| //! The `ObligationForest` is a utility data structure used in trait |
| //! matching to track the set of outstanding obligations (those not yet |
| //! resolved to success or error). It also tracks the "backtrace" of each |
| //! pending obligation (why we are trying to figure this out in the first |
| //! place). |
| //! |
| //! ### External view |
| //! |
| //! `ObligationForest` supports two main public operations (there are a |
| //! few others not discussed here): |
| //! |
| //! 1. Add a new root obligations (`register_obligation`). |
| //! 2. Process the pending obligations (`process_obligations`). |
| //! |
| //! When a new obligation `N` is added, it becomes the root of an |
| //! obligation tree. This tree can also carry some per-tree state `T`, |
| //! which is given at the same time. This tree is a singleton to start, so |
| //! `N` is both the root and the only leaf. Each time the |
| //! `process_obligations` method is called, it will invoke its callback |
| //! with every pending obligation (so that will include `N`, the first |
| //! time). The callback also receives a (mutable) reference to the |
| //! per-tree state `T`. The callback should process the obligation `O` |
| //! that it is given and return a `ProcessResult`: |
| //! |
| //! - `Unchanged` -> ambiguous result. Obligation was neither a success |
| //! nor a failure. It is assumed that further attempts to process the |
| //! obligation will yield the same result unless something in the |
| //! surrounding environment changes. |
| //! - `Changed(C)` - the obligation was *shallowly successful*. The |
| //! vector `C` is a list of subobligations. The meaning of this is that |
| //! `O` was successful on the assumption that all the obligations in `C` |
| //! are also successful. Therefore, `O` is only considered a "true" |
| //! success if `C` is empty. Otherwise, `O` is put into a suspended |
| //! state and the obligations in `C` become the new pending |
| //! obligations. They will be processed the next time you call |
| //! `process_obligations`. |
| //! - `Error(E)` -> obligation failed with error `E`. We will collect this |
| //! error and return it from `process_obligations`, along with the |
| //! "backtrace" of obligations (that is, the list of obligations up to |
| //! and including the root of the failed obligation). No further |
| //! obligations from that same tree will be processed, since the tree is |
| //! now considered to be in error. |
| //! |
| //! When the call to `process_obligations` completes, you get back an `Outcome`, |
| //! which includes two bits of information: |
| //! |
| //! - `completed`: a list of obligations where processing was fully |
| //! completed without error (meaning that all transitive subobligations |
| //! have also been completed). So, for example, if the callback from |
| //! `process_obligations` returns `Changed(C)` for some obligation `O`, |
| //! then `O` will be considered completed right away if `C` is the |
| //! empty vector. Otherwise it will only be considered completed once |
| //! all the obligations in `C` have been found completed. |
| //! - `errors`: a list of errors that occurred and associated backtraces |
| //! at the time of error, which can be used to give context to the user. |
| //! |
| //! Upon completion, none of the existing obligations were *shallowly |
| //! successful* (that is, no callback returned `Changed(_)`). This implies that |
| //! all obligations were either errors or returned an ambiguous result. |
| //! |
| //! ### Implementation details |
| //! |
| //! For the most part, comments specific to the implementation are in the |
| //! code. This file only contains a very high-level overview. Basically, |
| //! the forest is stored in a vector. Each element of the vector is a node |
| //! in some tree. Each node in the vector has the index of its dependents, |
| //! including the first dependent which is known as the parent. It also |
| //! has a current state, described by `NodeState`. After each processing |
