rustc_type_ir/search_graph/mod.rs
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/// The search graph is responsible for caching and cycle detection in the trait
/// solver. Making sure that caching doesn't result in soundness bugs or unstable
/// query results is very challenging and makes this one of the most-involved
/// self-contained components of the compiler.
///
/// We added fuzzing support to test its correctness. The fuzzers used to verify
/// the current implementation can be found in https://github.com/lcnr/search_graph_fuzz.
///
/// This is just a quick overview of the general design, please check out the relevant
/// [rustc-dev-guide chapter](https://rustc-dev-guide.rust-lang.org/solve/caching.html) for
/// more details. Caching is split between a global cache and the per-cycle `provisional_cache`.
/// The global cache has to be completely unobservable, while the per-cycle cache may impact
/// behavior as long as the resulting behavior is still correct.
use std::cmp::Ordering;
use std::collections::BTreeSet;
use std::fmt::Debug;
use std::hash::Hash;
use std::marker::PhantomData;
use derive_where::derive_where;
use rustc_index::{Idx, IndexVec};
use tracing::debug;
use crate::data_structures::HashMap;
use crate::solve::SolverMode;
mod global_cache;
use global_cache::CacheData;
pub use global_cache::GlobalCache;
/// The search graph does not simply use `Interner` directly
/// to enable its fuzzing without having to stub the rest of
/// the interner. We don't make this a super trait of `Interner`
/// as users of the shared type library shouldn't have to care
/// about `Input` and `Result` as they are implementation details
/// of the search graph.
pub trait Cx: Copy {
type Input: Debug + Eq + Hash + Copy;
type Result: Debug + Eq + Hash + Copy;
type DepNodeIndex;
type Tracked<T: Debug + Clone>: Debug;
fn mk_tracked<T: Debug + Clone>(
self,
data: T,
dep_node_index: Self::DepNodeIndex,
) -> Self::Tracked<T>;
fn get_tracked<T: Debug + Clone>(self, tracked: &Self::Tracked<T>) -> T;
fn with_cached_task<T>(self, task: impl FnOnce() -> T) -> (T, Self::DepNodeIndex);
fn with_global_cache<R>(
self,
mode: SolverMode,
f: impl FnOnce(&mut GlobalCache<Self>) -> R,
) -> R;
fn evaluation_is_concurrent(&self) -> bool;
}
pub trait Delegate {
type Cx: Cx;
/// Whether to use the provisional cache. Set to `false` by a fuzzer when
/// validating the search graph.
const ENABLE_PROVISIONAL_CACHE: bool;
type ValidationScope;
/// Returning `Some` disables the global cache for the current goal.
///
/// The `ValidationScope` is used when fuzzing the search graph to track
/// for which goals the global cache has been disabled. This is necessary
/// as we may otherwise ignore the global cache entry for some goal `G`
/// only to later use it, failing to detect a cycle goal and potentially
/// changing the result.
fn enter_validation_scope(
cx: Self::Cx,
input: <Self::Cx as Cx>::Input,
) -> Option<Self::ValidationScope>;
const FIXPOINT_STEP_LIMIT: usize;
type ProofTreeBuilder;
fn inspect_is_noop(inspect: &mut Self::ProofTreeBuilder) -> bool;
const DIVIDE_AVAILABLE_DEPTH_ON_OVERFLOW: usize;
fn initial_provisional_result(
cx: Self::Cx,
kind: PathKind,
input: <Self::Cx as Cx>::Input,
) -> <Self::Cx as Cx>::Result;
fn is_initial_provisional_result(
cx: Self::Cx,
kind: PathKind,
input: <Self::Cx as Cx>::Input,
result: <Self::Cx as Cx>::Result,
) -> bool;
fn on_stack_overflow(
cx: Self::Cx,
inspect: &mut Self::ProofTreeBuilder,
input: <Self::Cx as Cx>::Input,
) -> <Self::Cx as Cx>::Result;
fn on_fixpoint_overflow(
cx: Self::Cx,
input: <Self::Cx as Cx>::Input,
) -> <Self::Cx as Cx>::Result;
fn is_ambiguous_result(result: <Self::Cx as Cx>::Result) -> bool;
fn propagate_ambiguity(
cx: Self::Cx,
for_input: <Self::Cx as Cx>::Input,
from_result: <Self::Cx as Cx>::Result,
) -> <Self::Cx as Cx>::Result;
fn step_is_coinductive(cx: Self::Cx, input: <Self::Cx as Cx>::Input) -> bool;
}
/// In the initial iteration of a cycle, we do not yet have a provisional
/// result. In the case we return an initial provisional result depending
/// on the kind of cycle.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PathKind {
Coinductive,
Inductive,
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum UsageKind {
Single(PathKind),
Mixed,
}
impl UsageKind {
fn merge(self, other: Self) -> Self {
match (self, other) {
(UsageKind::Mixed, _) | (_, UsageKind::Mixed) => UsageKind::Mixed,
(UsageKind::Single(lhs), UsageKind::Single(rhs)) => {
if lhs == rhs {
UsageKind::Single(lhs)
} else {
UsageKind::Mixed
}
}
}
}
fn and_merge(&mut self, other: Self) {
*self = self.merge(other);
}
}
#[derive(Debug, Clone, Copy)]
struct AvailableDepth(usize);
impl AvailableDepth {
/// Returns the remaining depth allowed for nested goals.
