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(), to avoid allocating new vectors.
153 node_rewrites
: 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.
255 pub struct Outcome
<O
, E
> {
256 /// Obligations that were completely evaluated, including all
257 /// (transitive) subobligations. Only computed if requested.
258 pub completed
: Option
<Vec
<O
>>,
260 /// Backtrace of obligations that were found to be in error.
261 pub errors
: Vec
<Error
<O
, E
>>,
263 /// If true, then we saw no successful obligations, which means
264 /// there is no point in further iteration. This is based on the
265 /// assumption that when trait matching returns `Error` or
266 /// `Unchanged`, those results do not affect environmental
267 /// inference state. (Note that if we invoke `process_obligations`
268 /// with no pending obligations, stalled will be true.)
272 /// Should `process_obligations` compute the `Outcome::completed` field of its
275 pub enum DoCompleted
{
280 #[derive(Debug, PartialEq, Eq)]
281 pub struct Error
<O
, E
> {
283 pub backtrace
: Vec
<O
>,
286 impl<O
: ForestObligation
> ObligationForest
<O
> {
287 pub fn new() -> ObligationForest
<O
> {
290 done_cache
: Default
::default(),
291 active_cache
: Default
::default(),
292 node_rewrites
: vec
![],
293 obligation_tree_id_generator
: (0..).map(ObligationTreeId
),
294 error_cache
: Default
::default(),
298 /// Returns the total number of nodes in the forest that have not
299 /// yet been fully resolved.
300 pub fn len(&self) -> usize {
304 /// Registers an obligation.
305 pub fn register_obligation(&mut self, obligation
: O
) {
306 // Ignore errors here - there is no guarantee of success.
307 let _
= self.register_obligation_at(obligation
, None
);
310 // Returns Err(()) if we already know this obligation failed.
311 fn register_obligation_at(&mut self, obligation
: O
, parent
: Option
<usize>) -> Result
<(), ()> {
312 if self.done_cache
.contains(&obligation
.as_cache_key()) {
313 debug
!("register_obligation_at: ignoring already done obligation: {:?}", obligation
);
317 match self.active_cache
.entry(obligation
.as_cache_key()) {
318 Entry
::Occupied(o
) => {
319 let node
= &mut self.nodes
[*o
.get()];
320 if let Some(parent_index
) = parent
{
321 // If the node is already in `active_cache`, it has already
322 // had its chance to be marked with a parent. So if it's
323 // not already present, just dump `parent` into the
324 // dependents as a non-parent.
325 if !node
.dependents
.contains(&parent_index
) {
326 node
.dependents
.push(parent_index
);
329 if let NodeState
::Error
= node
.state
.get() { Err(()) }
else { Ok(()) }
331 Entry
::Vacant(v
) => {
332 let obligation_tree_id
= match parent
{
333 Some(parent_index
) => self.nodes
[parent_index
].obligation_tree_id
,
334 None
=> self.obligation_tree_id_generator
.next().unwrap(),
337 let already_failed
= parent
.is_some()
340 .get(&obligation_tree_id
)
341 .map(|errors
| errors
.contains(&obligation
.as_cache_key()))
347 let new_index
= self.nodes
.len();
349 self.nodes
.push(Node
::new(parent
, obligation
, obligation_tree_id
));
356 /// Converts all remaining obligations to the given error.
357 pub fn to_errors
<E
: Clone
>(&mut self, error
: E
) -> Vec
<Error
<O
, E
>> {
362 .filter(|(_index
, node
)| node
.state
.get() == NodeState
::Pending
)
363 .map(|(index
, _node
)| Error { error: error.clone(), backtrace: self.error_at(index) }
)
366 let successful_obligations
= self.compress(DoCompleted
::Yes
);
367 assert
!(successful_obligations
.unwrap().is_empty());
371 /// Returns the set of obligations that are in a pending state.
372 pub fn map_pending_obligations
<P
, F
>(&self, f
: F
) -> Vec
<P
>
378 .filter(|node
| node
.state
.get() == NodeState
::Pending
)
379 .map(|node
| f(&node
.obligation
))
383 fn insert_into_error_cache(&mut self, index
: usize) {
384 let node
= &self.nodes
[index
];
386 .entry(node
.obligation_tree_id
)
388 .insert(node
.obligation
.as_cache_key());
391 /// Performs a pass through the obligation list. This must
392 /// be called in a loop until `outcome.stalled` is false.
394 /// This _cannot_ be unrolled (presently, at least).
395 pub fn process_obligations
<P
>(
398 do_completed
: DoCompleted
,
399 ) -> Outcome
<O
, P
::Error
>
401 P
: ObligationProcessor
<Obligation
= O
>,
403 let mut errors
= vec
![];
404 let mut stalled
= true;
406 // Note that the loop body can append new nodes, and those new nodes
407 // will then be processed by subsequent iterations of the loop.
