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(&mut self,
99 obligation: &mut Self::Obligation)
100 -> ProcessResult<Self::Obligation, Self::Error>;
102 /// As we do the cycle check, we invoke this callback when we
103 /// encounter an actual cycle. `cycle` is an iterator that starts
104 /// at the start of the cycle in the stack and walks **toward the
107 /// In other words, if we had O1 which required O2 which required
108 /// O3 which required O1, we would give an iterator yielding O1,
109 /// O2, O3 (O1 is not yielded twice).
110 fn process_backedge<'c, I>(&mut self,
112 _marker: PhantomData<&'c Self::Obligation>)
113 where 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,
189 parent: Option<usize>,
191 obligation_tree_id: ObligationTreeId
195 state: Cell::new(NodeState::Pending),
197 if let Some(parent_index) = parent {
202 has_parent: parent.is_some(),
208 /// The state of one node in some tree within the forest. This
209 /// represents the current state of processing for the obligation (of
210 /// type `O`) associated with this node.
212 /// The non-`Error` state transitions are as follows.
216 /// | register_obligation_at() (called by process_obligations() and
217 /// v from outside the crate)
220 /// | process_obligations()
222 /// Success(not_waiting())
224 /// | | mark_still_waiting_nodes()
226 /// | Success(still_waiting())
232 /// The `Error` state can be introduced in several places, via `error_at()`.
234 /// Outside of `ObligationForest` methods, nodes should be either `Pending` or
236 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
238 /// This obligation has not yet been selected successfully. Cannot have
242 /// This obligation was selected successfully, but it may be waiting on one
243 /// or more pending subobligations, as indicated by the `WaitingState`.
244 Success(WaitingState),
246 /// This obligation was resolved to an error. It will be removed by the
247 /// next compression step.
251 /// Indicates when a `Success` node was last (if ever) waiting on one or more
252 /// `Pending` nodes. The notion of "when" comes from `ObligationForest::gen`.
253 /// - 0: "Not waiting". This is a special value, set by `process_obligation`,
254 /// and usable because generation counting starts at 1.
255 /// - 1..ObligationForest::gen: "Was waiting" in a previous generation, but
256 /// waiting no longer. In other words, finished.
257 /// - ObligationForest::gen: "Still waiting" in this generation.
259 /// Things to note about this encoding:
260 /// - Every time `ObligationForest::gen` is incremented, all the "still
261 /// waiting" nodes automatically become "was waiting".
262 /// - `ObligationForest::is_still_waiting` is very cheap.
264 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
265 struct WaitingState(u32);
268 pub struct Outcome<O, E> {
269 /// Obligations that were completely evaluated, including all
270 /// (transitive) subobligations. Only computed if requested.
271 pub completed: Option<Vec<O>>,
273 /// Backtrace of obligations that were found to be in error.
274 pub errors: Vec<Error<O, E>>,
276 /// If true, then we saw no successful obligations, which means
277 /// there is no point in further iteration. This is based on the
278 /// assumption that when trait matching returns `Error` or
279 /// `Unchanged`, those results do not affect environmental
280 /// inference state. (Note that if we invoke `process_obligations`
281 /// with no pending obligations, stalled will be true.)
285 /// Should `process_obligations` compute the `Outcome::completed` field of its
288 pub enum DoCompleted {
293 #[derive(Debug, PartialEq, Eq)]
294 pub struct Error<O, E> {
296 pub backtrace: Vec<O>,
299 impl<O: ForestObligation> ObligationForest<O> {
300 pub fn new() -> ObligationForest<O> {
304 done_cache: Default::default(),
305 active_cache: Default::default(),
306 node_rewrites: RefCell::new(vec![]),
307 obligation_tree_id_generator: (0..).map(ObligationTreeId),
308 error_cache: Default::default(),
312 /// Returns the total number of nodes in the forest that have not
313 /// yet been fully resolved.
314 pub fn len(&self) -> usize {
318 /// Registers an obligation.
319 pub fn register_obligation(&mut self, obligation: O) {
320 // Ignore errors here - there is no guarantee of success.
