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//! 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 (`push_tree`). //! 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 one of three results: //! //! - `Ok(None)` -> 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. //! - `Ok(Some(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`. //! - `Err(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 three 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 `Ok(Some(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. //! - `stalled`: if true, then none of the existing obligations were //! *shallowly successful* (that is, no callback returned `Ok(Some(_))`). //! This implies that all obligations were either errors or returned an //! ambiguous result, which means that any further calls to //! `process_obligations` would simply yield back further ambiguous //! results. This is used by the `FulfillmentContext` to decide when it //! has reached a steady state. //! //! #### Snapshots //! //! The `ObligationForest` supports a limited form of snapshots; see //! `start_snapshot`, `commit_snapshot`, and `rollback_snapshot`. In //! particular, you can use a snapshot to roll back new root //! obligations. However, it is an error to attempt to //! `process_obligations` during a snapshot. //! //! ### 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 an (optional) //! parent and (for convenience) its root (which may be itself). 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 node_index; use self::node_index::NodeIndex; mod graphviz; #[cfg(test)] mod test; pub trait ForestObligation : Clone + Debug { type Predicate : Clone + hash::Hash + Eq + Debug; fn as_predicate(&self) -> &Self::Predicate; } pub trait ObligationProcessor { type Obligation : ForestObligation; type Error : Debug; 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 /// `process_obligations`, this list only contains nodes in the /// `Pending` or `Success` state (with a non-zero number of /// incomplete children). During processing, some of those nodes /// may be changed to the error state, or we may find that they /// are completed (That is, `num_incomplete_children` drops to 0). /// At the end of processing, those nodes will be removed by a /// call to `compress`. /// /// At all times we maintain the invariant that every node appears /// at a higher index than its parent. This is needed by the /// backtrace iterator (which uses `split_at`). nodes: Vec<Node<O>>, /// A cache of predicates that have been successfully completed. done_cache: FxHashSet<O::Predicate>, /// An cache of the nodes in `nodes`, indexed by predicate. waiting_cache: FxHashMap<O::Predicate, NodeIndex>, scratch: Option<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::Predicate>>, } #[derive(Debug)] struct Node<O> { obligation: O, state: Cell<NodeState>, /// The parent of a node - the original obligation of /// which it is a subobligation. Except for error reporting, /// it is just like any member of `dependents`. parent: Option<NodeIndex>, /// Obligations that depend on this obligation for their /// completion. They must all be in a non-pending state. dependents: Vec<NodeIndex>, /// Identifier of the obligation tree to which this node belongs. obligation_tree_id: ObligationTreeId, } /// 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. /// /// Outside of ObligationForest methods, nodes should be either Pending /// or Waiting. #[derive(Debug, Copy, Clone, PartialEq, Eq)] enum NodeState { /// Obligations for which selection had not yet returned a /// non-ambiguous result. 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. Error nodes are /// removed from the vector by the compression step. Error, /// This is a temporary state used in DFS loops to detect cycles, /// it should not exist outside of these DFSes. OnDfsStack, } #[derive(Debug)] pub struct Outcome<O, E> { /// Obligations that were completely evaluated, including all /// (transitive) subobligations. Only computed if requested. pub completed: Option<Vec<O>>, /// Backtrace of obligations that were found to be in error. pub errors: Vec<Error<O, E>>, /// If true, 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 if we invoke `process_obligations` /// with no pending obligations, stalled will be true.) pub stalled: bool, } /// Should `process_obligations` compute the `Outcome::completed` field of its /// result? #[derive(PartialEq)] pub enum DoCompleted { No, Yes, } #[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(), waiting_cache: Default::default(), scratch: Some(vec![]), obligation_tree_id_generator: (0..).map(|i| ObligationTreeId(i)), 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. /// /// This CAN be done in a snapshot 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<NodeIndex>) -> Result<(), ()> { if self.done_cache.contains(obligation.as_predicate()) { return Ok(()); } match self.waiting_cache.entry(obligation.as_predicate().clone()) { Entry::Occupied(o) => { debug!("register_obligation_at({:?}, {:?}) - duplicate of {:?