409 // We can't use an iterator for the loop because `self.nodes` is
410 // appended to and the borrow checker would complain. We also can't use
411 // `for index in 0..self.nodes.len() { ... }` because the range would
412 // be computed with the initial length, and we would miss the appended
413 // nodes. Therefore we use a `while` loop.
415 while let Some(node
) = self.nodes
.get_mut(index
) {
416 // `processor.process_obligation` can modify the predicate within
417 // `node.obligation`, and that predicate is the key used for
418 // `self.active_cache`. This means that `self.active_cache` can get
419 // out of sync with `nodes`. It's not very common, but it does
420 // happen, and code in `compress` has to allow for it.
421 if node
.state
.get() != NodeState
::Pending
{
426 match processor
.process_obligation(&mut node
.obligation
) {
427 ProcessResult
::Unchanged
=> {
428 // No change in state.
430 ProcessResult
::Changed(children
) => {
431 // We are not (yet) stalled.
433 node
.state
.set(NodeState
::Success
);
435 for child
in children
{
436 let st
= self.register_obligation_at(child
, Some(index
));
437 if let Err(()) = st
{
438 // Error already reported - propagate it
440 self.error_at(index
);
444 ProcessResult
::Error(err
) => {
446 errors
.push(Error { error: err, backtrace: self.error_at(index) }
);
453 // There's no need to perform marking, cycle processing and compression when nothing
456 completed
: if do_completed
== DoCompleted
::Yes { Some(vec![]) }
else { None }
,
462 self.mark_successes();
463 self.process_cycles(processor
);
464 let completed
= self.compress(do_completed
);
466 Outcome { completed, errors, stalled }
469 /// Returns a vector of obligations for `p` and all of its
470 /// ancestors, putting them into the error state in the process.
471 fn error_at(&self, mut index
: usize) -> Vec
<O
> {
472 let mut error_stack
: Vec
<usize> = vec
![];
473 let mut trace
= vec
![];
476 let node
= &self.nodes
[index
];
477 node
.state
.set(NodeState
::Error
);
478 trace
.push(node
.obligation
.clone());
480 // The first dependent is the parent, which is treated
482 error_stack
.extend(node
.dependents
.iter().skip(1));
483 index
= node
.dependents
[0];
485 // No parent; treat all dependents non-specially.
486 error_stack
.extend(node
.dependents
.iter());
491 while let Some(index
) = error_stack
.pop() {
492 let node
= &self.nodes
[index
];
493 if node
.state
.get() != NodeState
::Error
{
494 node
.state
.set(NodeState
::Error
);
495 error_stack
.extend(node
.dependents
.iter());
502 /// Mark all `Waiting` nodes as `Success`, except those that depend on a
504 fn mark_successes(&self) {
505 // Convert all `Waiting` nodes to `Success`.
506 for node
in &self.nodes
{
507 if node
.state
.get() == NodeState
::Waiting
{
508 node
.state
.set(NodeState
::Success
);
512 // Convert `Success` nodes that depend on a pending node back to
514 for node
in &self.nodes
{
515 if node
.state
.get() == NodeState
::Pending
{
516 // This call site is hot.
517 self.inlined_mark_dependents_as_waiting(node
);
522 // This always-inlined function is for the hot call site.
524 fn inlined_mark_dependents_as_waiting(&self, node
: &Node
<O
>) {
525 for &index
in node
.dependents
.iter() {
526 let node
= &self.nodes
[index
];
527 let state
= node
.state
.get();
528 if state
== NodeState
::Success
{
529 node
.state
.set(NodeState
::Waiting
);
530 // This call site is cold.
531 self.uninlined_mark_dependents_as_waiting(node
);
533 debug_assert
!(state
== NodeState
::Waiting
|| state
== NodeState
::Error
)
538 // This never-inlined function is for the cold call site.
540 fn uninlined_mark_dependents_as_waiting(&self, node
: &Node
<O
>) {
541 self.inlined_mark_dependents_as_waiting(node
)
544 /// Report cycles between all `Success` nodes, and convert all `Success`
545 /// nodes to `Done`. This must be called after `mark_successes`.