321 let _ = self.register_obligation_at(obligation, None);
324 // Returns Err(()) if we already know this obligation failed.
325 fn register_obligation_at(&mut self, obligation: O, parent: Option<usize>) -> Result<(), ()> {
326 if self.done_cache.contains(obligation.as_predicate()) {
330 match self.active_cache.entry(obligation.as_predicate().clone()) {
331 Entry::Occupied(o) => {
332 let index = *o.get();
333 debug!("register_obligation_at({:?}, {:?}) - duplicate of {:?}!",
334 obligation, parent, index);
335 let node = &mut self.nodes[index];
336 if let Some(parent_index) = parent {
337 // If the node is already in `active_cache`, it has already
338 // had its chance to be marked with a parent. So if it's
339 // not already present, just dump `parent` into the
340 // dependents as a non-parent.
341 if !node.dependents.contains(&parent_index) {
342 node.dependents.push(parent_index);
345 if let NodeState::Error = node.state.get() {
351 Entry::Vacant(v) => {
352 debug!("register_obligation_at({:?}, {:?}) - ok, new index is {}",
353 obligation, parent, self.nodes.len());
355 let obligation_tree_id = match parent {
356 Some(parent_index) => self.nodes[parent_index].obligation_tree_id,
357 None => self.obligation_tree_id_generator.next().unwrap(),
363 .get(&obligation_tree_id)
364 .map(|errors| errors.contains(obligation.as_predicate()))
370 let new_index = self.nodes.len();
372 self.nodes.push(Node::new(parent, obligation, obligation_tree_id));
379 /// Converts all remaining obligations to the given error.
380 pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> {
381 let errors = self.nodes.iter().enumerate()
382 .filter(|(_index, node)| node.state.get() == NodeState::Pending)
383 .map(|(index, _node)| {
385 error: error.clone(),
386 backtrace: self.error_at(index),
391 let successful_obligations = self.compress(DoCompleted::Yes);
392 assert!(successful_obligations.unwrap().is_empty());
396 /// Returns the set of obligations that are in a pending state.
397 pub fn map_pending_obligations<P, F>(&self, f: F) -> Vec<P>
401 .filter(|node| node.state.get() == NodeState::Pending)
402 .map(|node| f(&node.obligation))
406 fn insert_into_error_cache(&mut self, index: usize) {
407 let node = &self.nodes[index];
409 .entry(node.obligation_tree_id)
411 .insert(node.obligation.as_predicate().clone());
414 fn not_waiting() -> WaitingState {
418 fn still_waiting(&self) -> WaitingState {
419 WaitingState(self.gen)
422 fn is_still_waiting(&self, waiting: WaitingState) -> bool {
423 waiting.0 == self.gen
426 /// Performs a pass through the obligation list. This must
427 /// be called in a loop until `outcome.stalled` is false.
429 /// This _cannot_ be unrolled (presently, at least).
430 pub fn process_obligations<P>(&mut self, processor: &mut P, do_completed: DoCompleted)
431 -> Outcome<O, P::Error>
432 where P: ObligationProcessor<Obligation=O>
434 debug!("process_obligations(len={})", self.nodes.len());
438 let mut errors = vec![];
439 let mut stalled = true;
441 // Note that the loop body can append new nodes, and those new nodes
442 // will then be processed by subsequent iterations of the loop.
444 // We can't use an iterator for the loop because `self.nodes` is
445 // appended to and the borrow checker would complain. We also can't use
446 // `for index in 0..self.nodes.len() { ... }` because the range would
447 // be computed with the initial length, and we would miss the appended
448 // nodes. Therefore we use a `while` loop.
450 while index < self.nodes.len() {
451 let node = &mut self.nodes[index];
453 debug!("process_obligations: node {} == {:?}", index, node);
455 // `processor.process_obligation` can modify the predicate within
456 // `node.obligation`, and that predicate is the key used for
457 // `self.active_cache`. This means that `self.active_cache` can get
458 // out of sync with `nodes`. It's not very common, but it does
459 // happen, and code in `compress` has to allow for it.
