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};
78 use std::collections::hash_map::Entry;
81 use std::marker::PhantomData;
88 pub trait ForestObligation: Clone + Debug {
89 type CacheKey: Clone + hash::Hash + Eq + Debug;
91 /// Converts this `ForestObligation` suitable for use as a cache key.
92 /// If two distinct `ForestObligations`s return the same cache key,
93 /// then it must be sound to use the result of processing one obligation
94 /// (e.g. success for error) for the other obligation
95 fn as_cache_key(&self) -> Self::CacheKey;
98 pub trait ObligationProcessor {
99 type Obligation: ForestObligation;
102 fn process_obligation(
104 obligation: &mut Self::Obligation,
105 ) -> ProcessResult<Self::Obligation, Self::Error>;
107 /// As we do the cycle check, we invoke this callback when we
108 /// encounter an actual cycle. `cycle` is an iterator that starts
109 /// at the start of the cycle in the stack and walks **toward the
112 /// In other words, if we had O1 which required O2 which required
113 /// O3 which required O1, we would give an iterator yielding O1,
114 /// O2, O3 (O1 is not yielded twice).
115 fn process_backedge<'c, I>(&mut self, cycle: I, _marker: PhantomData<&'c Self::Obligation>)
117 I: Clone + Iterator<Item = &'c Self::Obligation>;
120 /// The result type used by `process_obligation`.
122 pub enum ProcessResult<O, E> {
128 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
129 struct ObligationTreeId(usize);
131 type ObligationTreeIdGenerator =
132 std::iter::Map<std::ops::RangeFrom<usize>, fn(usize) -> ObligationTreeId>;
134 pub struct ObligationForest<O: ForestObligation> {
135 /// The list of obligations. In between calls to `process_obligations`,
136 /// this list only contains nodes in the `Pending` or `Waiting` state.
138 /// `usize` indices are used here and throughout this module, rather than
139 /// `rustc_index::newtype_index!` indices, because this code is hot enough
140 /// that the `u32`-to-`usize` conversions that would be required are
141 /// significant, and space considerations are not important.
144 /// A cache of predicates that have been successfully completed.
145 done_cache: FxHashSet<O::CacheKey>,
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::CacheKey, usize>,
152 /// A vector reused in compress() and find_cycles_from_node(), to avoid allocating new vectors.
153 reused_node_vec: 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::CacheKey>>,
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 represents the
200 /// current state of processing for the obligation (of type `O`) associated
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()
215 /// | | mark_successes()
219 /// | process_cycles()
227 /// The `Error` state can be introduced in several places, via `error_at()`.
229 /// Outside of `ObligationForest` methods, nodes should be either `Pending` or
231 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
233 /// This obligation has not yet been selected successfully. Cannot have
237 /// This obligation was selected successfully, but may or may not have
241 /// This obligation was selected successfully, but it has a pending
245 /// This obligation, along with its subobligations, are complete, and will
246 /// be removed in the next collection.
249 /// This obligation was resolved to an error. It will be removed by the
250 /// next compression step.
254 /// This trait allows us to have two different Outcome types:
255 /// - the normal one that does as little as possible
256 /// - one for tests that does some additional work and checking
257 pub trait OutcomeTrait {
262 fn mark_not_stalled(&mut self);
263 fn is_stalled(&self) -> bool;
264 fn record_completed(&mut self, outcome: &Self::Obligation);
265 fn record_error(&mut self, error: Self::Error);
269 pub struct Outcome<O, E> {
270 /// Backtrace of obligations that were found to be in error.
271 pub errors: Vec<Error<O, E>>,
273 /// If true, then we saw no successful obligations, which means
274 /// there is no point in further iteration. This is based on the
275 /// assumption that when trait matching returns `Error` or
276 /// `Unchanged`, those results do not affect environmental
277 /// inference state. (Note that if we invoke `process_obligations`
278 /// with no pending obligations, stalled will be true.)
