Check existence of pre comment explicitly

This commit fixes #1428.

src/lists.rs

@@ -350,7 +350,9 @@ fn next(&mut self) -> Option<Self::Item> {
                     .span_to_snippet(codemap::mk_sp(self.prev_span_end, (self.get_lo)(&item)))
                     .unwrap();
                 let trimmed_pre_snippet = pre_snippet.trim();
-                let pre_comment = if !trimmed_pre_snippet.is_empty() {
+                let has_pre_comment = trimmed_pre_snippet.contains("//") ||
+                                      trimmed_pre_snippet.contains("/*");
+                let pre_comment = if has_pre_comment {
                     Some(trimmed_pre_snippet.to_owned())
                 } else {
                     None

tests/source/structs.rs

@@ -178,3 +178,5 @@ struct Deep {
                                                                          Type,
                                                                          NodeType>,
 }
+
+struct Foo<C=()>(String);

tests/target/structs.rs

@@ -182,3 +182,5 @@ struct Deep {
     deeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep:
         node::Handle<IdRef<'id, Node<Key, Value>>, Type, NodeType>,
 }
+
+struct Foo<C = ()>(String);