1 //! The `ObligationForest` is a utility data structure used in trait
2 //! matching to track the set of outstanding obligations (those not yet
3 //! resolved to success or error). It also tracks the "backtrace" of each
4 //! pending obligation (why we are trying to figure this out in the first
9 //! `ObligationForest` supports two main public operations (there are a
10 //! few others not discussed here):
12 //! 1. Add a new root obligations (`register_obligation`).
13 //! 2. Process the pending obligations (`process_obligations`).
15 //! When a new obligation `N` is added, it becomes the root of an
16 //! obligation tree. This tree can also carry some per-tree state `T`,
17 //! which is given at the same time. This tree is a singleton to start, so
18 //! `N` is both the root and the only leaf. Each time the
19 //! `process_obligations` method is called, it will invoke its callback
20 //! with every pending obligation (so that will include `N`, the first
21 //! time). The callback also receives a (mutable) reference to the
22 //! per-tree state `T`. The callback should process the obligation `O`
23 //! that it is given and return a `ProcessResult`:
25 //! - `Unchanged` -> ambiguous result. Obligation was neither a success
26 //! nor a failure. It is assumed that further attempts to process the
27 //! obligation will yield the same result unless something in the
28 //! surrounding environment changes.
29 //! - `Changed(C)` - the obligation was *shallowly successful*. The
30 //! vector `C` is a list of subobligations. The meaning of this is that
31 //! `O` was successful on the assumption that all the obligations in `C`
32 //! are also successful. Therefore, `O` is only considered a "true"
33 //! success if `C` is empty. Otherwise, `O` is put into a suspended
34 //! state and the obligations in `C` become the new pending
35 //! obligations. They will be processed the next time you call
36 //! `process_obligations`.
37 //! - `Error(E)` -> obligation failed with error `E`. We will collect this
38 //! error and return it from `process_obligations`, along with the
39 //! "backtrace" of obligations (that is, the list of obligations up to
40 //! and including the root of the failed obligation). No further
41 //! obligations from that same tree will be processed, since the tree is
42 //! now considered to be in error.
44 //! When the call to `process_obligations` completes, you get back an `Outcome`,
45 //! which includes three bits of information:
47 //! - `completed`: a list of obligations where processing was fully
48 //! completed without error (meaning that all transitive subobligations
49 //! have also been completed). So, for example, if the callback from
50 //! `process_obligations` returns `Changed(C)` for some obligation `O`,
51 //! then `O` will be considered completed right away if `C` is the
52 //! empty vector. Otherwise it will only be considered completed once
53 //! all the obligations in `C` have been found completed.
54 //! - `errors`: a list of errors that occurred and associated backtraces
55 //! at the time of error, which can be used to give context to the user.
56 //! - `stalled`: if true, then none of the existing obligations were
57 //! *shallowly successful* (that is, no callback returned `Changed(_)`).
58 //! This implies that all obligations were either errors or returned an
59 //! ambiguous result, which means that any further calls to
60 //! `process_obligations` would simply yield back further ambiguous
61 //! results. This is used by the `FulfillmentContext` to decide when it
62 //! has reached a steady state.
64 //! ### Implementation details
66 //! For the most part, comments specific to the implementation are in the
67 //! code. This file only contains a very high-level overview. Basically,
68 //! the forest is stored in a vector. Each element of the vector is a node
69 //! in some tree. Each node in the vector has the index of its dependents,
70 //! including the first dependent which is known as the parent. It also
71 //! has a current state, described by `NodeState`. After each processing
72 //! step, we compress the vector to remove completed and error nodes, which
73 //! aren't needed anymore.
75 use crate::fx::{FxHashMap, FxHashSet};
77 use std::cell::{Cell, RefCell};
78 use std::collections::hash_map::Entry;
81 use std::marker::PhantomData;
88 pub trait ForestObligation: Clone + Debug {
89 type Predicate: Clone + hash::Hash + Eq + Debug;
91 fn as_predicate(&self) -> &Self::Predicate;
94 pub trait ObligationProcessor {
95 type Obligation: ForestObligation;
98 fn process_obligation(
100 obligation: &mut Self::Obligation,
101 ) -> ProcessResult<Self::Obligation, Self::Error>;
103 /// As we do the cycle check, we invoke this callback when we
104 /// encounter an actual cycle. `cycle` is an iterator that starts
105 /// at the start of the cycle in the stack and walks **toward the
108 /// In other words, if we had O1 which required O2 which required
109 /// O3 which required O1, we would give an iterator yielding O1,
110 /// O2, O3 (O1 is not yielded twice).