| //! step, we compress the vector to remove completed and error nodes, which |
| //! aren't needed anymore. |
| |
| use crate::fx::{FxHashMap, FxHashSet}; |
| |
| use std::cell::Cell; |
| use std::collections::hash_map::Entry; |
| use std::fmt::Debug; |
| use std::hash; |
| use std::marker::PhantomData; |
| |
| mod graphviz; |
| |
| #[cfg(test)] |
| mod tests; |
| |
| pub trait ForestObligation: Clone + Debug { |
| type CacheKey: Clone + hash::Hash + Eq + Debug; |
| |
| /// Converts this `ForestObligation` suitable for use as a cache key. |
| /// If two distinct `ForestObligations`s return the same cache key, |
| /// then it must be sound to use the result of processing one obligation |
| /// (e.g. success for error) for the other obligation |
| fn as_cache_key(&self) -> Self::CacheKey; |
| } |
| |
| pub trait ObligationProcessor { |
| type Obligation: ForestObligation; |
| type Error: Debug; |
| |
| fn needs_process_obligation(&self, obligation: &Self::Obligation) -> bool; |
| |
| fn process_obligation( |
| &mut self, |
| obligation: &mut Self::Obligation, |
| ) -> ProcessResult<Self::Obligation, Self::Error>; |
| |
| /// As we do the cycle check, we invoke this callback when we |
| /// encounter an actual cycle. `cycle` is an iterator that starts |
| /// at the start of the cycle in the stack and walks **toward the |
| /// top**. |
| /// |
| /// In other words, if we had O1 which required O2 which required |
| /// O3 which required O1, we would give an iterator yielding O1, |
| /// O2, O3 (O1 is not yielded twice). |
| fn process_backedge<'c, I>(&mut self, cycle: I, _marker: PhantomData<&'c Self::Obligation>) |
| where |
| I: Clone + Iterator<Item = &'c Self::Obligation>; |
| } |
| |
| /// The result type used by `process_obligation`. |
| #[derive(Debug)] |
| pub enum ProcessResult<O, E> { |
| Unchanged, |
| Changed(Vec<O>), |
| Error(E), |
| } |
| |
| #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)] |
| struct ObligationTreeId(usize); |
| |
| type ObligationTreeIdGenerator = |
| std::iter::Map<std::ops::RangeFrom<usize>, fn(usize) -> ObligationTreeId>; |
| |
| pub struct ObligationForest<O: ForestObligation> { |
| /// The list of obligations. In between calls to [Self::process_obligations], |
| /// this list only contains nodes in the `Pending` or `Waiting` state. |
| /// |
| /// `usize` indices are used here and throughout this module, rather than |
| /// [`rustc_index::newtype_index!`] indices, because this code is hot enough |
| /// that the `u32`-to-`usize` conversions that would be required are |
| /// significant, and space considerations are not important. |
| nodes: Vec<Node<O>>, |
| |
| /// A cache of predicates that have been successfully completed. |
| done_cache: FxHashSet<O::CacheKey>, |
| |
| /// 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 `Self::process_obligation` for details. |
| active_cache: FxHashMap<O::CacheKey, usize>, |
| |
| /// A vector reused in [Self::compress()] and [Self::find_cycles_from_node()], |
| /// to avoid allocating new vectors. |
| reused_node_vec: Vec<usize>, |
| |
| obligation_tree_id_generator: ObligationTreeIdGenerator, |
| |
| /// Per tree error cache. This is used to deduplicate errors, |
| /// which is necessary to avoid trait resolution overflow in |
| /// some cases. |
| /// |
| /// See [this][details] for details. |
| /// |
| /// [details]: https://github.com/rust-lang/rust/pull/53255#issuecomment-421184780 |
| error_cache: FxHashMap<ObligationTreeId, FxHashSet<O::CacheKey>>, |
| } |
| |
| #[derive(Debug)] |
| 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<usize>, |
| |
| /// If true, `dependents[0]` points to a "parent" node, which requires |
| /// special treatment upon error but is otherwise treated the same. |
| /// (It would be more idiomatic to store the parent node in a separate |
| /// `Option<usize>` field, but that slows down the common case of |
| /// iterating over the parent and other descendants together.) |
| has_parent: bool, |