460 if node.state.get() != NodeState::Pending {
464 let result = processor.process_obligation(&mut node.obligation);
466 debug!("process_obligations: node {} got result {:?}", index, result);
469 ProcessResult::Unchanged => {
470 // No change in state.
472 ProcessResult::Changed(children) => {
473 // We are not (yet) stalled.
475 node.state.set(NodeState::Success(Self::not_waiting()));
477 for child in children {
478 let st = self.register_obligation_at(
482 if let Err(()) = st {
483 // Error already reported - propagate it
485 self.error_at(index);
489 ProcessResult::Error(err) => {
493 backtrace: self.error_at(index),
501 // There's no need to perform marking, cycle processing and compression when nothing
504 completed: if do_completed == DoCompleted::Yes { Some(vec![]) } else { None },
510 self.mark_still_waiting_nodes();
511 self.process_cycles(processor);
512 let completed = self.compress(do_completed);
514 debug!("process_obligations: complete");
523 /// Returns a vector of obligations for `p` and all of its
524 /// ancestors, putting them into the error state in the process.
525 fn error_at(&self, mut index: usize) -> Vec<O> {
526 let mut error_stack: Vec<usize> = vec![];
527 let mut trace = vec![];
530 let node = &self.nodes[index];
531 node.state.set(NodeState::Error);
532 trace.push(node.obligation.clone());
534 // The first dependent is the parent, which is treated
536 error_stack.extend(node.dependents.iter().skip(1));
537 index = node.dependents[0];
539 // No parent; treat all dependents non-specially.
540 error_stack.extend(node.dependents.iter());
545 while let Some(index) = error_stack.pop() {
546 let node = &self.nodes[index];
547 if node.state.get() != NodeState::Error {
548 node.state.set(NodeState::Error);
549 error_stack.extend(node.dependents.iter());
556 /// Mark all `Success` nodes that depend on a pending node as still
557 /// waiting. Upon completion, any `Success` nodes that aren't still waiting
558 /// can be removed by `compress`.
559 fn mark_still_waiting_nodes(&self) {
560 for node in &self.nodes {
561 if node.state.get() == NodeState::Pending {
562 // This call site is hot.
563 self.inlined_mark_dependents_as_still_waiting(node);
568 // This always-inlined function is for the hot call site.
570 fn inlined_mark_dependents_as_still_waiting(&self, node: &Node<O>) {
571 for &index in node.dependents.iter() {
572 let node = &self.nodes[index];
573 if let NodeState::Success(waiting) = node.state.get() {
574 if !self.is_still_waiting(waiting) {
575 node.state.set(NodeState::Success(self.still_waiting()));
576 // This call site is cold.
577 self.uninlined_mark_dependents_as_still_waiting(node);
583 // This never-inlined function is for the cold call site.
585 fn uninlined_mark_dependents_as_still_waiting(&self, node: &Node<O>) {
586 self.inlined_mark_dependents_as_still_waiting(node)
589 /// Report cycles between all `Success` nodes that aren't still waiting.
590 /// This must be called after `mark_still_waiting_nodes`.
