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use super::inspect;
use super::inspect::ProofTreeBuilder;
use super::SolverMode;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::fx::FxHashSet;
use rustc_index::Idx;
use rustc_index::IndexVec;
use rustc_middle::dep_graph::dep_kinds;
use rustc_middle::traits::solve::inspect::CacheHit;
use rustc_middle::traits::solve::CacheData;
use rustc_middle::traits::solve::{CanonicalInput, Certainty, EvaluationCache, QueryResult};
use rustc_middle::ty::TyCtxt;
use rustc_session::Limit;
use std::collections::hash_map::Entry;
rustc_index::newtype_index! {
pub struct StackDepth {}
}
#[derive(Debug)]
struct StackEntry<'tcx> {
input: CanonicalInput<'tcx>,
available_depth: Limit,
// 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,
// In case of a cycle, the depth of the root.
cycle_root_depth: StackDepth,
encountered_overflow: bool,
has_been_used: bool,
/// Starts out as `None` and gets set when rerunning this
/// goal in case we encounter a cycle.
provisional_result: Option<QueryResult<'tcx>>,
/// We put only the root goal of a coinductive cycle into the global cache.
///
/// If we were to use that result when later trying to prove another cycle
/// participant, we can end up with unstable query results.
///
/// See tests/ui/new-solver/coinduction/incompleteness-unstable-result.rs for
/// an example of where this is needed.
cycle_participants: FxHashSet<CanonicalInput<'tcx>>,
}
pub(super) struct SearchGraph<'tcx> {
mode: SolverMode,
local_overflow_limit: usize,
/// The stack of goals currently being computed.
///
/// An element is *deeper* in the stack if its index is *lower*.
stack: IndexVec<StackDepth, StackEntry<'tcx>>,
stack_entries: FxHashMap<CanonicalInput<'tcx>, StackDepth>,
}
impl<'tcx> SearchGraph<'tcx> {
pub(super) fn new(tcx: TyCtxt<'tcx>, mode: SolverMode) -> SearchGraph<'tcx> {
Self {
mode,
local_overflow_limit: tcx.recursion_limit().0.checked_ilog2().unwrap_or(0) as usize,
stack: Default::default(),
stack_entries: Default::default(),
}
}
pub(super) fn solver_mode(&self) -> SolverMode {
self.mode
}
pub(super) fn local_overflow_limit(&self) -> usize {
self.local_overflow_limit
}
/// Update the stack and reached depths on cache hits.
#[instrument(level = "debug", skip(self))]
fn on_cache_hit(&mut self, additional_depth: usize, encountered_overflow: bool) {
let reached_depth = self.stack.next_index().plus(additional_depth);
if let Some(last) = self.stack.raw.last_mut() {
last.reached_depth = last.reached_depth.max(reached_depth);
last.encountered_overflow |= encountered_overflow;
}
}
/// Pops the highest goal from the stack, lazily updating the
/// the next goal in the stack.
///
/// Directly popping from the stack instead of using this method
/// would cause us to not track overflow and recursion depth correctly.
fn pop_stack(&mut self) -> StackEntry<'tcx> {
let elem = self.stack.pop().unwrap();
assert!(self.stack_entries.remove(&elem.input).is_some());
if let Some(last) = self.stack.raw.last_mut() {
last.reached_depth = last.reached_depth.max(elem.reached_depth);
last.encountered_overflow |= elem.encountered_overflow;
}
elem
}
/// The trait solver behavior is different for coherence
/// so we use a separate cache. Alternatively we could use
/// a single cache and share it between coherence and ordinary
/// trait solving.
pub(super) fn global_cache(&self, tcx: TyCtxt<'tcx>) -> &'tcx EvaluationCache<'tcx> {
match self.mode {
SolverMode::Normal => &tcx.new_solver_evaluation_cache,
SolverMode::Coherence => &tcx.new_solver_coherence_evaluation_cache,
}
}
pub(super) fn is_empty(&self) -> bool {
self.stack.is_empty()
}
/// Whether we're currently in a cycle. This should only be used
/// for debug assertions.
pub(super) fn in_cycle(&self) -> bool {
if let Some(stack_depth) = self.stack.last_index() {
// Either the current goal on the stack is the root of a cycle
// or it depends on a goal with a lower depth.
self.stack[stack_depth].has_been_used
|| self.stack[stack_depth].cycle_root_depth != stack_depth
} else {
false
}
}
/// Fetches whether the current goal encountered overflow.