| |
| /// Identifier of the obligation tree to which this node belongs. |
| obligation_tree_id: ObligationTreeId, |
| } |
| |
| impl<O> Node<O> { |
| fn new(parent: Option<usize>, obligation: O, obligation_tree_id: ObligationTreeId) -> Node<O> { |
| Node { |
| obligation, |
| state: Cell::new(NodeState::Pending), |
| dependents: if let Some(parent_index) = parent { vec![parent_index] } else { vec![] }, |
| has_parent: parent.is_some(), |
| obligation_tree_id, |
| } |
| } |
| } |
| |
| /// The state of one node in some tree within the forest. This represents the |
| /// current state of processing for the obligation (of type `O`) associated |
| /// with this node. |
| /// |
| /// The non-`Error` state transitions are as follows. |
| /// ```text |
| /// (Pre-creation) |
| /// | |
| /// | register_obligation_at() (called by process_obligations() and |
| /// v from outside the crate) |
| /// Pending |
| /// | |
| /// | process_obligations() |
| /// v |
| /// Success |
| /// | ^ |
| /// | | mark_successes() |
| /// | v |
| /// | Waiting |
| /// | |
| /// | process_cycles() |
| /// v |
| /// Done |
| /// | |
| /// | compress() |
| /// v |
| /// (Removed) |
| /// ``` |
| /// The `Error` state can be introduced in several places, via `error_at()`. |
| /// |
| /// Outside of `ObligationForest` methods, nodes should be either `Pending` or |
| /// `Waiting`. |
| #[derive(Debug, Copy, Clone, PartialEq, Eq)] |
| enum NodeState { |
| /// This obligation has not yet been selected successfully. Cannot have |
| /// subobligations. |
| Pending, |
| |
| /// This obligation was selected successfully, but may or may not have |
| /// subobligations. |
| Success, |
| |
| /// This obligation was selected successfully, but it has a pending |
| /// subobligation. |
| Waiting, |
| |
| /// This obligation, along with its subobligations, are complete, and will |
| /// be removed in the next collection. |
| Done, |
| |
| /// This obligation was resolved to an error. It will be removed by the |
| /// next compression step. |
| Error, |
| } |
| |
| /// This trait allows us to have two different Outcome types: |
| /// - the normal one that does as little as possible |
| /// - one for tests that does some additional work and checking |
| pub trait OutcomeTrait { |
| type Error; |
| type Obligation; |
| |
| fn new() -> Self; |
| fn record_completed(&mut self, outcome: &Self::Obligation); |
| fn record_error(&mut self, error: Self::Error); |
| } |
| |
| #[derive(Debug)] |
| pub struct Outcome<O, E> { |
| /// Backtrace of obligations that were found to be in error. |
| pub errors: Vec<Error<O, E>>, |
| } |
| |
| impl<O, E> OutcomeTrait for Outcome<O, E> { |
| type Error = Error<O, E>; |
| type Obligation = O; |
| |
| fn new() -> Self { |
| Self { errors: vec![] } |
| } |
| |
| fn record_completed(&mut self, _outcome: &Self::Obligation) { |
| // do nothing |
| } |
| |
| fn record_error(&mut self, error: Self::Error) { |
| self.errors.push(error) |
| } |
| } |
| |
| #[derive(Debug, PartialEq, Eq)] |
| pub struct Error<O, E> { |
| pub error: E, |
| pub backtrace: Vec<O>, |
| } |
| |
| impl<O: ForestObligation> ObligationForest<O> { |
| pub fn new() -> ObligationForest<O> { |
| ObligationForest { |
| nodes: vec![], |
| done_cache: Default::default(), |
| active_cache: Default::default(), |
| reused_node_vec: vec![], |
| obligation_tree_id_generator: (0..).map(ObligationTreeId), |
| error_cache: Default::default(), |
| } |
| } |
| |
| /// Returns the total number of nodes in the forest that have not |
| /// yet been fully resolved. |
| pub fn len(&self) -> usize { |
| self.nodes.len() |
| } |
| |
| /// Registers an obligation. |
| pub fn register_obligation(&mut self, obligation: O) { |
| // Ignore errors here - there is no guarantee of success. |
| let _ = self.register_obligation_at(obligation, None); |
| } |
| |
| // Returns Err(()) if we already know this obligation failed. |
| fn register_obligation_at(&mut self, obligation: O, parent: Option<usize>) -> Result<(), ()> { |