282 impl<O, E> OutcomeTrait for Outcome<O, E> {
283 type Error = Error<O, E>;
287 Self { stalled: true, errors: vec![] }
290 fn mark_not_stalled(&mut self) {
291 self.stalled = false;
294 fn is_stalled(&self) -> bool {
298 fn record_completed(&mut self, _outcome: &Self::Obligation) {
302 fn record_error(&mut self, error: Self::Error) {
303 self.errors.push(error)
307 #[derive(Debug, PartialEq, Eq)]
308 pub struct Error<O, E> {
310 pub backtrace: Vec<O>,
313 impl<O: ForestObligation> ObligationForest<O> {
314 pub fn new() -> ObligationForest<O> {
317 done_cache: Default::default(),
318 active_cache: Default::default(),
319 reused_node_vec: vec![],
320 obligation_tree_id_generator: (0..).map(ObligationTreeId),
321 error_cache: Default::default(),
325 /// Returns the total number of nodes in the forest that have not
326 /// yet been fully resolved.
327 pub fn len(&self) -> usize {
331 /// Registers an obligation.
332 pub fn register_obligation(&mut self, obligation: O) {
333 // Ignore errors here - there is no guarantee of success.
334 let _ = self.register_obligation_at(obligation, None);
337 // Returns Err(()) if we already know this obligation failed.
338 fn register_obligation_at(&mut self, obligation: O, parent: Option<usize>) -> Result<(), ()> {
339 let cache_key = obligation.as_cache_key();
340 if self.done_cache.contains(&cache_key) {
341 debug!("register_obligation_at: ignoring already done obligation: {:?}", obligation);
345 match self.active_cache.entry(cache_key) {
346 Entry::Occupied(o) => {
347 let node = &mut self.nodes[*o.get()];
348 if let Some(parent_index) = parent {
349 // If the node is already in `active_cache`, it has already
350 // had its chance to be marked with a parent. So if it's
351 // not already present, just dump `parent` into the
352 // dependents as a non-parent.
353 if !node.dependents.contains(&parent_index) {
354 node.dependents.push(parent_index);
357 if let NodeState::Error = node.state.get() { Err(()) } else { Ok(()) }
359 Entry::Vacant(v) => {
360 let obligation_tree_id = match parent {
361 Some(parent_index) => self.nodes[parent_index].obligation_tree_id,
362 None => self.obligation_tree_id_generator.next().unwrap(),
365 let already_failed = parent.is_some()
368 .get(&obligation_tree_id)
369 .map_or(false, |errors| errors.contains(v.key()));
374 let new_index = self.nodes.len();
376 self.nodes.push(Node::new(parent, obligation, obligation_tree_id));
383 /// Converts all remaining obligations to the given error.
384 pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> {
389 .filter(|(_index, node)| node.state.get() == NodeState::Pending)
390 .map(|(index, _node)| Error { error: error.clone(), backtrace: self.error_at(index) })
393 self.compress(|_| assert!(false));
397 /// Returns the set of obligations that are in a pending state.
398 pub fn map_pending_obligations<P, F>(&self, f: F) -> Vec<P>
404 .filter(|node| node.state.get() == NodeState::Pending)
405 .map(|node| f(&node.obligation))
409 fn insert_into_error_cache(&mut self, index: usize) {
410 let node = &self.nodes[index];
412 .entry(node.obligation_tree_id)
414 .insert(node.obligation.as_cache_key());
417 /// Performs a pass through the obligation list. This must
418 /// be called in a loop until `outcome.stalled` is false.
420 /// This _cannot_ be unrolled (presently, at least).
422 pub fn process_obligations<P, OUT>(&mut self, processor: &mut P) -> OUT
424 P: ObligationProcessor<Obligation = O>,
425 OUT: OutcomeTrait<Obligation = O, Error = Error<O, P::Error>>,
427 let mut outcome = OUT::new();
429 // Note that the loop body can append new nodes, and those new nodes
430 // will then be processed by subsequent iterations of the loop.
