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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 (`register_obligation`).
//! 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 a `ProcessResult`:
//!
//! - `Unchanged` -> 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.
//! - `Changed(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`.
//! - `Error(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 two 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 `Changed(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.
//!
//! Upon completion, none of the existing obligations were *shallowly
//! successful* (that is, no callback returned `Changed(_)`). This implies that
//! all obligations were either errors or returned an ambiguous result.
//!
//! ### 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 its dependents,
//! including the first dependent which is known as the parent. 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 graphviz;
#[cfg(test)]
mod tests;
pub trait ForestObligation: Clone + Debug {
type CacheKey: Clone + hash::Hash + Eq + Debug;
/// Converts this `ForestObligation` suitable for use as a cache key.
/// If two distinct `ForestObligations`s return the same cache key,
/// then it must be sound to use the result of processing one obligation
/// (e.g. success for error) for the other obligation
fn as_cache_key(&self) -> Self::CacheKey;
}
pub trait ObligationProcessor {
type Obligation: ForestObligation;
type Error: Debug;
type OUT: OutcomeTrait<Obligation = Self::Obligation, Error = Error<Self::Obligation, Self::Error>>;
/// Implementations can provide a fast-path to obligation-processing
/// by counting the prefix of the passed iterator for which
/// `needs_process_obligation` would return false.
fn skippable_obligations<'a>(
&'a self,
_it: impl Iterator<Item = &'a Self::Obligation>,
) -> usize {
0
}
fn needs_process_obligation(&self, _obligation: &Self::Obligation) -> bool;
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>,
) -> Result<(), Self::Error>
where
I: Clone + Iterator<Item = &'c Self::Obligation>;
}
/// The result type used by `process_obligation`.
// `repr(C)` to inhibit the niche filling optimization. Otherwise, the `match` appearing
// in `process_obligations` is significantly slower, which can substantially affect
// benchmarks like `rustc-perf`'s inflate and keccak.
#[repr(C)]
#[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 = impl Iterator<Item = ObligationTreeId>;
pub struct ObligationForest<O: ForestObligation> {
/// The list of obligations. In between calls to [Self::process_obligations],
/// this list only contains nodes in the `Pending` or `Waiting` state.
///
/// `usize` indices are used here and throughout this module, rather than
/// [`rustc_index::newtype_index!`] indices, because this code is hot enough
/// that the `u32`-to-`usize` conversions that would be required are
/// significant, and space considerations are not important.
nodes: Vec<Node<O>>,
/// A cache of predicates that have been successfully completed.
done_cache: FxHashSet<O::CacheKey>,
/// A cache of the nodes in `nodes`, indexed by predicate. Unfortunately,
/// its contents are not guaranteed to match those of `nodes`. See the
/// comments in `Self::process_obligation` for details.
active_cache: FxHashMap<O::CacheKey, usize>,
/// A vector reused in [Self::compress()] and [Self::find_cycles_from_node()],
/// to avoid allocating new vectors.
reused_node_vec: 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::CacheKey>>,
}
#[derive(Debug)]
struct Node<O> {
obligation: O,
state: Cell<NodeState>,
/// Obligations that depend on this obligation for their completion. They
/// must all be in a non-pending state.
dependents: Vec<usize>,
/// If true, `dependents[0]` points to a "parent" node, which requires
/// special treatment upon error but is otherwise treated the same.
/// (It would be more idiomatic to store the parent node in a separate
/// `Option<usize>` field, but that slows down the common case of
/// iterating over the parent and other descendants together.)
has_parent: bool,
/// Identifier of the obligation tree to which this node belongs.
obligation_tree_id: ObligationTreeId,
}
impl<O> Node<O> {
fn new(parent: Option<usize>, obligation: O, obligation_tree_id: ObligationTreeId) -> Node<O> {
Node {
obligation,
state: Cell::new(NodeState::Pending),
dependents: if let Some(parent_index) = parent { vec![parent_index] } else { vec![] },
has_parent: parent.is_some(),
obligation_tree_id,
}
}
}
/// 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.
///
/// The non-`Error` state transitions are as follows.