111 fn process_backedge<'c, I>(&mut self, cycle: I, _marker: PhantomData<&'c Self::Obligation>)
113 I: Clone + Iterator<Item = &'c Self::Obligation>;
116 /// The result type used by `process_obligation`.
118 pub enum ProcessResult<O, E> {
124 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
125 struct ObligationTreeId(usize);
127 type ObligationTreeIdGenerator =
128 ::std::iter::Map<::std::ops::RangeFrom<usize>, fn(usize) -> ObligationTreeId>;
130 pub struct ObligationForest<O: ForestObligation> {
131 /// The list of obligations. In between calls to `process_obligations`,
132 /// this list only contains nodes in the `Pending` or `Success` state.
134 /// `usize` indices are used here and throughout this module, rather than
135 /// `rustc_index::newtype_index!` indices, because this code is hot enough
136 /// that the `u32`-to-`usize` conversions that would be required are
137 /// significant, and space considerations are not important.
140 /// The process generation is 1 on the first call to `process_obligations`,
141 /// 2 on the second call, etc.
144 /// A cache of predicates that have been successfully completed.
145 done_cache: FxHashSet<O::Predicate>,
147 /// A cache of the nodes in `nodes`, indexed by predicate. Unfortunately,
148 /// its contents are not guaranteed to match those of `nodes`. See the
149 /// comments in `process_obligation` for details.
150 active_cache: FxHashMap<O::Predicate, usize>,
152 /// A vector reused in compress(), to avoid allocating new vectors.
153 node_rewrites: RefCell<Vec<usize>>,
155 obligation_tree_id_generator: ObligationTreeIdGenerator,
157 /// Per tree error cache. This is used to deduplicate errors,
158 /// which is necessary to avoid trait resolution overflow in
161 /// See [this][details] for details.
163 /// [details]: https://github.com/rust-lang/rust/pull/53255#issuecomment-421184780
164 error_cache: FxHashMap<ObligationTreeId, FxHashSet<O::Predicate>>,
170 state: Cell<NodeState>,
172 /// Obligations that depend on this obligation for their completion. They
173 /// must all be in a non-pending state.
174 dependents: Vec<usize>,
176 /// If true, dependents[0] points to a "parent" node, which requires
177 /// special treatment upon error but is otherwise treated the same.
178 /// (It would be more idiomatic to store the parent node in a separate
179 /// `Option<usize>` field, but that slows down the common case of
180 /// iterating over the parent and other descendants together.)
183 /// Identifier of the obligation tree to which this node belongs.
184 obligation_tree_id: ObligationTreeId,
188 fn new(parent: Option<usize>, obligation: O, obligation_tree_id: ObligationTreeId) -> Node<O> {
191 state: Cell::new(NodeState::Pending),
192 dependents: if let Some(parent_index) = parent { vec![parent_index] } else { vec![] },
193 has_parent: parent.is_some(),
199 /// The state of one node in some tree within the forest. This
200 /// represents the current state of processing for the obligation (of
201 /// type `O`) associated with this node.
203 /// The non-`Error` state transitions are as follows.
207 /// | register_obligation_at() (called by process_obligations() and
208 /// v from outside the crate)
211 /// | process_obligations()
213 /// Success(not_waiting())
215 /// | | mark_still_waiting_nodes()
217 /// | Success(still_waiting())
223 /// The `Error` state can be introduced in several places, via `error_at()`.
225 /// Outside of `ObligationForest` methods, nodes should be either `Pending` or
227 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
229 /// This obligation has not yet been selected successfully. Cannot have
233 /// This obligation was selected successfully, but it may be waiting on one
234 /// or more pending subobligations, as indicated by the `WaitingState`.
235 Success(WaitingState),
237 /// This obligation was resolved to an error. It will be removed by the
238 /// next compression step.
242 /// Indicates when a `Success` node was last (if ever) waiting on one or more
243 /// `Pending` nodes. The notion of "when" comes from `ObligationForest::gen`.
244 /// - 0: "Not waiting". This is a special value, set by `process_obligation`,
245 /// and usable because generation counting starts at 1.
246 /// - 1..ObligationForest::gen: "Was waiting" in a previous generation, but
247 /// waiting no longer. In other words, finished.
248 /// - ObligationForest::gen: "Still waiting" in this generation.
250 /// Things to note about this encoding:
251 /// - Every time `ObligationForest::gen` is incremented, all the "still
252 /// waiting" nodes automatically become "was waiting".