///
/// This is generally simply one less than the current depth.
/// However, if we encountered overflow, we significantly reduce
/// the remaining depth of all nested goals to prevent hangs
/// in case there is exponential blowup.
fn allowed_depth_for_nested<D: Delegate>(
root_depth: AvailableDepth,
stack: &IndexVec<StackDepth, StackEntry<D::Cx>>,
) -> Option<AvailableDepth> {
if let Some(last) = stack.raw.last() {
if last.available_depth.0 == 0 {
return None;
}
Some(if last.encountered_overflow {
AvailableDepth(last.available_depth.0 / D::DIVIDE_AVAILABLE_DEPTH_ON_OVERFLOW)
} else {
AvailableDepth(last.available_depth.0 - 1)
})
} else {
Some(root_depth)
}
}
/// Whether we're allowed to use a global cache entry which required
/// the given depth.
fn cache_entry_is_applicable(self, additional_depth: usize) -> bool {
self.0 >= additional_depth
}
}
/// All cycle heads a given goal depends on, ordered by their stack depth.
///
/// We therefore pop the cycle heads from highest to lowest.
#[derive(Clone, Debug, PartialEq, Eq, Default)]
struct CycleHeads {
heads: BTreeSet<StackDepth>,
}
impl CycleHeads {
fn is_empty(&self) -> bool {
self.heads.is_empty()
}
fn highest_cycle_head(&self) -> StackDepth {
*self.heads.last().unwrap()
}
fn opt_highest_cycle_head(&self) -> Option<StackDepth> {
self.heads.last().copied()
}
fn opt_lowest_cycle_head(&self) -> Option<StackDepth> {
self.heads.first().copied()
}
fn remove_highest_cycle_head(&mut self) {
let last = self.heads.pop_last();
debug_assert_ne!(last, None);
}
fn insert(&mut self, head: StackDepth) {
self.heads.insert(head);
}
fn merge(&mut self, heads: &CycleHeads) {
for &head in heads.heads.iter() {
self.insert(head);
}
}
/// Update the cycle heads of a goal at depth `this` given the cycle heads
/// of a nested goal. This merges the heads after filtering the parent goal
/// itself.
fn extend_from_child(&mut self, this: StackDepth, child: &CycleHeads) {
for &head in child.heads.iter() {
match head.cmp(&this) {
Ordering::Less => {}
Ordering::Equal => continue,
Ordering::Greater => unreachable!(),
}
self.insert(head);
}
}
}
/// The nested goals of each stack entry and the path from the
/// stack entry to that nested goal.
///
/// We only start tracking nested goals once we've either encountered
/// overflow or a solver cycle. This is a performance optimization to
/// avoid tracking nested goals on the happy path.
///
/// We use nested goals for two reasons:
/// - when rebasing provisional cache entries
/// - when checking whether we have to ignore a global cache entry as reevaluating
/// it would encounter a cycle or use a provisional cache entry.
///
/// We need to disable the global cache if using it would hide a cycle, as
/// cycles can impact behavior. The cycle ABA may have different final
/// results from a the cycle BAB depending on the cycle root.
#[derive_where(Debug, Default; X: Cx)]
struct NestedGoals<X: Cx> {
nested_goals: HashMap<X::Input, UsageKind>,
}
impl<X: Cx> NestedGoals<X> {
fn is_empty(&self) -> bool {
self.nested_goals.is_empty()
}
fn insert(&mut self, input: X::Input, path_from_entry: UsageKind) {
self.nested_goals.entry(input).or_insert(path_from_entry).and_merge(path_from_entry);
}
fn merge(&mut self, nested_goals: &NestedGoals<X>) {
#[allow(rustc::potential_query_instability)]
for (input, path_from_entry) in nested_goals.iter() {
self.insert(input, path_from_entry);
}
}
/// Adds the nested goals of a nested goal, given that the path `step_kind` from this goal
/// to the parent goal.