/// ```text
/// (Pre-creation)
/// |
/// | register_obligation_at() (called by process_obligations() and
/// v from outside the crate)
/// Pending
/// |
/// | process_obligations()
/// v
/// Success
/// | ^
/// | | mark_successes()
/// | v
/// | Waiting
/// |
/// | process_cycles()
/// v
/// Done
/// |
/// | compress()
/// v
/// (Removed)
/// ```
/// The `Error` state can be introduced in several places, via `error_at()`.
///
/// Outside of `ObligationForest` methods, nodes should be either `Pending` or
/// `Waiting`.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum NodeState {
/// This obligation has not yet been selected successfully. Cannot have
/// subobligations.
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. It will be removed by the
/// next compression step.
Error,
}
/// This trait allows us to have two different Outcome types:
/// - the normal one that does as little as possible
/// - one for tests that does some additional work and checking
pub trait OutcomeTrait {
type Error;
type Obligation;
fn new() -> Self;
fn record_completed(&mut self, outcome: &Self::Obligation);
fn record_error(&mut self, error: Self::Error);
}
#[derive(Debug)]
pub struct Outcome<O, E> {
/// Backtrace of obligations that were found to be in error.
pub errors: Vec<Error<O, E>>,
}
impl<O, E> OutcomeTrait for Outcome<O, E> {
type Error = Error<O, E>;
type Obligation = O;
fn new() -> Self {
Self { errors: vec![] }
}
fn record_completed(&mut self, _outcome: &Self::Obligation) {
// do nothing
}
fn record_error(&mut self, error: Self::Error) {
self.errors.push(error)
}
}
#[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(),
active_cache: Default::default(),
reused_node_vec: vec![],
obligation_tree_id_generator: (0..).map(ObligationTreeId),
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.
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<usize>) -> Result<(), ()> {
let cache_key = obligation.as_cache_key();
if self.done_cache.contains(&cache_key) {
debug!("register_obligation_at: ignoring already done obligation: {:?}", obligation);
return Ok(());
}
match self.active_cache.entry(cache_key) {
Entry::Occupied(o) => {
let node = &mut self.nodes[*o.get()];
if let Some(parent_index) = parent {
// If the node is already in `active_cache`, it has already
// had its chance to be marked with a parent. So if it's
// not already present, just dump `parent` into the
// dependents as a non-parent.
if !node.dependents.contains(&parent_index) {
node.dependents.push(parent_index);
}
}
if let NodeState::Error = node.state.get() { Err(()) } else { Ok(()) }
}
Entry::Vacant(v) => {
let obligation_tree_id = match parent {
Some(parent_index) => self.nodes[parent_index].obligation_tree_id,
None => self.obligation_tree_id_generator.next().unwrap(),
};
let already_failed = parent.is_some()
&& self
.error_cache
.get(&obligation_tree_id)
.is_some_and(|errors| errors.contains(v.key()));
if already_failed {
Err(())
} else {
let new_index = self.nodes.len();
v.insert(new_index);
self.nodes.push(Node::new(parent, obligation, obligation_tree_id));
Ok(())
}
}
}
}
/// Converts all remaining obligations to the given error.
pub fn to_errors<E: Clone>(&mut self, error: E) -> Vec<Error<O, E>> {
let errors = self
.nodes
.iter()
.enumerate()
.filter(|(_index, node)| node.state.get() == NodeState::Pending)
.map(|(index, _node)| Error { error: error.clone(), backtrace: self.error_at(index) })
.collect();
self.compress(|_| assert!(false));
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(|node| node.state.get() == NodeState::Pending)
.map(|node| f(&node.obligation))
.collect()
}
fn insert_into_error_cache(&mut self, index: usize) {
let node = &self.nodes[index];
self.error_cache
.entry(node.obligation_tree_id)
.or_default()
.insert(node.obligation.as_cache_key());
}
/// Performs a fixpoint computation over the obligation list.
#[inline(never)]
pub fn process_obligations<P>(&mut self, processor: &mut P) -> P::OUT
where
P: ObligationProcessor<Obligation = O>,
{
let mut outcome = P::OUT::new();
// Fixpoint computation: we repeat until the inner loop stalls.
loop {
let mut has_changed = false;
// This is the super fast path for cheap-to-check conditions.
let mut index =
processor.skippable_obligations(self.nodes.iter().map(|n| &n.obligation));
// Note that the loop body can append new nodes, and those new nodes
// will then be processed by subsequent iterations of the loop.