253 /// - `ObligationForest::is_still_waiting` is very cheap.
255 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
256 struct WaitingState(u32);
259 pub struct Outcome<O, E> {
260 /// Obligations that were completely evaluated, including all
261 /// (transitive) subobligations. Only computed if requested.
262 pub completed: Option<Vec<O>>,
264 /// Backtrace of obligations that were found to be in error.
265 pub errors: Vec<Error<O, E>>,
267 /// If true, then we saw no successful obligations, which means
268 /// there is no point in further iteration. This is based on the
269 /// assumption that when trait matching returns `Error` or
270 /// `Unchanged`, those results do not affect environmental
271 /// inference state. (Note that if we invoke `process_obligations`
272 /// with no pending obligations, stalled will be true.)
276 /// Should `process_obligations` compute the `Outcome::completed` field of its
279 pub enum DoCompleted {
284 #[derive(Debug, PartialEq, Eq)]
285 pub struct Error<O, E> {
287 pub backtrace: Vec<O>,
290 impl<O: ForestObligation> ObligationForest<O> {
291 pub fn new() -> ObligationForest<O> {
295 done_cache: Default::default(),
296 active_cache: Default::default(),
297 node_rewrites: RefCell::new(vec![]),
298 obligation_tree_id_generator: (0..).map(ObligationTreeId),
299 error_cache: Default::default(),
303 /// Returns the total number of nodes in the forest that have not
304 /// yet been fully resolved.
305 pub fn len(&self) -> usize {
309 /// Registers an obligation.
310 pub fn register_obligation(&mut self, obligation: O) {
311 // Ignore errors here - there is no guarantee of success.
312 let _ = self.register_obligation_at(obligation, None);
315 // Returns Err(()) if we already know this obligation failed.
316 fn register_obligation_at(&mut self, obligation: O, parent: Option<usize>) -> Result<(), ()> {
317 if self.done_cache.contains(obligation.as_predicate()) {
321 match self.active_cache.entry(obligation.as_predicate().clone()) {
322 Entry::Occupied(o) => {
323 let node = &mut self.nodes[*o.get()];
324 if let Some(parent_index) = parent {
325 // If the node is already in `active_cache`, it has already
326 // had its chance to be marked with a parent. So if it's
327 // not already present, just dump `parent` into the
328 // dependents as a non-parent.
329 if !node.dependents.contains(&parent_index) {
330 node.dependents.push(parent_index);
333 if let NodeState::Error = node.state.get() { Err(()) } else { Ok(()) }
335 Entry::Vacant(v) => {
336 let obligation_tree_id = match parent {
337 Some(parent_index) => self.nodes[parent_index].obligation_tree_id,
338 None => self.obligation_tree_id_generator.next().unwrap(),
341 let already_failed = parent.is_some()
344 .get(&obligation_tree_id)
345 .map(|errors| errors.contains(obligation.as_predicate()))
351 let new_index = self.nodes.len();
353 self.nodes.push(Node::new(parent, obligation, obligation_tree_id));
360 /// Converts all remaining obligations to the given error.
361 pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> {
366 .filter(|(_index, node)| node.state.get() == NodeState::Pending)
367 .map(|(index, _node)| Error { error: error.clone(), backtrace: self.error_at(index) })
370 let successful_obligations = self.compress(DoCompleted::Yes);
371 assert!(successful_obligations.unwrap().is_empty());
375 /// Returns the set of obligations that are in a pending state.
376 pub fn map_pending_obligations<P, F>(&self, f: F) -> Vec<P>
382 .filter(|node| node.state.get() == NodeState::Pending)
383 .map(|node| f(&node.obligation))
387 fn insert_into_error_cache(&mut self, index: usize) {
388 let node = &self.nodes[index];
390 .entry(node.obligation_tree_id)
392 .insert(node.obligation.as_predicate().clone());
395 fn not_waiting() -> WaitingState {
399 fn still_waiting(&self) -> WaitingState {
400 WaitingState(self.gen)
403 fn is_still_waiting(&self, waiting: WaitingState) -> bool {
404 waiting.0 == self.gen
407 /// Performs a pass through the obligation list. This must
408 /// be called in a loop until `outcome.stalled` is false.
410 /// This _cannot_ be unrolled (presently, at least).
411 pub fn process_obligations<P>(
414 do_completed: DoCompleted,
415 ) -> Outcome<O, P::Error>
417 P: ObligationProcessor<Obligation = O>,
421 let mut errors = vec![];
422 let mut stalled = true;
424 // Note that the loop body can append new nodes, and those new nodes
425 // will then be processed by subsequent iterations of the loop.