| let cache_key = obligation.as_cache_key(); |
| if self.done_cache.contains(&cache_key) { |
| debug!("register_obligation_at: ignoring already done obligation: {:?}", obligation); |
| return Ok(()); |
| } |
| |
| match self.active_cache.entry(cache_key) { |
| Entry::Occupied(o) => { |
| let node = &mut self.nodes[*o.get()]; |
| if let Some(parent_index) = parent { |
| // If the node is already in `active_cache`, it has already |
| // had its chance to be marked with a parent. So if it's |
| // not already present, just dump `parent` into the |
| // dependents as a non-parent. |
| if !node.dependents.contains(&parent_index) { |
| node.dependents.push(parent_index); |
| } |
| } |
| if let NodeState::Error = node.state.get() { Err(()) } else { Ok(()) } |
| } |
| Entry::Vacant(v) => { |
| let obligation_tree_id = match parent { |
| Some(parent_index) => self.nodes[parent_index].obligation_tree_id, |
| None => self.obligation_tree_id_generator.next().unwrap(), |
| }; |
| |
| let already_failed = parent.is_some() |
| && self |
| .error_cache |
| .get(&obligation_tree_id) |
| .map_or(false, |errors| errors.contains(v.key())); |
| |
| if already_failed { |
| Err(()) |
| } else { |
| let new_index = self.nodes.len(); |
| v.insert(new_index); |
| self.nodes.push(Node::new(parent, obligation, obligation_tree_id)); |
| Ok(()) |
| } |
| } |
| } |
| } |
| |
| /// Converts all remaining obligations to the given error. |
| pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> { |
| let errors = self |
| .nodes |
| .iter() |
| .enumerate() |
| .filter(|(_index, node)| node.state.get() == NodeState::Pending) |
| .map(|(index, _node)| Error { error: error.clone(), backtrace: self.error_at(index) }) |
| .collect(); |
| |
| self.compress(|_| assert!(false)); |
| errors |
| } |
| |
| /// Returns the set of obligations that are in a pending state. |
| pub fn map_pending_obligations<P, F>(&self, f: F) -> Vec<P> |
| where |
| F: Fn(&O) -> P, |
| { |
| self.nodes |
| .iter() |
| .filter(|node| node.state.get() == NodeState::Pending) |
| .map(|node| f(&node.obligation)) |
| .collect() |
| } |
| |
| fn insert_into_error_cache(&mut self, index: usize) { |
| let node = &self.nodes[index]; |
| self.error_cache |
| .entry(node.obligation_tree_id) |
| .or_default() |
| .insert(node.obligation.as_cache_key()); |
| } |
| |
| /// Performs a fixpoint computation over the obligation list. |
| #[inline(never)] |
| pub fn process_obligations<P, OUT>(&mut self, processor: &mut P) -> OUT |
| where |
| P: ObligationProcessor<Obligation = O>, |
| OUT: OutcomeTrait<Obligation = O, Error = Error<O, P::Error>>, |
| { |
| let mut outcome = OUT::new(); |
| |
| // Fixpoint computation: we repeat until the inner loop stalls. |
| loop { |
| let mut has_changed = false; |
| |
| // Note that the loop body can append new nodes, and those new nodes |
| // will then be processed by subsequent iterations of the loop. |
| // |
| // We can't use an iterator for the loop because `self.nodes` is |
| // appended to and the borrow checker would complain. We also can't use |
| // `for index in 0..self.nodes.len() { ... }` because the range would |
| // be computed with the initial length, and we would miss the appended |
| // nodes. Therefore we use a `while` loop. |
| let mut index = 0; |
| while let Some(node) = self.nodes.get_mut(index) { |
| if node.state.get() != NodeState::Pending |
| || !processor.needs_process_obligation(&node.obligation) |
| { |
| index += 1; |
| continue; |
| } |
| |
| // `processor.process_obligation` can modify the predicate within |
| // `node.obligation`, and that predicate is the key used for |
| // `self.active_cache`. This means that `self.active_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. |
| |
| match processor.process_obligation(&mut node.obligation) { |
| ProcessResult::Unchanged => { |
| // No change in state. |
| } |
| ProcessResult::Changed(children) => { |
| // We are not (yet) stalled. |
| has_changed = true; |
| node.state.set(NodeState::Success); |