591 fn process_cycles<P>(&self, processor: &mut P)
592 where P: ObligationProcessor<Obligation=O>
594 let mut stack = vec![];
596 debug!("process_cycles()");
598 for (index, node) in self.nodes.iter().enumerate() {
599 // For some benchmarks this state test is extremely hot. It's a win
600 // to handle the no-op cases immediately to avoid the cost of the
602 if let NodeState::Success(waiting) = node.state.get() {
603 if !self.is_still_waiting(waiting) {
604 self.find_cycles_from_node(&mut stack, processor, index);
609 debug!("process_cycles: complete");
611 debug_assert!(stack.is_empty());
614 fn find_cycles_from_node<P>(&self, stack: &mut Vec<usize>, processor: &mut P, index: usize)
615 where P: ObligationProcessor<Obligation=O>
617 let node = &self.nodes[index];
618 if let NodeState::Success(waiting) = node.state.get() {
619 if !self.is_still_waiting(waiting) {
620 match stack.iter().rposition(|&n| n == index) {
623 for &index in node.dependents.iter() {
624 self.find_cycles_from_node(stack, processor, index);
630 processor.process_backedge(
631 stack[rpos..].iter().map(GetObligation(&self.nodes)),
640 /// Compresses the vector, removing all popped nodes. This adjusts the
641 /// indices and hence invalidates any outstanding indices. `process_cycles`
642 /// must be run beforehand to remove any cycles on not-still-waiting
645 fn compress(&mut self, do_completed: DoCompleted) -> Option<Vec<O>> {
646 let orig_nodes_len = self.nodes.len();
647 let mut node_rewrites: Vec<_> = self.node_rewrites.replace(vec![]);
648 debug_assert!(node_rewrites.is_empty());
649 node_rewrites.extend(0..orig_nodes_len);
650 let mut dead_nodes = 0;
651 let mut removed_success_obligations: Vec<O> = vec![];
653 // Move removable nodes to the end, preserving the order of the
657 // self.nodes[0..index - dead_nodes] are the first remaining nodes
658 // self.nodes[index - dead_nodes..index] are all dead
659 // self.nodes[index..] are unchanged
660 for index in 0..orig_nodes_len {
661 let node = &self.nodes[index];
662 match node.state.get() {
663 NodeState::Pending => {
665 self.nodes.swap(index, index - dead_nodes);
666 node_rewrites[index] -= dead_nodes;
669 NodeState::Success(waiting) if self.is_still_waiting(waiting) => {
671 self.nodes.swap(index, index - dead_nodes);
672 node_rewrites[index] -= dead_nodes;
675 NodeState::Success(_) => {
676 // This lookup can fail because the contents of
677 // `self.active_cache` are not guaranteed to match those of
678 // `self.nodes`. See the comment in `process_obligation`
680 if let Some((predicate, _)) =
681 self.active_cache.remove_entry(node.obligation.as_predicate())
683 self.done_cache.insert(predicate);
685 self.done_cache.insert(node.obligation.as_predicate().clone());
687 if do_completed == DoCompleted::Yes {
688 // Extract the success stories.
689 removed_success_obligations.push(node.obligation.clone());
691 node_rewrites[index] = orig_nodes_len;
694 NodeState::Error => {
695 // We *intentionally* remove the node from the cache at this point. Otherwise
696 // tests must come up with a different type on every type error they
698 self.active_cache.remove(node.obligation.as_predicate());
699 self.insert_into_error_cache(index);
700 node_rewrites[index] = orig_nodes_len;
707 // Remove the dead nodes and rewrite indices.
708 self.nodes.truncate(orig_nodes_len - dead_nodes);
709 self.apply_rewrites(&node_rewrites);
712 node_rewrites.truncate(0);
713 self.node_rewrites.replace(node_rewrites);
715 if do_completed == DoCompleted::Yes {
716 Some(removed_success_obligations)
722 fn apply_rewrites(&mut self, node_rewrites: &[usize]) {
723 let orig_nodes_len = node_rewrites.len();
725 for node in &mut self.nodes {
727 while i < node.dependents.len() {
728 let new_index = node_rewrites[node.dependents[i]];
729 if new_index >= orig_nodes_len {
730 node.dependents.swap_remove(i);
731 if i == 0 && node.has_parent {
732 // We just removed the parent.
733 node.has_parent = false;
736 node.dependents[i] = new_index;
742 // This updating of `self.active_cache` is necessary because the
743 // removal of nodes within `compress` can fail. See above.
744 self.active_cache.retain(|_predicate, index| {
745 let new_index = node_rewrites[*index];
746 if new_index >= orig_nodes_len {
756 // I need a Clone closure.
758 struct GetObligation<'a, O>(&'a [Node<O>]);
760 impl<'a, 'b, O> FnOnce<(&'b usize,)> for GetObligation<'a, O> {
762 extern "rust-call" fn call_once(self, args: (&'b usize,)) -> &'a O {
763 &self.0[*args.0].obligation
767 impl<'a, 'b, O> FnMut<(&'b usize,)> for GetObligation<'a, O> {
768 extern "rust-call" fn call_mut(&mut self, args: (&'b usize,)) -> &'a O {
769 &self.0[*args.0].obligation