///
/// If the path from this goal to the nested goal is inductive, the paths from this goal
/// to all nested goals of that nested goal are also inductive. Otherwise the paths are
/// the same as for the child.
fn extend_from_child(&mut self, step_kind: PathKind, nested_goals: &NestedGoals<X>) {
#[allow(rustc::potential_query_instability)]
for (input, path_from_entry) in nested_goals.iter() {
let path_from_entry = match step_kind {
PathKind::Coinductive => path_from_entry,
PathKind::Inductive => UsageKind::Single(PathKind::Inductive),
};
self.insert(input, path_from_entry);
}
}
#[cfg_attr(feature = "nightly", rustc_lint_query_instability)]
#[allow(rustc::potential_query_instability)]
fn iter(&self) -> impl Iterator<Item = (X::Input, UsageKind)> + '_ {
self.nested_goals.iter().map(|(i, p)| (*i, *p))
}
fn get(&self, input: X::Input) -> Option<UsageKind> {
self.nested_goals.get(&input).copied()
}
fn contains(&self, input: X::Input) -> bool {
self.nested_goals.contains_key(&input)
}
}
rustc_index::newtype_index! {
#[orderable]
#[gate_rustc_only]
pub struct StackDepth {}
}
/// Stack entries of the evaluation stack. Its fields tend to be lazily
/// when popping a child goal or completely immutable.
#[derive_where(Debug; X: Cx)]
struct StackEntry<X: Cx> {
input: X::Input,
/// The available depth of a given goal, immutable.
available_depth: AvailableDepth,
/// The maximum depth reached by this stack entry, only up-to date
/// for the top of the stack and lazily updated for the rest.
reached_depth: StackDepth,
/// All cycle heads this goal depends on. Lazily updated and only
/// up-to date for the top of the stack.
heads: CycleHeads,
/// Whether evaluating this goal encountered overflow. Lazily updated.
encountered_overflow: bool,
/// Whether this goal has been used as the root of a cycle. This gets
/// eagerly updated when encountering a cycle.
has_been_used: Option<UsageKind>,
/// The nested goals of this goal, see the doc comment of the type.
nested_goals: NestedGoals<X>,
/// Starts out as `None` and gets set when rerunning this
/// goal in case we encounter a cycle.
provisional_result: Option<X::Result>,
}
/// A provisional result of an already computed goals which depends on other
/// goals still on the stack.
#[derive_where(Debug; X: Cx)]
struct ProvisionalCacheEntry<X: Cx> {
/// Whether evaluating the goal encountered overflow. This is used to
/// disable the cache entry except if the last goal on the stack is
/// already involved in this cycle.
encountered_overflow: bool,
/// All cycle heads this cache entry depends on.
heads: CycleHeads,
/// The path from the highest cycle head to this goal.
path_from_head: PathKind,
nested_goals: NestedGoals<X>,
result: X::Result,
}
pub struct SearchGraph<D: Delegate<Cx = X>, X: Cx = <D as Delegate>::Cx> {
mode: SolverMode,
root_depth: AvailableDepth,
/// The stack of goals currently being computed.
///
/// An element is *deeper* in the stack if its index is *lower*.
stack: IndexVec<StackDepth, StackEntry<X>>,
/// The provisional cache contains entries for already computed goals which
/// still depend on goals higher-up in the stack. We don't move them to the
/// global cache and track them locally instead. A provisional cache entry
/// is only valid until the result of one of its cycle heads changes.
provisional_cache: HashMap<X::Input, Vec<ProvisionalCacheEntry<X>>>,
_marker: PhantomData<D>,
}
impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
pub fn new(mode: SolverMode, root_depth: usize) -> SearchGraph<D> {
Self {
mode,
root_depth: AvailableDepth(root_depth),
stack: Default::default(),
provisional_cache: Default::default(),
_marker: PhantomData,
}
}
pub fn solver_mode(&self) -> SolverMode {
self.mode
}
/// Lazily update the stack entry for the parent goal.
