fix: position=None keys broke key order when inserted into the table without sort_by_key

Author: mxndtaylor

crates/toml_edit/src/table.rs

@@ -519,14 +519,21 @@ fn decorate_table(table: &mut Table) {
     }
 }
 
+/// sort_by_key works because we add the position to _every_ item's key during parsing,
+/// so keys without positions would be either:
+///    1. non-leaf keys (i.e. "foo" or "bar" in dotted key "foo.bar.baz")
+///    2. custom keys added to the doc without an explicit position
+/// and in the case of (1), we'd never see it since we only look at the last
+/// key in a dotted-key. So, we can safely return a constant value for these cases.
+///
+/// To support the most intuitive behavior, we return the maximum usize, placing
+/// position=None items at the end, so when you insert it without position, it
+/// appends it to the end.
 pub(crate) fn sort_values_by_position<'s>(values: &mut [(Vec<&'s Key>, &'s Value)]) {
-    values.sort_by(|(left_kp, _), (right_kp, _)| {
-        return match (left_kp.last().map(|x| x.position),
-                      right_kp.last().map(|x| x.position)) {
-            // first if both have Some() position, its easy
-            (Some(Some(p1)), Some(Some(p2))) => p1.cmp(&p2),
-            // if one or more do not, preserve order
-            _ => std::cmp::Ordering::Equal
+    values.sort_by_key(|(left_kp, _)| {
+        return match left_kp.last().map(|x| x.position) {
+            Some(Some(pos)) => pos,
+            _ => usize::MAX
         }
     });
 }