432 // We can't use an iterator for the loop because `self.nodes` is
433 // appended to and the borrow checker would complain. We also can't use
434 // `for index in 0..self.nodes.len() { ... }` because the range would
435 // be computed with the initial length, and we would miss the appended
436 // nodes. Therefore we use a `while` loop.
438 while let Some(node) = self.nodes.get_mut(index) {
439 // `processor.process_obligation` can modify the predicate within
440 // `node.obligation`, and that predicate is the key used for
441 // `self.active_cache`. This means that `self.active_cache` can get
442 // out of sync with `nodes`. It's not very common, but it does
443 // happen, and code in `compress` has to allow for it.
444 if node.state.get() != NodeState::Pending {
449 match processor.process_obligation(&mut node.obligation) {
450 ProcessResult::Unchanged => {
451 // No change in state.
453 ProcessResult::Changed(children) => {
454 // We are not (yet) stalled.
455 outcome.mark_not_stalled();
456 node.state.set(NodeState::Success);
458 for child in children {
459 let st = self.register_obligation_at(child, Some(index));
460 if let Err(()) = st {
461 // Error already reported - propagate it
463 self.error_at(index);
467 ProcessResult::Error(err) => {
468 outcome.mark_not_stalled();
469 outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
475 // There's no need to perform marking, cycle processing and compression when nothing
477 if !outcome.is_stalled() {
478 self.mark_successes();
479 self.process_cycles(processor);
480 self.compress(|obl| outcome.record_completed(obl));
486 /// Returns a vector of obligations for `p` and all of its
487 /// ancestors, putting them into the error state in the process.
488 fn error_at(&self, mut index: usize) -> Vec<O> {
489 let mut error_stack: Vec<usize> = vec![];
490 let mut trace = vec![];
493 let node = &self.nodes[index];
494 node.state.set(NodeState::Error);
495 trace.push(node.obligation.clone());
497 // The first dependent is the parent, which is treated
499 error_stack.extend(node.dependents.iter().skip(1));
500 index = node.dependents[0];
502 // No parent; treat all dependents non-specially.
503 error_stack.extend(node.dependents.iter());
508 while let Some(index) = error_stack.pop() {
509 let node = &self.nodes[index];
510 if node.state.get() != NodeState::Error {
511 node.state.set(NodeState::Error);
512 error_stack.extend(node.dependents.iter());
519 /// Mark all `Waiting` nodes as `Success`, except those that depend on a
521 fn mark_successes(&self) {
522 // Convert all `Waiting` nodes to `Success`.
523 for node in &self.nodes {
524 if node.state.get() == NodeState::Waiting {
525 node.state.set(NodeState::Success);
529 // Convert `Success` nodes that depend on a pending node back to
531 for node in &self.nodes {
532 if node.state.get() == NodeState::Pending {
533 // This call site is hot.
534 self.inlined_mark_dependents_as_waiting(node);
539 // This always-inlined function is for the hot call site.
541 fn inlined_mark_dependents_as_waiting(&self, node: &Node<O>) {
542 for &index in node.dependents.iter() {
543 let node = &self.nodes[index];
544 let state = node.state.get();
545 if state == NodeState::Success {
546 // This call site is cold.
547 self.uninlined_mark_dependents_as_waiting(node);
549 debug_assert!(state == NodeState::Waiting || state == NodeState::Error)
554 // This never-inlined function is for the cold call site.
556 fn uninlined_mark_dependents_as_waiting(&self, node: &Node<O>) {
557 // Mark node Waiting in the cold uninlined code instead of the hot inlined
558 node.state.set(NodeState::Waiting);
559 self.inlined_mark_dependents_as_waiting(node)
562 /// Report cycles between all `Success` nodes, and convert all `Success`
563 /// nodes to `Done`. This must be called after `mark_successes`.