}!", obligation, parent, o.get()); let node = &mut self.nodes[o.get().get()]; if let Some(parent) = parent { // If the node is already in `waiting_cache`, it's already // been marked with a parent. (It's possible that parent // has been cleared by `apply_rewrites`, though.) So just // dump `parent` into `node.dependents`... unless it's // already in `node.dependents` or `node.parent`. if !node.dependents.contains(&parent) && Some(parent) != node.parent { node.dependents.push(parent); } } if let NodeState::Error = node.state.get() { Err(()) } else { Ok(()) } } Entry::Vacant(v) => { debug!("register_obligation_at({:?}, {:?}) - ok, new index is {}", obligation, parent, self.nodes.len()); let obligation_tree_id = match parent { Some(p) => { let parent_node = &self.nodes[p.get()]; parent_node.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(|errors| errors.contains(obligation.as_predicate())) .unwrap_or(false); if already_failed { Err(()) } else { v.insert(NodeIndex::new(self.nodes.len())); self.nodes.push(Node::new(parent, obligation, obligation_tree_id)); Ok(()) } } } } /// Converts all remaining obligations to the given error. /// /// This cannot be done during a snapshot. pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> { let mut errors = vec![]; for index in 0..self.nodes.len() { if let NodeState::Pending = self.nodes[index].state.get() { let backtrace = self.error_at(index); errors.push(Error { error: error.clone(), backtrace, }); } } let successful_obligations = self.compress(DoCompleted::Yes); assert!(successful_obligations.unwrap().is_empty()); 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(|n| n.state.get() == NodeState::Pending) .map(|n| f(&n.obligation)) .collect() } fn insert_into_error_cache(&mut self, node_index: usize) { let node = &self.nodes[node_index]; self.error_cache .entry(node.obligation_tree_id) .or_default() .insert(node.obligation.as_predicate().clone()); } /// Performs a pass through the obligation list. This must /// be called in a loop until `outcome.stalled` is false. /// /// This _cannot_ be unrolled (presently, at least). pub fn process_obligations<P>(&mut self, processor: &mut P, do_completed: DoCompleted) -> Outcome<O, P::Error> where P: ObligationProcessor<Obligation=O> { debug!("process_obligations(len={})", self.nodes.len()); let mut errors = vec![]; let mut stalled = true; for index in 0..self.nodes.len() { debug!("process_obligations: node {} == {:?}", index, self.nodes[index]); let result = match self.nodes[index] { Node { ref state, ref mut obligation, .. } if state.get() == NodeState::Pending => processor.process_obligation(obligation), _ => continue }; debug!("process_obligations: node {} got result {:?}", index, result); match result { ProcessResult::Unchanged => { // No change in state. } ProcessResult::Changed(children) => { // We are not (yet) stalled. stalled = false; self.nodes[index].state.set(NodeState::Success); for child in children { let st = self.register_obligation_at( child, Some(NodeIndex::new(index)) ); if let Err(()) = st { // error already reported - propagate it // to our node. self.error_at(index); } } } ProcessResult::Error(err) => { stalled = false; let backtrace = self.error_at(index); errors.push(Error { error: err, backtrace, }); } } } if stalled { // There's no need to perform marking, cycle processing and compression when nothing // changed. return Outcome { completed: if do_completed == DoCompleted::Yes { Some(vec![]) } else { None }, errors, stalled, }; } self.mark_as_waiting(); self.process_cycles(processor); // Now we have to compress the result let completed = self.compress(do_completed); debug!("process_obligations: complete"); Outcome { completed, errors, stalled, } } /// Mark all `NodeState::Success` nodes as `NodeState::Done` and /// report all cycles between them. This should be called /// after `mark_as_waiting` marks all nodes with pending /// subobligations as NodeState::Waiting. fn process_cycles<P>(&mut self, processor: &mut P) where P: ObligationProcessor<Obligation=O> { let mut stack = self.scratch.take().unwrap(); debug_assert!(stack.is_empty()); debug!("process_cycles()"); for index in 0..self.nodes.len() { // For rustc-benchmarks/inflate-0.1.0 this state test is extremely // hot and the state is almost always `Pending` or `Waiting`. It's // a win to handle the no-op cases immediately to avoid the cost of // the function call. let state = self.nodes[index].state.get(); match state { NodeState::Waiting | NodeState::Pending | NodeState::Done | NodeState::Error => {}, _ => self.find_cycles_from_node(&mut stack, processor, index), } } debug!("process_cycles: complete"); debug_assert!(stack.is_empty()); self.scratch = Some(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]; let state = node.state.get(); match state { NodeState::OnDfsStack => { let index = stack.iter().rposition(|n| *n == index).unwrap(); processor.process_backedge(stack[index..].iter().map(GetObligation(&self.nodes)), PhantomData); } NodeState::Success => { node.state.set(NodeState::OnDfsStack); stack.push(index); for dependent in node.parent.iter().chain(node.dependents.iter()) { self.find_cycles_from_node(stack, processor, dependent.get()); } stack.pop(); node.state.set(NodeState::Done); }, NodeState::Waiting | NodeState::Pending => { // this node is still reachable from some pending node. We // will get to it when they are all processed. } NodeState::Done | NodeState::Error => { // already processed that node } }; } /// Returns a vector of obligations for `p` and all of its /// ancestors, putting them into the error state in the process. fn error_at(&mut self, p: usize) -> Vec<O> { let mut error_stack = self.scratch.take().unwrap(); let mut trace = vec![]; let mut n = p; loop { self.nodes[n].state.set(NodeState::Error); trace.push(self.nodes[n].obligation.clone()); error_stack.extend(self.nodes[n].dependents.iter().map(|x| x.get())); // loop to the parent match self.nodes[n].parent { Some(q) => n = q.get(), None => break } } while let Some(i) = error_stack.pop() { match self.nodes[i].state.get() { NodeState::Error => continue, _ => self.nodes[i].state.set(NodeState::Error), } let node = &self.nodes[i]; error_stack.extend( node.parent.iter().chain(node.dependents.iter()).map(|x| x.get()) ); } self.scratch = Some(error_stack); trace } #[inline] fn mark_neighbors_as_waiting_from(&self, node: &Node<O>) { for dependent in node.parent.iter().chain(node.dependents.iter()) { self.mark_as_waiting_from(&self.nodes[dependent.get()]); } } /// Marks all nodes that depend on a pending node as `NodeState::Waiting`. fn mark_as_waiting(&self) { for node in &self.nodes { if node.state.get() == NodeState::Waiting { node.state.set(NodeState::Success); } } for node in &self.nodes { if node.state.get() == NodeState::Pending { self.mark_neighbors_as_waiting_from(node); } } } fn mark_as_waiting_from(&self, node: &Node<O>) { match node.state.get() { NodeState::Waiting | NodeState::Error | NodeState::OnDfsStack => return, NodeState::Success => node.state.set(NodeState::Waiting), NodeState::Pending | NodeState::Done => {}, } self.mark_neighbors_as_waiting_from(node); } /// Compresses the vector, removing all popped nodes. This adjusts /// the indices and hence invalidates any outstanding /// indices. Cannot be used during a transaction. /// /// Beforehand, all nodes must be marked as `Done` and no cycles /// on these nodes may be present. This is done by e.g., `process_cycles`. #[inline(never)] fn compress(&mut self, do_completed: DoCompleted) -> Option<Vec<O>> { let nodes_len = self.nodes.len(); let mut node_rewrites: Vec<_> = self.scratch.take().unwrap(); node_rewrites.extend(0..nodes_len); let mut dead_nodes = 0; // Now move all popped nodes to the end. Try to keep the order. // // LOOP INVARIANT: // self.nodes[0..i - dead_nodes] are the first remaining nodes // self.nodes[i - dead_nodes..i] are all dead // self.nodes[i..] are unchanged for i in 0..self.nodes.len() { match self.nodes[i].state.get() { NodeState::Pending | NodeState::Waiting => { if dead_nodes > 0 { self.nodes.swap(i, i - dead_nodes); node_rewrites[i] -= dead_nodes; } } NodeState::Done => { // Avoid cloning the key (predicate) in case it exists in the waiting cache if let Some((predicate, _)) = self.waiting_cache .remove_entry(self.nodes[i].obligation.as_predicate()) { self.done_cache.insert(predicate); } else { self.done_cache.insert(self.nodes[i].obligation.as_predicate().clone()); } node_rewrites[i] = 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.waiting_cache.remove(self.nodes[i].obligation.as_predicate()); node_rewrites[i] = nodes_len; dead_nodes += 1; self.insert_into_error_cache(i); } NodeState::OnDfsStack | NodeState::Success => unreachable!() } } // No compression needed. if dead_nodes == 0 { node_rewrites.truncate(0); self.scratch = Some(node_rewrites); return if do_completed == DoCompleted::Yes { Some(vec![]) } else { None }; } // Pop off all the nodes we killed and extract the success // stories. let successful = if do_completed == DoCompleted::Yes { Some((0..dead_nodes) .map(|_| self.nodes.pop().unwrap()) .flat_map(|node| { match node.state.get() { NodeState::Error => None, NodeState::Done => Some(node.obligation), _ => unreachable!() } }) .collect()) } else { self.nodes.truncate(self.nodes.len() - dead_nodes); None }; self.apply_rewrites(&node_rewrites); node_rewrites.truncate(0); self.scratch = Some(node_rewrites); successful } fn apply_rewrites(&mut self, node_rewrites: &[usize]) { let nodes_len = node_rewrites.len(); for node in &mut self.nodes { if let Some(index) = node.parent { let new_index = node_rewrites[index.get()]; if new_index >= nodes_len { // parent dead due to error node.parent = None; } else { node.parent = Some(NodeIndex::new(new_index)); } } let mut i = 0; while i < node.dependents.len() { let new_index = node_rewrites[node.dependents[i].get()]; if new_index >= nodes_len { node.dependents.swap_remove(i); } else { node.dependents[i] = NodeIndex::new(new_index); i += 1; } } } let mut kill_list = vec![]; for (predicate, index) in &mut self.waiting_cache { let new_index = node_rewrites[index.get()]; if new_index >= nodes_len { kill_list.push(predicate.clone()); } else { *index = NodeIndex::new(new_index); } } for predicate in kill_list { self.waiting_cache.remove(&predicate); } } } impl<O> Node<O> { fn new( parent: Option<NodeIndex>, obligation: O, obligation_tree_id: ObligationTreeId ) -> Node<O> { Node { obligation, state: Cell::new(NodeState::Pending), parent, dependents: vec![], obligation_tree_id, } } } // I need a Clone closure #[derive(Clone)] struct GetObligation<'a, O>(&'a [Node<O>]); impl<'a, 'b, O> FnOnce<(&'b usize,)> for GetObligation<'a, O> { type Output = &'a O; extern "rust-call" fn call_once(self, args: (&'b usize,)) -> &'a O { &self.0[*args.0].obligation } } impl<'a, 'b, O> FnMut<(&'b usize,)> for GetObligation<'a, O> { extern "rust-call" fn call_mut(&mut self, args: (&'b usize,)) -> &'a O { &self.0[*args.0].obligation } }