/// This behavior is shared between actually evaluating goals
/// and using existing global cache entries to make sure they
/// have the same impact on the remaining evaluation.
fn update_parent_goal(
cx: X,
stack: &mut IndexVec<StackDepth, StackEntry<X>>,
reached_depth: StackDepth,
heads: &CycleHeads,
encountered_overflow: bool,
nested_goals: &NestedGoals<X>,
) {
if let Some(parent_index) = stack.last_index() {
let parent = &mut stack[parent_index];
parent.reached_depth = parent.reached_depth.max(reached_depth);
parent.encountered_overflow |= encountered_overflow;
parent.heads.extend_from_child(parent_index, heads);
let step_kind = Self::step_kind(cx, parent.input);
parent.nested_goals.extend_from_child(step_kind, nested_goals);
// Once we've got goals which encountered overflow or a cycle,
// we track all goals whose behavior may depend depend on these
// goals as this change may cause them to now depend on additional
// goals, resulting in new cycles. See the dev-guide for examples.
if !nested_goals.is_empty() {
parent.nested_goals.insert(parent.input, UsageKind::Single(PathKind::Coinductive))
}
}
}
pub fn is_empty(&self) -> bool {
if self.stack.is_empty() {
debug_assert!(self.provisional_cache.is_empty());
true
} else {
false
}
}
/// The number of goals currently in the search graph. This should only be
/// used for debugging purposes.
pub fn debug_current_depth(&self) -> usize {
self.stack.len()
}
fn step_kind(cx: X, input: X::Input) -> PathKind {
if D::step_is_coinductive(cx, input) { PathKind::Coinductive } else { PathKind::Inductive }
}
/// Whether the path from `head` to the current stack entry is inductive or coinductive.
fn stack_path_kind(
cx: X,
stack: &IndexVec<StackDepth, StackEntry<X>>,
head: StackDepth,
) -> PathKind {
if stack.raw[head.index()..].iter().all(|entry| D::step_is_coinductive(cx, entry.input)) {
PathKind::Coinductive
} else {
PathKind::Inductive
}
}
/// Probably the most involved method of the whole solver.
///
/// Given some goal which is proven via the `prove_goal` closure, this
/// handles caching, overflow, and coinductive cycles.
pub fn with_new_goal(
&mut self,
cx: X,
input: X::Input,
inspect: &mut D::ProofTreeBuilder,
mut evaluate_goal: impl FnMut(&mut Self, &mut D::ProofTreeBuilder) -> X::Result,
) -> X::Result {
let Some(available_depth) =
AvailableDepth::allowed_depth_for_nested::<D>(self.root_depth, &self.stack)
else {
return self.handle_overflow(cx, input, inspect);
};
// We check the provisional cache before checking the global cache. This simplifies
// the implementation as we can avoid worrying about cases where both the global and
// provisional cache may apply, e.g. consider the following example
//
// - xxBA overflow
// - A
// - BA cycle
// - CB :x:
if let Some(result) = self.lookup_provisional_cache(cx, input) {
return result;
}
// Lookup the global cache unless we're building proof trees or are currently
// fuzzing.
let validate_cache = if !D::inspect_is_noop(inspect) {
None
} else if let Some(scope) = D::enter_validation_scope(cx, input) {
// When validating the global cache we need to track the goals for which the
// global cache has been disabled as it may otherwise change the result for
// cyclic goals. We don't care about goals which are not on the current stack
// so it's fine to drop their scope eagerly.
self.lookup_global_cache_untracked(cx, input, available_depth)
.inspect(|expected| debug!(?expected, "validate cache entry"))
.map(|r| (scope, r))
} else if let Some(result) = self.lookup_global_cache(cx, input, available_depth) {
return result;
} else {
None
};
// Detect cycles on the stack. We do this after the global cache lookup to
// avoid iterating over the stack in case a goal has already been computed.
// This may not have an actual performance impact and we could reorder them
// as it may reduce the number of `nested_goals` we need to track.
if let Some(result) = self.check_cycle_on_stack(cx, input) {
debug_assert!(validate_cache.is_none(), "global cache and cycle on stack");
return result;
}
// Unfortunate, it looks like we actually have to compute this goalrar.
let depth = self.stack.next_index();
let entry = StackEntry {
input,
available_depth,
reached_depth: depth,
heads: Default::default(),
encountered_overflow: false,
has_been_used: None,
nested_goals: Default::default(),
provisional_result: None,
};
assert_eq!(self.stack.push(entry), depth);
// This is for global caching, so we properly track query dependencies.
