diff --git a/rust/hg-core/src/ancestors.rs b/rust/hg-core/src/ancestors.rs
--- a/rust/hg-core/src/ancestors.rs
+++ b/rust/hg-core/src/ancestors.rs
@@ -321,7 +321,7 @@
 
         // TODO heuristics for with_capacity()?
         let mut missing: Vec<Revision> = Vec::new();
-        for curr in (0..=start).map(|i| start - i) {
+        for curr in (0..=start).rev() {
             if revs_visit.is_empty() {
                 break;
             }