//
// We can't use an iterator for the loop because `self.nodes` is
// appended to and the borrow checker would complain. We also can't use
// `for index in 0..self.nodes.len() { ... }` because the range would
// be computed with the initial length, and we would miss the appended
// nodes. Therefore we use a `while` loop.
while let Some(node) = self.nodes.get_mut(index) {
// This is the moderately fast path when the prefix skipping above didn't work out.
if node.state.get() != NodeState::Pending
|| !processor.needs_process_obligation(&node.obligation)
{
index += 1;
continue;
}
// `processor.process_obligation` can modify the predicate within
// `node.obligation`, and that predicate is the key used for
// `self.active_cache`. This means that `self.active_cache` can get
// out of sync with `nodes`. It's not very common, but it does
// happen, and code in `compress` has to allow for it.
// This code is much less hot.
match processor.process_obligation(&mut node.obligation) {
ProcessResult::Unchanged => {
// No change in state.
}
ProcessResult::Changed(children) => {
// We are not (yet) stalled.
has_changed = true;
node.state.set(NodeState::Success);
for child in children {
let st = self.register_obligation_at(child, Some(index));
if let Err(()) = st {
// Error already reported - propagate it
// to our node.
self.error_at(index);
}
}
}
ProcessResult::Error(err) => {
has_changed = true;
outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
}
}
index += 1;
}
// If unchanged, 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 this will occur if we invoke
// `process_obligations` with no pending obligations.)
if !has_changed {
break;
}
self.mark_successes();
self.process_cycles(processor, &mut outcome);
self.compress(|obl| outcome.record_completed(obl));
}
outcome
}
/// Returns a vector of obligations for `p` and all of its
/// ancestors, putting them into the error state in the process.
fn error_at(&self, mut index: usize) -> Vec<O> {
let mut error_stack: Vec<usize> = vec![];
let mut trace = vec![];
loop {
let node = &self.nodes[index];
node.state.set(NodeState::Error);
trace.push(node.obligation.clone());
if node.has_parent {
// The first dependent is the parent, which is treated
// specially.
error_stack.extend(node.dependents.iter().skip(1));
index = node.dependents[0];
} else {
// No parent; treat all dependents non-specially.
error_stack.extend(node.dependents.iter());
break;
}
}
while let Some(index) = error_stack.pop() {
let node = &self.nodes[index];
if node.state.get() != NodeState::Error {
node.state.set(NodeState::Error);
error_stack.extend(node.dependents.iter());
}
}
trace
}
/// Mark all `Waiting` nodes as `Success`, except those that depend on a
/// pending node.
fn mark_successes(&self) {
// Convert all `Waiting` nodes to `Success`.
for node in &self.nodes {
if node.state.get() == NodeState::Waiting {
node.state.set(NodeState::Success);
}
}
// Convert `Success` nodes that depend on a pending node back to
// `Waiting`.
for node in &self.nodes {
if node.state.get() == NodeState::Pending {
// This call site is hot.
self.inlined_mark_dependents_as_waiting(node);
}
}
}
// This always-inlined function is for the hot call site.
#[inline(always)]
fn inlined_mark_dependents_as_waiting(&self, node: &Node<O>) {
for &index in node.dependents.iter() {
let node = &self.nodes[index];
let state = node.state.get();
if state == NodeState::Success {
// This call site is cold.
self.uninlined_mark_dependents_as_waiting(node);
} else {
debug_assert!(state == NodeState::Waiting || state == NodeState::Error)
}
}
}
// This never-inlined function is for the cold call site.