// Everything that affects the `result` should be performed within this
// `with_anon_task` closure. If computing this goal depends on something
// not tracked by the cache key and from outside of this anon task, it
// must not be added to the global cache. Notably, this is the case for
// trait solver cycles participants.
let ((final_entry, result), dep_node) = cx.with_cached_task(|| {
self.evaluate_goal_in_task(cx, input, inspect, &mut evaluate_goal)
});
// We've finished computing the goal and have popped it from the stack,
// lazily update its parent goal.
Self::update_parent_goal(
cx,
&mut self.stack,
final_entry.reached_depth,
&final_entry.heads,
final_entry.encountered_overflow,
&final_entry.nested_goals,
);
// We're now done with this goal. We only add the root of cycles to the global cache.
// In case this goal is involved in a larger cycle add it to the provisional cache.
if final_entry.heads.is_empty() {
if let Some((_scope, expected)) = validate_cache {
// Do not try to move a goal into the cache again if we're testing
// the global cache.
assert_eq!(result, expected, "input={input:?}");
} else if D::inspect_is_noop(inspect) {
self.insert_global_cache(cx, input, final_entry, result, dep_node)
}
} else if D::ENABLE_PROVISIONAL_CACHE {
debug_assert!(validate_cache.is_none());
let entry = self.provisional_cache.entry(input).or_default();
let StackEntry { heads, nested_goals, encountered_overflow, .. } = final_entry;
let path_from_head = Self::stack_path_kind(cx, &self.stack, heads.highest_cycle_head());
entry.push(ProvisionalCacheEntry {
encountered_overflow,
heads,
path_from_head,
nested_goals,
result,
});
} else {
debug_assert!(validate_cache.is_none());
}
result
}
fn handle_overflow(
&mut self,
cx: X,
input: X::Input,
inspect: &mut D::ProofTreeBuilder,
) -> X::Result {
if let Some(last) = self.stack.raw.last_mut() {
last.encountered_overflow = true;
// If computing a goal `B` depends on another goal `A` and
// `A` has a nested goal which overflows, then computing `B`
// at the same depth, but with `A` already on the stack,
// would encounter a solver cycle instead, potentially
// changing the result.
//
// We must therefore not use the global cache entry for `B` in that case.
// See tests/ui/traits/next-solver/cycles/hidden-by-overflow.rs
last.nested_goals.insert(last.input, UsageKind::Single(PathKind::Coinductive));
}
debug!("encountered stack overflow");
D::on_stack_overflow(cx, inspect, input)
}
/// When reevaluating a goal with a changed provisional result, all provisional cache entry
/// which depend on this goal get invalidated.
fn clear_dependent_provisional_results(&mut self) {
let head = self.stack.next_index();
#[allow(rustc::potential_query_instability)]
self.provisional_cache.retain(|_, entries| {
entries.retain(|entry| entry.heads.highest_cycle_head() != head);
!entries.is_empty()
});
}
/// A necessary optimization to handle complex solver cycles. A provisional cache entry
/// relies on a set of cycle heads and the path towards these heads. When popping a cycle
/// head from the stack after we've finished computing it, we can't be sure that the
/// provisional cache entry is still applicable. We need to keep the cache entries to
/// prevent hangs.
///
/// What we therefore do is check whether the cycle kind of all cycles the goal of a
/// provisional cache entry is involved in would stay the same when computing the
/// goal without its cycle head on the stack. For more details, see the relevant
/// [rustc-dev-guide chapter](https://rustc-dev-guide.rust-lang.org/solve/caching.html).
///
/// This can be thought of rotating the sub-tree of this provisional result and changing
/// its entry point while making sure that all paths through this sub-tree stay the same.
///
///
/// In case the popped cycle head failed to reach a fixpoint anything which depends on
/// its provisional result is invalid. Actually discarding provisional cache entries in
/// this case would cause hangs, so we instead change the result of dependant provisional
/// cache entries to also be ambiguous. This causes some undesirable ambiguity for nested
/// goals whose result doesn't actually depend on this cycle head, but that's acceptable
/// to me.
fn rebase_provisional_cache_entries(
&mut self,
cx: X,
stack_entry: &StackEntry<X>,
mut mutate_result: impl FnMut(X::Input, X::Result) -> X::Result,
) {
let head = self.stack.next_index();
#[allow(rustc::potential_query_instability)]
self.provisional_cache.retain(|&input, entries| {
entries.retain_mut(|entry| {
let ProvisionalCacheEntry {
encountered_overflow: _,
heads,
path_from_head,
nested_goals,
result,
} = entry;
if heads.highest_cycle_head() != head {
return true;
}
// We don't try rebasing if the path from the current head
// to the cache entry is not coinductive or if the path from
// the cache entry to the current head is not coinductive.