427 // We can't use an iterator for the loop because `self.nodes` is
428 // appended to and the borrow checker would complain. We also can't use
429 // `for index in 0..self.nodes.len() { ... }` because the range would
430 // be computed with the initial length, and we would miss the appended
431 // nodes. Therefore we use a `while` loop.
433 while index < self.nodes.len() {
434 let node = &mut self.nodes[index];
436 // `processor.process_obligation` can modify the predicate within
437 // `node.obligation`, and that predicate is the key used for
438 // `self.active_cache`. This means that `self.active_cache` can get
439 // out of sync with `nodes`. It's not very common, but it does
440 // happen, and code in `compress` has to allow for it.
441 if node.state.get() != NodeState::Pending {
446 match processor.process_obligation(&mut node.obligation) {
447 ProcessResult::Unchanged => {
448 // No change in state.
450 ProcessResult::Changed(children) => {
451 // We are not (yet) stalled.
453 node.state.set(NodeState::Success(Self::not_waiting()));
455 for child in children {
456 let st = self.register_obligation_at(child, Some(index));
457 if let Err(()) = st {
458 // Error already reported - propagate it
460 self.error_at(index);
464 ProcessResult::Error(err) => {
466 errors.push(Error { error: err, backtrace: self.error_at(index) });
473 // There's no need to perform marking, cycle processing and compression when nothing
476 completed: if do_completed == DoCompleted::Yes { Some(vec![]) } else { None },
482 self.mark_still_waiting_nodes();
483 self.process_cycles(processor);
484 let completed = self.compress(do_completed);
486 Outcome { completed, errors, stalled }
489 /// Returns a vector of obligations for `p` and all of its
490 /// ancestors, putting them into the error state in the process.
491 fn error_at(&self, mut index: usize) -> Vec<O> {
492 let mut error_stack: Vec<usize> = vec![];
493 let mut trace = vec![];
496 let node = &self.nodes[index];
497 node.state.set(NodeState::Error);
498 trace.push(node.obligation.clone());
500 // The first dependent is the parent, which is treated
502 error_stack.extend(node.dependents.iter().skip(1));
503 index = node.dependents[0];
505 // No parent; treat all dependents non-specially.
506 error_stack.extend(node.dependents.iter());
511 while let Some(index) = error_stack.pop() {
512 let node = &self.nodes[index];
513 if node.state.get() != NodeState::Error {
514 node.state.set(NodeState::Error);
515 error_stack.extend(node.dependents.iter());
522 /// Mark all `Success` nodes that depend on a pending node as still
523 /// waiting. Upon completion, any `Success` nodes that aren't still waiting
524 /// can be removed by `compress`.
525 fn mark_still_waiting_nodes(&self) {
526 for node in &self.nodes {
527 if node.state.get() == NodeState::Pending {
528 // This call site is hot.
529 self.inlined_mark_dependents_as_still_waiting(node);
534 // This always-inlined function is for the hot call site.
536 fn inlined_mark_dependents_as_still_waiting(&self, node: &Node<O>) {
537 for &index in node.dependents.iter() {
538 let node = &self.nodes[index];
539 if let NodeState::Success(waiting) = node.state.get() {
540 if !self.is_still_waiting(waiting) {
541 node.state.set(NodeState::Success(self.still_waiting()));
542 // This call site is cold.
543 self.uninlined_mark_dependents_as_still_waiting(node);
549 // This never-inlined function is for the cold call site.
551 fn uninlined_mark_dependents_as_still_waiting(&self, node: &Node<O>) {
552 self.inlined_mark_dependents_as_still_waiting(node)
555 /// Report cycles between all `Success` nodes that aren't still waiting.
556 /// This must be called after `mark_still_waiting_nodes`.
557 fn process_cycles<P>(&self, processor: &mut P)
559 P: ObligationProcessor<Obligation = O>,
561 let mut stack = vec![];
563 for (index, node) in self.nodes.iter().enumerate() {
564 // For some benchmarks this state test is extremely hot. It's a win
565 // to handle the no-op cases immediately to avoid the cost of the
567 if let NodeState::Success(waiting) = node.state.get() {
568 if !self.is_still_waiting(waiting) {
569 self.find_cycles_from_node(&mut stack, processor, index, index);
574 debug_assert!(stack.is_empty());
577 fn find_cycles_from_node<P>(
579 stack: &mut Vec<usize>,
584 P: ObligationProcessor<Obligation = O>,
586 let node = &self.nodes[index];
587 if let NodeState::Success(waiting) = node.state.get() {
588 if !self.is_still_waiting(waiting) {
589 match stack.iter().rposition(|&n| n == index) {
592 for &dep_index in node.dependents.iter() {
593 // The index check avoids re-considering a node.
