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Do not increase depth when evaluating nested goals of NormalizesTo #157718

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Do not increase depth when evaluating nested goals of NormalizesTo #157718

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7 changes: 5 additions & 2 deletions compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ use rustc_type_ir::inherent::*;
use rustc_type_ir::region_constraint::RegionConstraint;
use rustc_type_ir::relate::Relate;
use rustc_type_ir::relate::solver_relating::RelateExt;
use rustc_type_ir::search_graph::{CandidateHeadUsages, PathKind};
use rustc_type_ir::search_graph::{CandidateHeadUsages, IncreaseDepthForNested, PathKind};
use rustc_type_ir::solve::{
AccessedOpaques, ExternalRegionConstraints, FetchEligibleAssocItemResponse, MaybeInfo,
NoSolutionOrRerunNonErased, OpaqueTypesJank, QueryResultOrRerunNonErased, RerunCondition,
Expand Down Expand Up @@ -483,7 +483,7 @@ where
stalled_on: Option<GoalStalledOn<I>>,
) -> Result<GoalEvaluation<I>, NoSolutionOrRerunNonErased> {
let (normalization_nested_goals, goal_evaluation) =
self.evaluate_goal_raw(source, goal, stalled_on)?;
self.evaluate_goal_raw(source, goal, stalled_on, IncreaseDepthForNested::Yes)?;
assert!(normalization_nested_goals.is_empty());
Ok(goal_evaluation)
}
Expand Down Expand Up @@ -575,6 +575,7 @@ where
source: GoalSource,
goal: Goal<I, I::Predicate>,
stalled_on: Option<GoalStalledOn<I>>,
increase_depth_for_nested: IncreaseDepthForNested,
) -> Result<(NestedNormalizationGoals<I>, GoalEvaluation<I>), NoSolutionOrRerunNonErased> {
if let RerunStalled::WontMakeProgress(stalled_certainty) =
self.rerunning_stalled_goal_may_make_progress(stalled_on.as_ref())
Expand Down Expand Up @@ -652,6 +653,7 @@ where
self.cx(),
canonical_goal,
step_kind,
increase_depth_for_nested,
&mut inspect::ProofTreeBuilder::new_noop(),
);

Expand Down Expand Up @@ -691,6 +693,7 @@ where
self.cx(),
canonical_goal,
step_kind,
increase_depth_for_nested,
&mut inspect::ProofTreeBuilder::new_noop(),
);
assert!(
Expand Down
15 changes: 14 additions & 1 deletion compiler/rustc_next_trait_solver/src/solve/project_goals/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ mod free_alias;
mod inherent;
mod opaque_types;

use rustc_type_ir::search_graph::IncreaseDepthForNested;
use rustc_type_ir::solve::QueryResultOrRerunNonErased;
use rustc_type_ir::{self as ty, Interner, ProjectionPredicate};
use tracing::{instrument, trace};
Expand Down Expand Up @@ -67,7 +68,19 @@ where
let (
NestedNormalizationGoals(nested_goals),
GoalEvaluation { goal: _, certainty, stalled_on: _, has_changed: _ },
) = self.evaluate_goal_raw(GoalSource::TypeRelating, normalizes_to, None)?;
) = self.evaluate_goal_raw(
GoalSource::TypeRelating,
normalizes_to,
None,
// We don't increase depth for nested goals for this `NormalizesTo` goal, as
// evaluating `NormalizesTo` is an extra step only exists in the new solver
// that behaves like a function call rather than an independent nested goal
// evaluation, so increasing the depth may end up regressions which hit the
// recursion limits for crates compiled well with the old solver. Furthermore,
// those nested goals from `NormalizesTo` will be evaluated again as the
// caller's nested goals with increased depths anyway.
IncreaseDepthForNested::No,
)?;

trace!(?nested_goals);