//
// Both of these constraints could be weakened, but by only
// accepting coinductive paths we don't have to worry about
// changing the cycle kind of the remaining cycles. We can
// extend this in the future once there's a known issue
// caused by it.
if *path_from_head != PathKind::Coinductive
|| nested_goals.get(stack_entry.input).unwrap()
!= UsageKind::Single(PathKind::Coinductive)
{
return false;
}
// Merge the cycle heads of the provisional cache entry and the
// popped head. If the popped cycle head was a root, discard all
// provisional cache entries which depend on it.
heads.remove_highest_cycle_head();
heads.merge(&stack_entry.heads);
let Some(head) = heads.opt_highest_cycle_head() else {
return false;
};
// As we've made sure that the path from the new highest cycle
// head to the uses of the popped cycle head are fully coinductive,
// we can be sure that the paths to all nested goals of the popped
// cycle head remain the same. We can simply merge them.
nested_goals.merge(&stack_entry.nested_goals);
// We now care about the path from the next highest cycle head to the
// provisional cache entry.
*path_from_head = Self::stack_path_kind(cx, &self.stack, head);
// Mutate the result of the provisional cache entry in case we did
// not reach a fixpoint.
*result = mutate_result(input, *result);
true
});
!entries.is_empty()
});
}
fn lookup_provisional_cache(&mut self, cx: X, input: X::Input) -> Option<X::Result> {
if !D::ENABLE_PROVISIONAL_CACHE {
return None;
}
let entries = self.provisional_cache.get(&input)?;
for &ProvisionalCacheEntry {
encountered_overflow,
ref heads,
path_from_head,
ref nested_goals,
result,
} in entries
{
let head = heads.highest_cycle_head();
if encountered_overflow {
// This check is overly strict and very subtle. We need to make sure that if
// a global cache entry depends on some goal without adding it to its
// `nested_goals`, that goal must never have an applicable provisional
// cache entry to avoid incorrectly applying the cache entry.
//
// As we'd have to otherwise track literally all nested goals, we only
// apply provisional cache entries which encountered overflow once the
// current goal is already part of the same cycle. This check could be
// improved but seems to be good enough for now.
let last = self.stack.raw.last().unwrap();
if !last.heads.opt_lowest_cycle_head().is_some_and(|lowest| lowest <= head) {
continue;
}
}
// A provisional cache entry is only valid if the current path from its
// highest cycle head to the goal is the same.
if path_from_head == Self::stack_path_kind(cx, &self.stack, head) {
// While we don't have to track the full depth of the provisional cache entry,
// we do have to increment the required depth by one as we'd have already failed
// with overflow otherwise
let next_index = self.stack.next_index();
let last = &mut self.stack.raw.last_mut().unwrap();
let path_from_entry = Self::step_kind(cx, last.input);
last.nested_goals.insert(input, UsageKind::Single(path_from_entry));
Self::update_parent_goal(
cx,
&mut self.stack,
next_index,
heads,
false,
nested_goals,
);
debug_assert!(self.stack[head].has_been_used.is_some());
debug!(?head, ?path_from_head, "provisional cache hit");
return Some(result);
}
}
None
}
/// Even if there is a global cache entry for a given goal, we need to make sure
/// evaluating this entry would not have ended up depending on either a goal
/// already on the stack or a provisional cache entry.
fn candidate_is_applicable(
cx: X,
stack: &IndexVec<StackDepth, StackEntry<X>>,
provisional_cache: &HashMap<X::Input, Vec<ProvisionalCacheEntry<X>>>,
nested_goals: &NestedGoals<X>,
) -> bool {
// If the global cache entry didn't depend on any nested goals, it always
// applies.
if nested_goals.is_empty() {
return true;
}
// If a nested goal of the global cache entry is on the stack, we would
// definitely encounter a cycle.
if stack.iter().any(|e| nested_goals.contains(e.input)) {
debug!("cache entry not applicable due to stack");
return false;
}
// The global cache entry is also invalid if there's a provisional cache entry
// would apply for any of its nested goals.