594 if dep_index >= min_index {
595 self.find_cycles_from_node(stack, processor, min_index, dep_index);
602 processor.process_backedge(
603 stack[rpos..].iter().map(GetObligation(&self.nodes)),
612 /// Compresses the vector, removing all popped nodes. This adjusts the
613 /// indices and hence invalidates any outstanding indices. `process_cycles`
614 /// must be run beforehand to remove any cycles on not-still-waiting
617 fn compress(&mut self, do_completed: DoCompleted) -> Option<Vec<O>> {
618 let orig_nodes_len = self.nodes.len();
619 let mut node_rewrites: Vec<_> = self.node_rewrites.replace(vec![]);
620 debug_assert!(node_rewrites.is_empty());
621 node_rewrites.extend(0..orig_nodes_len);
622 let mut dead_nodes = 0;
623 let mut removed_success_obligations: Vec<O> = vec![];
625 // Move removable nodes to the end, preserving the order of the
629 // self.nodes[0..index - dead_nodes] are the first remaining nodes
630 // self.nodes[index - dead_nodes..index] are all dead
631 // self.nodes[index..] are unchanged
632 for index in 0..orig_nodes_len {
633 let node = &self.nodes[index];
634 match node.state.get() {
635 NodeState::Pending => {
637 self.nodes.swap(index, index - dead_nodes);
638 node_rewrites[index] -= dead_nodes;
641 NodeState::Success(waiting) if self.is_still_waiting(waiting) => {
643 self.nodes.swap(index, index - dead_nodes);
644 node_rewrites[index] -= dead_nodes;
647 NodeState::Success(_) => {
648 // This lookup can fail because the contents of
649 // `self.active_cache` are not guaranteed to match those of
650 // `self.nodes`. See the comment in `process_obligation`
652 if let Some((predicate, _)) =
653 self.active_cache.remove_entry(node.obligation.as_predicate())
655 self.done_cache.insert(predicate);
657 self.done_cache.insert(node.obligation.as_predicate().clone());
659 if do_completed == DoCompleted::Yes {
660 // Extract the success stories.
661 removed_success_obligations.push(node.obligation.clone());
663 node_rewrites[index] = orig_nodes_len;
666 NodeState::Error => {
667 // We *intentionally* remove the node from the cache at this point. Otherwise
668 // tests must come up with a different type on every type error they
670 self.active_cache.remove(node.obligation.as_predicate());
671 self.insert_into_error_cache(index);
672 node_rewrites[index] = orig_nodes_len;
679 // Remove the dead nodes and rewrite indices.
680 self.nodes.truncate(orig_nodes_len - dead_nodes);
681 self.apply_rewrites(&node_rewrites);
684 node_rewrites.truncate(0);
685 self.node_rewrites.replace(node_rewrites);
687 if do_completed == DoCompleted::Yes { Some(removed_success_obligations) } else { None }
690 fn apply_rewrites(&mut self, node_rewrites: &[usize]) {
691 let orig_nodes_len = node_rewrites.len();
693 for node in &mut self.nodes {
695 while i < node.dependents.len() {
696 let new_index = node_rewrites[node.dependents[i]];
697 if new_index >= orig_nodes_len {
698 node.dependents.swap_remove(i);
699 if i == 0 && node.has_parent {
700 // We just removed the parent.
701 node.has_parent = false;
704 node.dependents[i] = new_index;
710 // This updating of `self.active_cache` is necessary because the
711 // removal of nodes within `compress` can fail. See above.
712 self.active_cache.retain(|_predicate, index| {
713 let new_index = node_rewrites[*index];
714 if new_index >= orig_nodes_len {
724 // I need a Clone closure.
726 struct GetObligation<'a, O>(&'a [Node<O>]);
728 impl<'a, 'b, O> FnOnce<(&'b usize,)> for GetObligation<'a, O> {
730 extern "rust-call" fn call_once(self, args: (&'b usize,)) -> &'a O {
731 &self.0[*args.0].obligation
735 impl<'a, 'b, O> FnMut<(&'b usize,)> for GetObligation<'a, O> {
736 extern "rust-call" fn call_mut(&mut self, args: (&'b usize,)) -> &'a O {
737 &self.0[*args.0].obligation