#[inline(never)]
fn uninlined_mark_dependents_as_waiting(&self, node: &Node<O>) {
// Mark node Waiting in the cold uninlined code instead of the hot inlined
node.state.set(NodeState::Waiting);
self.inlined_mark_dependents_as_waiting(node)
}
/// Report cycles between all `Success` nodes, and convert all `Success`
/// nodes to `Done`. This must be called after `mark_successes`.
fn process_cycles<P>(&mut self, processor: &mut P, outcome: &mut P::OUT)
where
P: ObligationProcessor<Obligation = O>,
{
let mut stack = std::mem::take(&mut self.reused_node_vec);
for (index, node) in self.nodes.iter().enumerate() {
// For some benchmarks this state test is extremely hot. It's a win
// to handle the no-op cases immediately to avoid the cost of the
// function call.
if node.state.get() == NodeState::Success {
self.find_cycles_from_node(&mut stack, processor, index, outcome);
}
}
debug_assert!(stack.is_empty());
self.reused_node_vec = stack;
}
fn find_cycles_from_node<P>(
&self,
stack: &mut Vec<usize>,
processor: &mut P,
index: usize,
outcome: &mut P::OUT,
) where
P: ObligationProcessor<Obligation = O>,
{
let node = &self.nodes[index];
if node.state.get() == NodeState::Success {
match stack.iter().rposition(|&n| n == index) {
None => {
stack.push(index);
for &dep_index in node.dependents.iter() {
self.find_cycles_from_node(stack, processor, dep_index, outcome);
}
stack.pop();
node.state.set(NodeState::Done);
}
Some(rpos) => {
// Cycle detected.
let result = processor.process_backedge(
stack[rpos..].iter().map(|&i| &self.nodes[i].obligation),
PhantomData,
);
if let Err(err) = result {
outcome.record_error(Error { error: err, backtrace: self.error_at(index) });
}
}
}
}
}
/// Compresses the vector, removing all popped nodes. This adjusts the
/// indices and hence invalidates any outstanding indices. `process_cycles`
/// must be run beforehand to remove any cycles on `Success` nodes.
#[inline(never)]
fn compress(&mut self, mut outcome_cb: impl FnMut(&O)) {
let orig_nodes_len = self.nodes.len();
let mut node_rewrites: Vec<_> = std::mem::take(&mut self.reused_node_vec);
debug_assert!(node_rewrites.is_empty());
node_rewrites.extend(0..orig_nodes_len);
let mut dead_nodes = 0;
// Move removable nodes to the end, preserving the order of the
// remaining nodes.
//
// LOOP INVARIANT:
// self.nodes[0..index - dead_nodes] are the first remaining nodes
// self.nodes[index - dead_nodes..index] are all dead
// self.nodes[index..] are unchanged
for index in 0..orig_nodes_len {
let node = &self.nodes[index];
match node.state.get() {
NodeState::Pending | NodeState::Waiting => {
if dead_nodes > 0 {
self.nodes.swap(index, index - dead_nodes);
node_rewrites[index] -= dead_nodes;
}
}
NodeState::Done => {
// The removal lookup might fail because the contents of
// `self.active_cache` are not guaranteed to match those of
// `self.nodes`. See the comment in `process_obligation`
// for more details.
let cache_key = node.obligation.as_cache_key();
self.active_cache.remove(&cache_key);
self.done_cache.insert(cache_key);
// Extract the success stories.
outcome_cb(&node.obligation);
node_rewrites[index] = orig_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.active_cache.remove(&node.obligation.as_cache_key());
self.insert_into_error_cache(index);
node_rewrites[index] = orig_nodes_len;
dead_nodes += 1;
}
NodeState::Success => unreachable!(),
}
}
if dead_nodes > 0 {
// Remove the dead nodes and rewrite indices.
self.nodes.truncate(orig_nodes_len - dead_nodes);
self.apply_rewrites(&node_rewrites);
}
node_rewrites.truncate(0);
self.reused_node_vec = node_rewrites;
}
#[inline(never)]
fn apply_rewrites(&mut self, node_rewrites: &[usize]) {
let orig_nodes_len = node_rewrites.len();
for node in &mut self.nodes {
let mut i = 0;
while let Some(dependent) = node.dependents.get_mut(i) {
let new_index = node_rewrites[*dependent];
if new_index >= orig_nodes_len {
node.dependents.swap_remove(i);
if i == 0 && node.has_parent {
// We just removed the parent.
node.has_parent = false;
}
} else {
*dependent = new_index;
i += 1;
}
}
}
// This updating of `self.active_cache` is necessary because the
// removal of nodes within `compress` can fail. See above.
self.active_cache.retain(|_predicate, index| {
let new_index = node_rewrites[*index];
if new_index >= orig_nodes_len {
false
} else {
*index = new_index;
true
}
});
}
}