduced since …

…I made this PR

src/formatting/attr.rs

@@ -187,7 +187,7 @@ fn format_derive(
     } else if let SeparatorTactic::Always = context.config.trailing_comma() {
         // Retain the trailing comma.
         result.push_str(&item_str);
-    } else if item_str.ends_with(",") {
+    } else if item_str.ends_with(',') {
         // Remove the trailing comma.
         result.push_str(&item_str[..item_str.len() - 1]);
     } else {

src/formatting/comment.rs

@@ -1704,8 +1704,8 @@ impl<'a> Iterator for CommentReducer<'a> {
 fn remove_comment_header(comment: &str) -> &str {
     if comment.starts_with("///") || comment.starts_with("//!") {
         &comment[3..]
-    } else if comment.starts_with("//") {
-        &comment[2..]
+    } else if let Some(stripped) = comment.strip_prefix("//") {
+        &stripped
     } else if (comment.starts_with("/**") && !comment.starts_with("/**/"))
         || comment.starts_with("/*!")
     {

src/formatting/vertical.rs

@@ -50,16 +50,15 @@ impl AlignedItem for ast::StructField {
             mk_sp(self.attrs.last().unwrap().span.hi(), self.span.lo())
         };
         let attrs_extendable = self.ident.is_none() && is_attributes_extendable(&attrs_str);
-        Some(rewrite_struct_field_prefix(context, self)).and_then(|field_str| {
-            combine_strs_with_missing_comments(
-                context,
-                &attrs_str,
-                &field_str,
-                missing_span,
-                shape,
-                attrs_extendable,
-            )
-        })
+        let prefix = rewrite_struct_field_prefix(context, self);
+        combine_strs_with_missing_comments(
+            context,
+            &attrs_str,
+            &prefix,
+            missing_span,
+            shape,
+            attrs_extendable,
+        )
     }
 
     fn rewrite_aligned_item(