Add test demonstrating problem.

org_mode_samples/comment/require_whitespace_after_hash.org

@@ -0,0 +1,3 @@
+# Comment line
+#not a comment
+# Comment again

src/parser/comment.rs

@@ -43,3 +43,33 @@ fn comment_line<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str
     let source = get_consumed(input, remaining);
     Ok((remaining, source))
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::parser::parser_context::ContextElement;
+    use crate::parser::parser_context::ContextTree;
+    use crate::parser::parser_with_context::parser_with_context;
+
+    use super::*;
+
+    #[test]
+    fn require_space_after_hash() {
+        let input = "# Comment line
+#not a comment
+# Comment again";
+        let initial_context: ContextTree<'_, '_> = ContextTree::new();
+        let document_context =
+            initial_context.with_additional_node(ContextElement::DocumentRoot(input));
+        let comment_matcher =
+            parser_with_context!(comment)(&document_context);
+        let (remaining, first_comment) =
+            comment_matcher(input).expect("Parse first comment");
+        assert_eq!(remaining, r#"#not a comment
+# Comment again"#);
+        assert_eq!(
+            first_comment.source,
+            "# Comment line
+"
+        );
+    }
+}