Auto merge of #51613 - nnethercote:ob-forest-cleanup, r=nikomatsakis

Obligation forest cleanup

While looking at this code I was scratching my head about whether a node could appear in both `parent` and `dependents`. Turns out it can, but it's not useful to do so, so this PR cleans things up so it's no longer possible.

Author: bors

src/librustc_data_structures/obligation_forest/mod.rs

@@ -91,13 +91,14 @@ struct Node<O> {
     obligation: O,
     state: Cell<NodeState>,
 
-    /// Obligations that depend on this obligation for their
-    /// completion. They must all be in a non-pending state.
-    dependents: Vec<NodeIndex>,
     /// The parent of a node - the original obligation of
     /// which it is a subobligation. Except for error reporting,
-    /// this is just another member of `dependents`.
+    /// it is just like any member of `dependents`.
     parent: Option<NodeIndex>,
+
+    /// Obligations that depend on this obligation for their
+    /// completion. They must all be in a non-pending state.
+    dependents: Vec<NodeIndex>,
 }
 
 /// The state of one node in some tree within the forest. This
@@ -193,15 +194,18 @@ impl<O: ForestObligation> ObligationForest<O> {
             Entry::Occupied(o) => {
                 debug!("register_obligation_at({:?}, {:?}) - duplicate of {:?}!",
                        obligation, parent, o.get());
+                let node = &mut self.nodes[o.get().get()];
                 if let Some(parent) = parent {
-                    if self.nodes[o.get().get()].dependents.contains(&parent) {
-                        debug!("register_obligation_at({:?}, {:?}) - duplicate subobligation",
-                               obligation, parent);
-                    } else {
-                        self.nodes[o.get().get()].dependents.push(parent);
+                    // If the node is already in `waiting_cache`, it's already
+                    // been marked with a parent. (It's possible that parent
+                    // has been cleared by `apply_rewrites`, though.) So just
+                    // dump `parent` into `node.dependents`... unless it's
+                    // already in `node.dependents` or `node.parent`.
+                    if !node.dependents.contains(&parent) && Some(parent) != node.parent {
+                        node.dependents.push(parent);
                     }
                 }
-                if let NodeState::Error = self.nodes[o.get().get()].state.get() {
+                if let NodeState::Error = node.state.get() {
                     Err(())
                 } else {
                     Ok(())
@@ -380,10 +384,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             NodeState::Success => {
                 node.state.set(NodeState::OnDfsStack);
                 stack.push(index);
-                if let Some(parent) = node.parent {
-                    self.find_cycles_from_node(stack, processor, parent.get());
-                }
-                for dependent in &node.dependents {
+                for dependent in node.parent.iter().chain(node.dependents.iter()) {
                     self.find_cycles_from_node(stack, processor, dependent.get());
                 }
                 stack.pop();
@@ -427,7 +428,7 @@ impl<O: ForestObligation> ObligationForest<O> {
             }
 
             error_stack.extend(
-                node.dependents.iter().cloned().chain(node.parent).map(|x| x.get())
+                node.parent.iter().chain(node.dependents.iter()).map(|x| x.get())
             );
         }
 
@@ -437,11 +438,7 @@ impl<O: ForestObligation> ObligationForest<O> {
 
     #[inline]
     fn mark_neighbors_as_waiting_from(&self, node: &Node<O>) {
-        if let Some(parent) = node.parent {
-            self.mark_as_waiting_from(&self.nodes[parent.get()]);
-        }
-
-        for dependent in &node.dependents {
+        for dependent in node.parent.iter().chain(node.dependents.iter()) {
             self.mark_as_waiting_from(&self.nodes[dependent.get()]);
         }
     }
@@ -588,8 +585,8 @@ impl<O> Node<O> {
     fn new(parent: Option<NodeIndex>, obligation: O) -> Node<O> {
         Node {
             obligation,
-            parent,
             state: Cell::new(NodeState::Pending),
+            parent,
             dependents: vec![],
         }
     }