564 fn process_cycles<P>(&mut self, processor: &mut P)
566 P: ObligationProcessor<Obligation = O>,
568 let mut stack = std::mem::take(&mut self.reused_node_vec);
569 for (index, node) in self.nodes.iter().enumerate() {
570 // For some benchmarks this state test is extremely hot. It's a win
571 // to handle the no-op cases immediately to avoid the cost of the
573 if node.state.get() == NodeState::Success {
574 self.find_cycles_from_node(&mut stack, processor, index);
578 debug_assert!(stack.is_empty());
579 self.reused_node_vec = stack;
582 fn find_cycles_from_node<P>(&self, stack: &mut Vec<usize>, processor: &mut P, index: usize)
584 P: ObligationProcessor<Obligation = O>,
586 let node = &self.nodes[index];
587 if node.state.get() == NodeState::Success {
588 match stack.iter().rposition(|&n| n == index) {
591 for &dep_index in node.dependents.iter() {
592 self.find_cycles_from_node(stack, processor, dep_index);
595 node.state.set(NodeState::Done);
599 processor.process_backedge(
600 stack[rpos..].iter().map(|&i| &self.nodes[i].obligation),
608 /// Compresses the vector, removing all popped nodes. This adjusts the
609 /// indices and hence invalidates any outstanding indices. `process_cycles`
610 /// must be run beforehand to remove any cycles on `Success` nodes.
612 fn compress(&mut self, mut outcome_cb: impl FnMut(&O)) {
613 let orig_nodes_len = self.nodes.len();
614 let mut node_rewrites: Vec<_> = std::mem::take(&mut self.reused_node_vec);
615 debug_assert!(node_rewrites.is_empty());
616 node_rewrites.extend(0..orig_nodes_len);
617 let mut dead_nodes = 0;
619 // Move removable nodes to the end, preserving the order of the
623 // self.nodes[0..index - dead_nodes] are the first remaining nodes
624 // self.nodes[index - dead_nodes..index] are all dead
625 // self.nodes[index..] are unchanged
626 for index in 0..orig_nodes_len {
627 let node = &self.nodes[index];
628 match node.state.get() {
629 NodeState::Pending | NodeState::Waiting => {
631 self.nodes.swap(index, index - dead_nodes);
632 node_rewrites[index] -= dead_nodes;
636 // This lookup can fail because the contents of
637 // `self.active_cache` are not guaranteed to match those of
638 // `self.nodes`. See the comment in `process_obligation`
640 if let Some((predicate, _)) =
641 self.active_cache.remove_entry(&node.obligation.as_cache_key())
643 self.done_cache.insert(predicate);
645 self.done_cache.insert(node.obligation.as_cache_key().clone());
647 // Extract the success stories.
648 outcome_cb(&node.obligation);
649 node_rewrites[index] = orig_nodes_len;
652 NodeState::Error => {
653 // We *intentionally* remove the node from the cache at this point. Otherwise
654 // tests must come up with a different type on every type error they
656 self.active_cache.remove(&node.obligation.as_cache_key());
657 self.insert_into_error_cache(index);
658 node_rewrites[index] = orig_nodes_len;
661 NodeState::Success => unreachable!(),
666 // Remove the dead nodes and rewrite indices.
667 self.nodes.truncate(orig_nodes_len - dead_nodes);
668 self.apply_rewrites(&node_rewrites);
671 node_rewrites.truncate(0);
672 self.reused_node_vec = node_rewrites;
676 fn apply_rewrites(&mut self, node_rewrites: &[usize]) {
677 let orig_nodes_len = node_rewrites.len();
679 for node in &mut self.nodes {
681 while let Some(dependent) = node.dependents.get_mut(i) {
682 let new_index = node_rewrites[*dependent];
683 if new_index >= orig_nodes_len {
684 node.dependents.swap_remove(i);
685 if i == 0 && node.has_parent {
686 // We just removed the parent.
687 node.has_parent = false;
690 *dependent = new_index;
696 // This updating of `self.active_cache` is necessary because the
697 // removal of nodes within `compress` can fail. See above.
698 self.active_cache.retain(|_predicate, index| {
699 let new_index = node_rewrites[*index];
700 if new_index >= orig_nodes_len {