Handle nil values for compare_property_object_tree.

org_mode_samples/affiliated_keyword/empty_caption.org

@@ -0,0 +1,5 @@
+#+caption:
+#+caption: *foo*
+#+caption[bar]:
+#+begin_src bash
+#+end_src

src/compare/compare_field.rs

@@ -548,6 +548,21 @@ where
     let mut full_status: Vec<DiffEntry<'b, 's>> = Vec::with_capacity(outer_rust_list.len());
 
     for (kw_e, kw_r) in outer_emacs_list.iter().zip(outer_rust_list) {
+        match (kw_e.as_atom(), kw_r) {
+            (Ok("nil"), (None, mandatory_value)) if mandatory_value.is_empty() => {
+                // If its an empty keyword then it becomes nil in the elisp.
+                continue;
+            }
+            (Ok("nil"), _) => {
+                let this_status = DiffStatus::Bad;
+                let message = Some(format!(
+                    "{} mismatch (emacs != rust) {:?} != {:?}",
+                    emacs_field, kw_e, kw_r
+                ));
+                return Ok(ComparePropertiesResult::SelfChange(this_status, message));
+            }
+            _ => {}
+        }
         let kw_e = kw_e.as_list()?;
         let child_status_length = kw_r.1.len() + kw_r.0.as_ref().map(|opt| opt.len()).unwrap_or(0);
         let mut child_status: Vec<DiffEntry<'b, 's>> = Vec::with_capacity(child_status_length);
@@ -556,18 +571,31 @@ where
             let mut kw_e = kw_e.iter();
             // First element is a list representing the mandatory value.
             if let Some(val_e) = kw_e.next() {
-                let el = val_e.as_list()?;
-                if el.len() != kw_r.1.len() {
-                    let this_status = DiffStatus::Bad;
-                    let message = Some(format!(
-                        "{} mismatch (emacs != rust) {:?} != {:?}",
-                        emacs_field, kw_e, kw_r
-                    ));
-                    return Ok(ComparePropertiesResult::SelfChange(this_status, message));
-                }
-                for (e, r) in el.iter().zip(kw_r.1.iter()) {
-                    child_status.push(compare_ast_node(source, e, r.into())?);
-                }
+                match (val_e.as_atom(), kw_r) {
+                    (Ok("nil"), (_, mandatory_value)) if mandatory_value.is_empty() => {}
+                    (Ok("nil"), _) => {
+                        let this_status = DiffStatus::Bad;
+                        let message = Some(format!(
+                            "{} mismatch (emacs != rust) {:?} != {:?}",
+                            emacs_field, kw_e, kw_r
+                        ));
+                        return Ok(ComparePropertiesResult::SelfChange(this_status, message));
+                    }
+                    _ => {
+                        let el = val_e.as_list()?;
+                        if el.len() != kw_r.1.len() {
+                            let this_status = DiffStatus::Bad;
+                            let message = Some(format!(
+                                "{} mismatch (emacs != rust) {:?} != {:?}",
+                                emacs_field, kw_e, kw_r
+                            ));
+                            return Ok(ComparePropertiesResult::SelfChange(this_status, message));
+                        }
+                        for (e, r) in el.iter().zip(kw_r.1.iter()) {
+                            child_status.push(compare_ast_node(source, e, r.into())?);
+                        }
+                    }
+                };
             } else {
                 let this_status = DiffStatus::Bad;
                 let message = Some(format!(