546 fn process_cycles
<P
>(&self, processor
: &mut P
)
548 P
: ObligationProcessor
<Obligation
= O
>,
550 let mut stack
= vec
![];
552 for (index
, node
) in self.nodes
.iter().enumerate() {
553 // For some benchmarks this state test is extremely hot. It's a win
554 // to handle the no-op cases immediately to avoid the cost of the
556 if node
.state
.get() == NodeState
::Success
{
557 self.find_cycles_from_node(&mut stack
, processor
, index
);
561 debug_assert
!(stack
.is_empty());
564 fn find_cycles_from_node
<P
>(&self, stack
: &mut Vec
<usize>, processor
: &mut P
, index
: usize)
566 P
: ObligationProcessor
<Obligation
= O
>,
568 let node
= &self.nodes
[index
];
569 if node
.state
.get() == NodeState
::Success
{
570 match stack
.iter().rposition(|&n
| n
== index
) {
573 for &dep_index
in node
.dependents
.iter() {
574 self.find_cycles_from_node(stack
, processor
, dep_index
);
577 node
.state
.set(NodeState
::Done
);
581 processor
.process_backedge(
582 stack
[rpos
..].iter().map(GetObligation(&self.nodes
)),
590 /// Compresses the vector, removing all popped nodes. This adjusts the
591 /// indices and hence invalidates any outstanding indices. `process_cycles`
592 /// must be run beforehand to remove any cycles on `Success` nodes.
594 fn compress(&mut self, do_completed
: DoCompleted
) -> Option
<Vec
<O
>> {
595 let orig_nodes_len
= self.nodes
.len();
596 let mut node_rewrites
: Vec
<_
> = std
::mem
::take(&mut self.node_rewrites
);
597 debug_assert
!(node_rewrites
.is_empty());
598 node_rewrites
.extend(0..orig_nodes_len
);
599 let mut dead_nodes
= 0;
600 let mut removed_done_obligations
: Vec
<O
> = vec
![];
602 // Move removable nodes to the end, preserving the order of the
606 // self.nodes[0..index - dead_nodes] are the first remaining nodes
607 // self.nodes[index - dead_nodes..index] are all dead
608 // self.nodes[index..] are unchanged
609 for index
in 0..orig_nodes_len
{
610 let node
= &self.nodes
[index
];
611 match node
.state
.get() {
612 NodeState
::Pending
| NodeState
::Waiting
=> {
614 self.nodes
.swap(index
, index
- dead_nodes
);
615 node_rewrites
[index
] -= dead_nodes
;
619 // This lookup can fail because the contents of
620 // `self.active_cache` are not guaranteed to match those of
621 // `self.nodes`. See the comment in `process_obligation`
623 if let Some((predicate
, _
)) =
624 self.active_cache
.remove_entry(&node
.obligation
.as_cache_key())
626 self.done_cache
.insert(predicate
);
628 self.done_cache
.insert(node
.obligation
.as_cache_key().clone());
630 if do_completed
== DoCompleted
::Yes
{
631 // Extract the success stories.
632 removed_done_obligations
.push(node
.obligation
.clone());
634 node_rewrites
[index
] = orig_nodes_len
;
637 NodeState
::Error
=> {
638 // We *intentionally* remove the node from the cache at this point. Otherwise
639 // tests must come up with a different type on every type error they
641 self.active_cache
.remove(&node
.obligation
.as_cache_key());
642 self.insert_into_error_cache(index
);
643 node_rewrites
[index
] = orig_nodes_len
;
646 NodeState
::Success
=> unreachable
!(),
651 // Remove the dead nodes and rewrite indices.
652 self.nodes
.truncate(orig_nodes_len
- dead_nodes
);
653 self.apply_rewrites(&node_rewrites
);
656 node_rewrites
.truncate(0);
657 self.node_rewrites
= node_rewrites
;
659 if do_completed
== DoCompleted
::Yes { Some(removed_done_obligations) }
else { None }
662 fn apply_rewrites(&mut self, node_rewrites
: &[usize]) {
663 let orig_nodes_len
= node_rewrites
.len();
665 for node
in &mut self.nodes
{
667 while let Some(dependent
) = node
.dependents
.get_mut(i
) {
668 let new_index
= node_rewrites
[*dependent
];
669 if new_index
>= orig_nodes_len
{
670 node
.dependents
.swap_remove(i
);
671 if i
== 0 && node
.has_parent
{
672 // We just removed the parent.
673 node
.has_parent
= false;
676 *dependent
= new_index
;
682 // This updating of `self.active_cache` is necessary because the
683 // removal of nodes within `compress` can fail. See above.
684 self.active_cache
.retain(|_predicate
, index
| {
685 let new_index
= node_rewrites
[*index
];
686 if new_index
>= orig_nodes_len
{
696 // I need a Clone closure.
698 struct GetObligation
<'a
, O
>(&'a
[Node
<O
>]);
700 impl<'a
, 'b
, O
> FnOnce
<(&'b
usize,)> for GetObligation
<'a
, O
> {
702 extern "rust-call" fn call_once(self, args
: (&'b
usize,)) -> &'a O
{
703 &self.0[*args
.0].obligation
707 impl<'a
, 'b
, O
> FnMut
<(&'b
usize,)> for GetObligation
<'a
, O
> {
708 extern "rust-call" fn call_mut(&mut self, args
: (&'b
usize,)) -> &'a O
{
709 &self.0[*args
.0].obligation