| |
| for child in children { |
| let st = self.register_obligation_at(child, Some(index)); |
| if let Err(()) = st { |
| // Error already reported - propagate it |
| // to our node. |
| self.error_at(index); |
| } |
| } |
| } |
| ProcessResult::Error(err) => { |
| has_changed = true; |
| outcome.record_error(Error { error: err, backtrace: self.error_at(index) }); |
| } |
| } |
| index += 1; |
| } |
| |
| // If unchanged, then we saw no successful obligations, which means |
| // there is no point in further iteration. This is based on the |
| // assumption that when trait matching returns `Error` or |
| // `Unchanged`, those results do not affect environmental inference |
| // state. (Note that this will occur if we invoke |
| // `process_obligations` with no pending obligations.) |
| if !has_changed { |
| break; |
| } |
| |
| self.mark_successes(); |
| self.process_cycles(processor); |
| self.compress(|obl| outcome.record_completed(obl)); |
| } |
| |
| outcome |
| } |
| |
| /// Returns a vector of obligations for `p` and all of its |
| /// ancestors, putting them into the error state in the process. |
| fn error_at(&self, mut index: usize) -> Vec<O> { |
| let mut error_stack: Vec<usize> = vec![]; |
| let mut trace = vec![]; |
| |
| loop { |
| let node = &self.nodes[index]; |
| node.state.set(NodeState::Error); |
| trace.push(node.obligation.clone()); |
| if node.has_parent { |
| // The first dependent is the parent, which is treated |
| // specially. |
| error_stack.extend(node.dependents.iter().skip(1)); |
| index = node.dependents[0]; |
| } else { |
| // No parent; treat all dependents non-specially. |
| error_stack.extend(node.dependents.iter()); |
| break; |
| } |
| } |
| |
| while let Some(index) = error_stack.pop() { |
| let node = &self.nodes[index]; |
| if node.state.get() != NodeState::Error { |
| node.state.set(NodeState::Error); |
| error_stack.extend(node.dependents.iter()); |
| } |
| } |
| |
| trace |
| } |
| |
| /// Mark all `Waiting` nodes as `Success`, except those that depend on a |
| /// pending node. |
| fn mark_successes(&self) { |
| // Convert all `Waiting` nodes to `Success`. |
| for node in &self.nodes { |
| if node.state.get() == NodeState::Waiting { |
| node.state.set(NodeState::Success); |
| } |
| } |
| |
| // Convert `Success` nodes that depend on a pending node back to |
| // `Waiting`. |
| for node in &self.nodes { |
| if node.state.get() == NodeState::Pending { |
| // This call site is hot. |
| self.inlined_mark_dependents_as_waiting(node); |
| } |
| } |
| } |
| |
| // This always-inlined function is for the hot call site. |
| #[inline(always)] |
| fn inlined_mark_dependents_as_waiting(&self, node: &Node<O>) { |
| for &index in node.dependents.iter() { |
| let node = &self.nodes[index]; |
| let state = node.state.get(); |
| if state == NodeState::Success { |
| // This call site is cold. |
| self.uninlined_mark_dependents_as_waiting(node); |
| } else { |
| debug_assert!(state == NodeState::Waiting || state == NodeState::Error) |
| } |
| } |
| } |
| |
| // This never-inlined function is for the cold call site. |
| #[inline(never)] |
| fn uninlined_mark_dependents_as_waiting(&self, node: &Node<O>) { |
| // Mark node Waiting in the cold uninlined code instead of the hot inlined |
| node.state.set(NodeState::Waiting); |
| self.inlined_mark_dependents_as_waiting(node) |
| } |
| |
| /// Report cycles between all `Success` nodes, and convert all `Success` |
| /// nodes to `Done`. This must be called after `mark_successes`. |
| fn process_cycles<P>(&mut self, processor: &mut P) |
| where |
| P: ObligationProcessor<Obligation = O>, |
| { |
| let mut stack = std::mem::take(&mut self.reused_node_vec); |
| for (index, node) in self.nodes.iter().enumerate() { |
| // For some benchmarks this state test is extremely hot. It's a win |
| // to handle the no-op cases immediately to avoid the cost of the |
| // function call. |
| if node.state.get() == NodeState::Success { |
| self.find_cycles_from_node(&mut stack, processor, index); |