#[allow(rustc::potential_query_instability)]
for (input, path_from_global_entry) in nested_goals.iter() {
let Some(entries) = provisional_cache.get(&input) else {
continue;
};
debug!(?input, ?path_from_global_entry, ?entries, "candidate_is_applicable");
// A provisional cache entry is applicable if the path to
// its highest cycle head is equal to the expected path.
for &ProvisionalCacheEntry {
encountered_overflow,
ref heads,
path_from_head,
nested_goals: _,
result: _,
} in entries.iter()
{
// We don't have to worry about provisional cache entries which encountered
// overflow, see the relevant comment in `lookup_provisional_cache`.
if encountered_overflow {
continue;
}
// A provisional cache entry only applies if the path from its highest head
// matches the path when encountering the goal.
let head = heads.highest_cycle_head();
let full_path = match Self::stack_path_kind(cx, stack, head) {
PathKind::Coinductive => path_from_global_entry,
PathKind::Inductive => UsageKind::Single(PathKind::Inductive),
};
match (full_path, path_from_head) {
(UsageKind::Mixed, _)
| (UsageKind::Single(PathKind::Coinductive), PathKind::Coinductive)
| (UsageKind::Single(PathKind::Inductive), PathKind::Inductive) => {
debug!(
?full_path,
?path_from_head,
"cache entry not applicable due to matching paths"
);
return false;
}
_ => debug!(?full_path, ?path_from_head, "paths don't match"),
}
}
}
true
}
/// Used when fuzzing the global cache. Accesses the global cache without
/// updating the state of the search graph.
fn lookup_global_cache_untracked(
&self,
cx: X,
input: X::Input,
available_depth: AvailableDepth,
) -> Option<X::Result> {
cx.with_global_cache(self.mode, |cache| {
cache
.get(cx, input, available_depth, |nested_goals| {
Self::candidate_is_applicable(
cx,
&self.stack,
&self.provisional_cache,
nested_goals,
)
})
.map(|c| c.result)
})
}
/// Try to fetch a previously computed result from the global cache,
/// making sure to only do so if it would match the result of reevaluating
/// this goal.
fn lookup_global_cache(
&mut self,
cx: X,
input: X::Input,
available_depth: AvailableDepth,
) -> Option<X::Result> {
cx.with_global_cache(self.mode, |cache| {
let CacheData { result, additional_depth, encountered_overflow, nested_goals } = cache
.get(cx, input, available_depth, |nested_goals| {
Self::candidate_is_applicable(
cx,
&self.stack,
&self.provisional_cache,
nested_goals,
)
})?;
// Update the reached depth of the current goal to make sure
// its state is the same regardless of whether we've used the
// global cache or not.
let reached_depth = self.stack.next_index().plus(additional_depth);
// We don't move cycle participants to the global cache, so the
// cycle heads are always empty.
let heads = Default::default();
Self::update_parent_goal(
cx,
&mut self.stack,
reached_depth,
&heads,
encountered_overflow,
nested_goals,
);
debug!(?additional_depth, "global cache hit");
Some(result)
})
}
fn check_cycle_on_stack(&mut self, cx: X, input: X::Input) -> Option<X::Result> {
let (head, _stack_entry) = self.stack.iter_enumerated().find(|(_, e)| e.input == input)?;
debug!("encountered cycle with depth {head:?}");
// We have a nested goal which directly relies on a goal deeper in the stack.
//
// We start by tagging all cycle participants, as that's necessary for caching.
//
// Finally we can return either the provisional response or the initial response
// in case we're in the first fixpoint iteration for this goal.
let path_kind = Self::stack_path_kind(cx, &self.stack, head);
let usage_kind = UsageKind::Single(path_kind);
self.stack[head].has_been_used =
Some(self.stack[head].has_been_used.map_or(usage_kind, |prev| prev.merge(usage_kind)));
// Subtle: when encountering a cyclic goal, we still first checked for overflow,
// so we have to update the reached depth.
let next_index = self.stack.next_index();
let last_index = self.stack.last_index().unwrap();
let last = &mut self.stack[last_index];
last.reached_depth = last.reached_depth.max(next_index);
let path_from_entry = Self::step_kind(cx, last.input);
last.nested_goals.insert(input, UsageKind::Single(path_from_entry));
last.nested_goals.insert(last.input, UsageKind::Single(PathKind::Coinductive));
if last_index != head {
last.heads.insert(head);
}
// Return the provisional result or, if we're in the first iteration,
// start with no constraints.
if let Some(result) = self.stack[head].provisional_result {
Some(result)
} else {
Some(D::initial_provisional_result(cx, path_kind, input))
}
}
/// Whether we've reached a fixpoint when evaluating a cycle head.