///
/// This should only be used for the check in `evaluate_goal`.
pub(super) fn encountered_overflow(&self) -> bool {
if let Some(last) = self.stack.raw.last() { last.encountered_overflow } else { false }
}
/// Resets `encountered_overflow` of the current goal.
///
/// This should only be used for the check in `evaluate_goal`.
pub(super) fn reset_encountered_overflow(&mut self, encountered_overflow: bool) -> bool {
if let Some(last) = self.stack.raw.last_mut() {
let prev = last.encountered_overflow;
last.encountered_overflow = encountered_overflow;
prev
} else {
false
}
}
/// 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(
tcx: TyCtxt<'tcx>,
stack: &IndexVec<StackDepth, StackEntry<'tcx>>,
) -> Option<Limit> {
if let Some(last) = stack.raw.last() {
if last.available_depth.0 == 0 {
return None;
}
Some(if last.encountered_overflow {
Limit(last.available_depth.0 / 4)
} else {
Limit(last.available_depth.0 - 1)
})
} else {
Some(tcx.recursion_limit())
}
}
/// 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(super) fn with_new_goal(
&mut self,
tcx: TyCtxt<'tcx>,
input: CanonicalInput<'tcx>,
inspect: &mut ProofTreeBuilder<'tcx>,
mut prove_goal: impl FnMut(&mut Self, &mut ProofTreeBuilder<'tcx>) -> QueryResult<'tcx>,
) -> QueryResult<'tcx> {
// Check for overflow.
let Some(available_depth) = Self::allowed_depth_for_nested(tcx, &self.stack) else {
if let Some(last) = self.stack.raw.last_mut() {
last.encountered_overflow = true;
}
inspect.goal_evaluation_kind(inspect::WipCanonicalGoalEvaluationKind::Overflow);
return Self::response_no_constraints(tcx, input, Certainty::OVERFLOW);
};
// Try to fetch the goal from the global cache.
if inspect.use_global_cache() {
if let Some(CacheData { result, reached_depth, encountered_overflow }) =
self.global_cache(tcx).get(
tcx,
input,
|cycle_participants| {
self.stack.iter().any(|entry| cycle_participants.contains(&entry.input))
},
available_depth,
)
{
inspect.goal_evaluation_kind(inspect::WipCanonicalGoalEvaluationKind::CacheHit(
CacheHit::Global,
));
self.on_cache_hit(reached_depth, encountered_overflow);
return result;
}
}
// Check whether we're in a cycle.
match self.stack_entries.entry(input) {
// No entry, we push this goal on the stack and try to prove it.
Entry::Vacant(v) => {
let depth = self.stack.next_index();
let entry = StackEntry {
input,
available_depth,
reached_depth: depth,
cycle_root_depth: depth,
encountered_overflow: false,
has_been_used: false,
provisional_result: None,
cycle_participants: Default::default(),
};
assert_eq!(self.stack.push(entry), depth);
v.insert(depth);
}
// We have a nested goal which relies on a goal `root` deeper in the stack.
//
// We first store that we may have to reprove `root` in case the provisional
// response is not equal to the final response. We also update the depth of all
// goals which recursively depend on our current goal to depend on `root`
// instead.
//
// Finally we can return either the provisional response for that goal if we have a
// coinductive cycle or an ambiguous result if the cycle is inductive.
