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Add comments about waiting_cache.

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This commit is contained in:
Nicholas Nethercote 2019-09-16 12:41:36 +10:00
parent 6391ef4d6e
commit e2492b7163
1 changed files with 14 additions and 2 deletions
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16
src/librustc_data_structures/obligation_forest/mod.rs
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@ -151,7 +151,9 @@ pub struct ObligationForest<O: ForestObligation> {
/// 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 @@ pub fn process_obligations<P>(&mut self, processor: &mut P, do_completed: DoComp
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 @@ fn compress(&mut self, do_completed: DoCompleted) -> Option<Vec<O>> {
}
}
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 @@ fn apply_rewrites(&mut self, node_rewrites: &[usize]) {
}
}
// 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()];