| } |
| } |
| |
| debug_assert!(stack.is_empty()); |
| self.reused_node_vec = stack; |
| } |
| |
| fn find_cycles_from_node<P>(&self, stack: &mut Vec<usize>, processor: &mut P, index: usize) |
| where |
| P: ObligationProcessor<Obligation = O>, |
| { |
| let node = &self.nodes[index]; |
| if node.state.get() == NodeState::Success { |
| match stack.iter().rposition(|&n| n == index) { |
| None => { |
| stack.push(index); |
| for &dep_index in node.dependents.iter() { |
| self.find_cycles_from_node(stack, processor, dep_index); |
| } |
| stack.pop(); |
| node.state.set(NodeState::Done); |
| } |
| Some(rpos) => { |
| // Cycle detected. |
| processor.process_backedge( |
| stack[rpos..].iter().map(|&i| &self.nodes[i].obligation), |
| PhantomData, |
| ); |
| } |
| } |
| } |
| } |
| |
| /// Compresses the vector, removing all popped nodes. This adjusts the |
| /// indices and hence invalidates any outstanding indices. `process_cycles` |
| /// must be run beforehand to remove any cycles on `Success` nodes. |
| #[inline(never)] |
| fn compress(&mut self, mut outcome_cb: impl FnMut(&O)) { |
| let orig_nodes_len = self.nodes.len(); |
| let mut node_rewrites: Vec<_> = std::mem::take(&mut self.reused_node_vec); |
| debug_assert!(node_rewrites.is_empty()); |
| node_rewrites.extend(0..orig_nodes_len); |
| let mut dead_nodes = 0; |
| |
| // Move removable nodes to the end, preserving the order of the |
| // remaining nodes. |
| // |
| // LOOP INVARIANT: |
| // self.nodes[0..index - dead_nodes] are the first remaining nodes |
| // self.nodes[index - dead_nodes..index] are all dead |
| // self.nodes[index..] are unchanged |
| for index in 0..orig_nodes_len { |
| let node = &self.nodes[index]; |
| match node.state.get() { |
| NodeState::Pending | NodeState::Waiting => { |
| if dead_nodes > 0 { |
| self.nodes.swap(index, index - dead_nodes); |
| node_rewrites[index] -= dead_nodes; |
| } |
| } |
| NodeState::Done => { |
| // The removal lookup might fail because the contents of |
| // `self.active_cache` are not guaranteed to match those of |
| // `self.nodes`. See the comment in `process_obligation` |
| // for more details. |
| let cache_key = node.obligation.as_cache_key(); |
| self.active_cache.remove(&cache_key); |
| self.done_cache.insert(cache_key); |
| |
| // Extract the success stories. |
| outcome_cb(&node.obligation); |
| node_rewrites[index] = orig_nodes_len; |
| dead_nodes += 1; |
| } |
| NodeState::Error => { |
| // We *intentionally* remove the node from the cache at this point. Otherwise |
| // tests must come up with a different type on every type error they |
| // check against. |
| self.active_cache.remove(&node.obligation.as_cache_key()); |
| self.insert_into_error_cache(index); |
| node_rewrites[index] = orig_nodes_len; |
| dead_nodes += 1; |
| } |
| NodeState::Success => unreachable!(), |
| } |
| } |
| |
| if dead_nodes > 0 { |
| // Remove the dead nodes and rewrite indices. |
| self.nodes.truncate(orig_nodes_len - dead_nodes); |
| self.apply_rewrites(&node_rewrites); |
| } |
| |
| node_rewrites.truncate(0); |
| self.reused_node_vec = node_rewrites; |
| } |
| |
| #[inline(never)] |
| fn apply_rewrites(&mut self, node_rewrites: &[usize]) { |
| let orig_nodes_len = node_rewrites.len(); |
| |
| for node in &mut self.nodes { |
| let mut i = 0; |
| while let Some(dependent) = node.dependents.get_mut(i) { |
| let new_index = node_rewrites[*dependent]; |
| if new_index >= orig_nodes_len { |
| node.dependents.swap_remove(i); |
| if i == 0 && node.has_parent { |
| // We just removed the parent. |
| node.has_parent = false; |
| } |
| } else { |
| *dependent = new_index; |
| i += 1; |
| } |
| } |
| } |
| |
| // This updating of `self.active_cache` is necessary because the |
| // removal of nodes within `compress` can fail. See above. |
| self.active_cache.retain(|_predicate, index| { |
| let new_index = node_rewrites[*index]; |
| if new_index >= orig_nodes_len { |
| false |
| } else { |
| *index = new_index; |
| true |
| } |
| }); |
| } |
| } |
