diff --git a/rust/hg-core/src/discovery.rs b/rust/hg-core/src/discovery.rs
--- a/rust/hg-core/src/discovery.rs
+++ b/rust/hg-core/src/discovery.rs
@@ -256,6 +256,11 @@
         &mut self,
         missing: impl IntoIterator<Item = Revision>,
     ) -> Result<(), GraphError> {
+        let missing: VecDeque<Revision> = missing.into_iter().collect();
+        if missing.is_empty() {
+            return Ok(());
+        }
+
         self.mutate_undecided(
             |oc| oc.compute_undecided(),
             |wu| wu.add_missing_revisions(missing),
@@ -431,20 +436,14 @@
     ///
     /// # Performance note
     ///
-    /// Except in the most trivial case, the first call of this method has
-    /// the side effect of computing `self.undecided` set for the first time,
-    /// and the related caches it might need for efficiency of its internal
-    /// computation. This is typically faster if more information is
-    /// available in `self.common`. Therefore, for good performance, the
-    /// caller should avoid calling this too early.
+    /// `tovisit` is provided as a `VecDeque` already so that a potential
+    /// caller from `PartialDiscovery` can avoid the undecided set in case
+    /// it turns out to be empty (hence restrain
+    /// from performing the transition to `WithUndecided`)
     pub fn add_missing_revisions(
         &mut self,
-        missing: impl IntoIterator<Item = Revision>,
+        mut tovisit: VecDeque<Revision>,
     ) -> Result<(), GraphError> {
-        let mut tovisit: VecDeque<Revision> = missing.into_iter().collect();
-        if tovisit.is_empty() {
-            return Ok(());
-        }
         self.ensure_children_cache()?;
         let children = self.children_cache.as_ref().unwrap();
         let mut seen: HashSet<Revision> = HashSet::new();
@@ -773,7 +772,7 @@
         // of `undecided`, let's check that, force the mutation and
         // ask for them
         assert_disco_is_only_commons(&disco);
-        disco.add_missing_revisions(Vec::new())?;
+        disco.mutate_undecided(|oc| oc.compute_undecided(), |_| Ok(()))?;
         assert_disco_is_b(&disco);
         assert_eq!(sorted_undecided(&disco), vec![5, 8, 10, 13]);
         assert_eq!(disco.stats().undecided, Some(4));