fn reached_fixpoint(
&mut self,
cx: X,
stack_entry: &StackEntry<X>,
usage_kind: UsageKind,
result: X::Result,
) -> bool {
if let Some(prev) = stack_entry.provisional_result {
prev == result
} else if let UsageKind::Single(kind) = usage_kind {
D::is_initial_provisional_result(cx, kind, stack_entry.input, result)
} else {
false
}
}
/// When we encounter a coinductive cycle, we have to fetch the
/// result of that cycle while we are still computing it. Because
/// of this we continuously recompute the cycle until the result
/// of the previous iteration is equal to the final result, at which
/// point we are done.
fn evaluate_goal_in_task(
&mut self,
cx: X,
input: X::Input,
inspect: &mut D::ProofTreeBuilder,
mut evaluate_goal: impl FnMut(&mut Self, &mut D::ProofTreeBuilder) -> X::Result,
) -> (StackEntry<X>, X::Result) {
let mut i = 0;
loop {
let result = evaluate_goal(self, inspect);
let stack_entry = self.stack.pop().unwrap();
debug_assert_eq!(stack_entry.input, input);
// If the current goal is not the root of a cycle, we are done.
//
// There are no provisional cache entries which depend on this goal.
let Some(usage_kind) = stack_entry.has_been_used else {
return (stack_entry, result);
};
// If it is a cycle head, we have to keep trying to prove it until
// we reach a fixpoint. We need to do so for all cycle heads,
// not only for the root.
//
// See tests/ui/traits/next-solver/cycles/fixpoint-rerun-all-cycle-heads.rs
// for an example.
//
// Check whether we reached a fixpoint, either because the final result
// is equal to the provisional result of the previous iteration, or because
// this was only the root of either coinductive or inductive cycles, and the
// final result is equal to the initial response for that case.
if self.reached_fixpoint(cx, &stack_entry, usage_kind, result) {
self.rebase_provisional_cache_entries(cx, &stack_entry, |_, result| result);
return (stack_entry, result);
}
// If computing this goal results in ambiguity with no constraints,
// we do not rerun it. It's incredibly difficult to get a different
// response in the next iteration in this case. These changes would
// likely either be caused by incompleteness or can change the maybe
// cause from ambiguity to overflow. Returning ambiguity always
// preserves soundness and completeness even if the goal is be known
// to succeed or fail.
//
// This prevents exponential blowup affecting multiple major crates.
// As we only get to this branch if we haven't yet reached a fixpoint,
// we also taint all provisional cache entries which depend on the
// current goal.
if D::is_ambiguous_result(result) {
self.rebase_provisional_cache_entries(cx, &stack_entry, |input, _| {
D::propagate_ambiguity(cx, input, result)
});
return (stack_entry, result);
};
// If we've reached the fixpoint step limit, we bail with overflow and taint all
// provisional cache entries which depend on the current goal.
i += 1;
if i >= D::FIXPOINT_STEP_LIMIT {
debug!("canonical cycle overflow");
let result = D::on_fixpoint_overflow(cx, input);
self.rebase_provisional_cache_entries(cx, &stack_entry, |input, _| {
D::on_fixpoint_overflow(cx, input)
});
return (stack_entry, result);
}
// Clear all provisional cache entries which depend on a previous provisional
// result of this goal and rerun.
self.clear_dependent_provisional_results();
debug!(?result, "fixpoint changed provisional results");
self.stack.push(StackEntry {
has_been_used: None,
provisional_result: Some(result),
..stack_entry
});
}
}
/// When encountering a cycle, both inductive and coinductive, we only
/// move the root into the global cache. We also store all other cycle
/// participants involved.
///
/// We must not use the global cache entry of a root goal if a cycle
/// participant is on the stack. This is necessary to prevent unstable
/// results. See the comment of `StackEntry::nested_goals` for
/// more details.
fn insert_global_cache(
&mut self,
cx: X,
input: X::Input,
final_entry: StackEntry<X>,
result: X::Result,
dep_node: X::DepNodeIndex,
) {
let additional_depth = final_entry.reached_depth.as_usize() - self.stack.len();
debug!(?final_entry, ?result, "insert global cache");
cx.with_global_cache(self.mode, |cache| {
cache.insert(
cx,
input,
result,
dep_node,
additional_depth,
final_entry.encountered_overflow,
final_entry.nested_goals,
)
})
}
}