Fix compilation.

src/parser/headline.rs

@@ -107,6 +107,7 @@ pub(crate) fn detect_headline<'s>(input: OrgSource<'s>) -> Res<OrgSource<'s>, ()
 /// Fields from a not-yet-fully-parsed Headline.
 ///
 /// This struct exists to give names to the fields of a partially-parsed Headline to avoid returning a large tuple of nameless fields.
+#[derive(Debug)]
 struct PreHeadline<'s> {
     headline_level: HeadlineLevel,
     star_count: HeadlineLevel,