Entry::Occupied(entry) => {
inspect.goal_evaluation_kind(inspect::WipCanonicalGoalEvaluationKind::CacheHit(
CacheHit::Provisional,
));
let stack_depth = *entry.get();
debug!("encountered cycle with depth {stack_depth:?}");
// We start by updating the root depth of all cycle participants, and
// add all cycle participants to the root.
let root_depth = self.stack[stack_depth].cycle_root_depth;
let (prev, participants) = self.stack.raw.split_at_mut(stack_depth.as_usize() + 1);
let root = &mut prev[root_depth.as_usize()];
for entry in participants {
debug_assert!(entry.cycle_root_depth >= root_depth);
entry.cycle_root_depth = root_depth;
root.cycle_participants.insert(entry.input);
// FIXME(@lcnr): I believe that this line is needed as we could
// otherwise access a cache entry for the root of a cycle while
// computing the result for a cycle participant. This can result
// in unstable results due to incompleteness.
//
// However, a test for this would be an even more complex version of
// tests/ui/traits/new-solver/coinduction/incompleteness-unstable-result.rs.
// I did not bother to write such a test and we have no regression test
// for this. It would be good to have such a test :)
#[allow(rustc::potential_query_instability)]
root.cycle_participants.extend(entry.cycle_participants.drain());
}
// If we're in a cycle, we have to retry proving the cycle head
// until we reach a fixpoint. It is not enough to simply retry the
// `root` goal of this cycle.
//
// See tests/ui/traits/new-solver/cycles/fixpoint-rerun-all-cycle-heads.rs
// for an example.
self.stack[stack_depth].has_been_used = true;
return if let Some(result) = self.stack[stack_depth].provisional_result {
result
} else {
// If we don't have a provisional result yet we're in the first iteration,
// so we start with no constraints.
let is_coinductive = self.stack.raw[stack_depth.index()..]
.iter()
.all(|entry| entry.input.value.goal.predicate.is_coinductive(tcx));
if is_coinductive {
Self::response_no_constraints(tcx, input, Certainty::Yes)
} else {
Self::response_no_constraints(tcx, input, Certainty::OVERFLOW)
}
};
}
}
// 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.
let ((final_entry, result), dep_node) =
tcx.dep_graph.with_anon_task(tcx, dep_kinds::TraitSelect, || {
// 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.
for _ in 0..self.local_overflow_limit() {
let result = prove_goal(self, inspect);
// Check whether the current goal is the root of a cycle and whether
// we have to rerun because its provisional result differed from the
// final result.
let stack_entry = self.pop_stack();
debug_assert_eq!(stack_entry.input, input);
if stack_entry.has_been_used
&& stack_entry.provisional_result.map_or(true, |r| r != result)
{
// If so, update its provisional result and reevaluate it.
let depth = self.stack.push(StackEntry {
has_been_used: false,
provisional_result: Some(result),
..stack_entry
});
assert_eq!(self.stack_entries.insert(input, depth), None);
} else {
return (stack_entry, result);
}
}
debug!("canonical cycle overflow");
let current_entry = self.pop_stack();
let result = Self::response_no_constraints(tcx, input, Certainty::OVERFLOW);
(current_entry, result)
});
// We're now done with this goal. In case this goal is involved in a larger cycle
// do not remove it from the provisional cache and update its provisional result.
// We only add the root of cycles to the global cache.
//
// It is not possible for any nested goal to depend on something deeper on the
// stack, as this would have also updated the depth of the current goal.
if final_entry.cycle_root_depth == self.stack.next_index() {
// 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 disable the global cache entry of the root goal if a cycle
// participant is on the stack. This is necessary to prevent unstable
// results. See the comment of `StackEntry::cycle_participants` for
// more details.
let reached_depth = final_entry.reached_depth.as_usize() - self.stack.len();
self.global_cache(tcx).insert(
input,
reached_depth,
final_entry.encountered_overflow,
final_entry.cycle_participants,
dep_node,
result,
)
}
result
}
fn response_no_constraints(
tcx: TyCtxt<'tcx>,
goal: CanonicalInput<'tcx>,
certainty: Certainty,
) -> QueryResult<'tcx> {
Ok(super::response_no_constraints_raw(tcx, goal.max_universe, goal.variables, certainty))
}
}