Add comments about `waiting_cache`.
diff --git a/src/librustc_data_structures/obligation_forest/mod.rs b/src/librustc_data_structures/obligation_forest/mod.rs
index 7904808..9dbae95 100644
--- a/src/librustc_data_structures/obligation_forest/mod.rs
+++ b/src/librustc_data_structures/obligation_forest/mod.rs
@@ -151,7 +151,9 @@
/// A cache of predicates that have been successfully completed.
done_cache: FxHashSet<O::Predicate>,
- /// An cache of the nodes in `nodes`, indexed by predicate.
+ /// A cache of the nodes in `nodes`, indexed by predicate. Unfortunately,
+ /// its contents are not guaranteed to match those of `nodes`. See the
+ /// comments in `process_obligation` for details.
waiting_cache: FxHashMap<O::Predicate, NodeIndex>,
scratch: Option<Vec<usize>>,
@@ -394,6 +396,11 @@
debug!("process_obligations: node {} == {:?}", i, node);
+ // `processor.process_obligation` can modify the predicate within
+ // `node.obligation`, and that predicate is the key used for
+ // `self.waiting_cache`. This means that `self.waiting_cache` can
+ // get out of sync with `nodes`. It's not very common, but it does
+ // happen, and code in `compress` has to allow for it.
let result = match node.state.get() {
NodeState::Pending => processor.process_obligation(&mut node.obligation),
_ => continue
@@ -621,7 +628,10 @@
}
}
NodeState::Done => {
- // Avoid cloning the key (predicate) in case it exists in the waiting cache
+ // This lookup can fail because the contents of
+ // `self.waiting_cache` is not guaranteed to match those of
+ // `self.nodes`. See the comment in `process_obligation`
+ // for more details.
if let Some((predicate, _)) = self.waiting_cache
.remove_entry(node.obligation.as_predicate())
{
@@ -703,6 +713,8 @@
}
}
+ // This updating of `self.waiting_cache` is necessary because the
+ // removal of nodes within `compress` can fail. See above.
let mut kill_list = vec![];
for (predicate, index) in &mut self.waiting_cache {
let new_i = node_rewrites[index.index()];
