Do not allow plain lists to start with an asterisk with no indent.

src/parser/plain_list.rs

@@ -59,7 +59,8 @@ pub fn plain_list_item<'r, 's>(
 
     let element_matcher = parser_with_context!(element)(&parser_context);
     let exit_matcher = parser_with_context!(exit_matcher_parser)(&parser_context);
-    let (remaining, bull) = bullet(remaining)?;
+    let (remaining, bull) =
+        verify(bullet, |bull: &str| bull != "*" || indent_level > 0)(remaining)?;
     let maybe_contentless_item: Res<&str, &str> = alt((eof, line_ending))(remaining);
     match maybe_contentless_item {
         Ok((rem, _ws)) => {
@@ -94,7 +95,6 @@ pub fn plain_list_item<'r, 's>(
 
 #[tracing::instrument(ret, level = "debug")]
 fn bullet<'s>(i: &'s str) -> Res<&'s str, &'s str> {
-    // TODO: If asterisk, it cannot be at start of line or it would be a headline
     alt((
         tag("*"),
         tag("-"),
@@ -208,4 +208,28 @@ mod tests {
         assert_eq!(remaining, "");
         assert_eq!(result.source, "1. foo");
     }
+
+    #[test]
+    fn plain_list_cant_start_line_with_asterisk() {
+        // Plain lists with an asterisk bullet must be indented or else they would be a headline
+        let input = "* foo";
+        let initial_context: ContextTree<'_, '_> = ContextTree::new();
+        let document_context =
+            initial_context.with_additional_node(ContextElement::DocumentRoot(input));
+        let plain_list_matcher = parser_with_context!(plain_list)(&document_context);
+        let result = plain_list_matcher(input);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn indented_can_start_line_with_asterisk() {
+        // Plain lists with an asterisk bullet must be indented or else they would be a headline
+        let input = " * foo";
+        let initial_context: ContextTree<'_, '_> = ContextTree::new();
+        let document_context =
+            initial_context.with_additional_node(ContextElement::DocumentRoot(input));
+        let plain_list_matcher = parser_with_context!(plain_list)(&document_context);
+        let result = plain_list_matcher(input);
+        assert!(result.is_ok());
+    }
 }