Expand Down
41 changes: 31 additions & 10 deletions compiler/rustc_type_ir/src/search_graph/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -262,6 +262,12 @@ impl CandidateHeadUsages {
}
}

#[derive(Debug, Clone, Copy)]
pub enum IncreaseDepthForNested {
Yes,
No,
}

#[derive(Debug, Clone, Copy)]
struct AvailableDepth(usize);
impl AvailableDepth {
Expand All @@ -276,6 +282,13 @@ impl AvailableDepth {
stack: &Stack<D::Cx>,
) -> Option<AvailableDepth> {
if let Some(last) = stack.last() {
match last.increase_depth_for_nested {
IncreaseDepthForNested::Yes => {}
IncreaseDepthForNested::No => {
return Some(last.available_depth);
}
}

if last.available_depth.0 == 0 {
return None;
}
Expand Down Expand Up @@ -566,7 +579,7 @@ impl<X: Cx> EvaluationResult<X> {
encountered_overflow,
// Unlike `encountered_overflow`, we share `heads`, `required_depth`,
// and `nested_goals` between evaluations.
required_depth: final_entry.required_depth,
required_depth: final_entry.required_depth(),
heads: final_entry.heads,
nested_goals: final_entry.nested_goals,
// We only care about the final result.
Expand Down Expand Up @@ -619,13 +632,17 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
fn update_parent_goal(
stack: &mut Stack<X>,
step_kind_from_parent: PathKind,
required_depth_for_nested: usize,
min_reachable_for_nested: Option<AvailableDepth>,
heads: impl Iterator<Item = (StackDepth, CycleHead)>,
encountered_overflow: bool,
context: UpdateParentGoalCtxt<'_, X>,
) {
if let Some((parent_index, parent)) = stack.last_mut_with_index() {
parent.required_depth = parent.required_depth.max(required_depth_for_nested + 1);
if let Some(min_reachable_for_nested) = min_reachable_for_nested {
parent.min_reached_available_depth = AvailableDepth(
parent.min_reached_available_depth.0.min(min_reachable_for_nested.0),
);
}
parent.encountered_overflow |= encountered_overflow;

for (head_index, head) in heads {
Expand Down Expand Up @@ -739,8 +756,9 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
input,
step_kind_from_parent,
available_depth,
min_reached_available_depth: available_depth,
provisional_result: None,
required_depth: 0,
increase_depth_for_nested: IncreaseDepthForNested::Yes,
heads: Default::default(),
encountered_overflow: false,
usages: None,
Expand All @@ -761,6 +779,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
cx: X,
input: X::Input,
step_kind_from_parent: PathKind,
increase_depth_for_nested: IncreaseDepthForNested,
inspect: &mut D::ProofTreeBuilder,
) -> X::Result {
let Some(available_depth) =
Expand Down Expand Up @@ -816,7 +835,8 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
step_kind_from_parent,
available_depth,
provisional_result: None,
required_depth: 0,
min_reached_available_depth: available_depth,
increase_depth_for_nested,
heads: Default::default(),
encountered_overflow: false,
usages: None,
Expand All @@ -838,7 +858,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
Self::update_parent_goal(
&mut self.stack,
step_kind_from_parent,
evaluation_result.required_depth,
Some(AvailableDepth(available_depth.0 - evaluation_result.required_depth)),
evaluation_result.heads.iter(),
evaluation_result.encountered_overflow,
UpdateParentGoalCtxt::Ordinary(&evaluation_result.nested_goals),
Expand Down Expand Up @@ -1116,7 +1136,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
Self::update_parent_goal(
&mut self.stack,
step_kind_from_parent,
0,
None,
heads.iter(),
encountered_overflow,
UpdateParentGoalCtxt::ProvisionalCacheHit,
Expand Down Expand Up @@ -1239,7 +1259,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
Self::update_parent_goal(
&mut self.stack,
step_kind_from_parent,
required_depth,
Some(AvailableDepth(available_depth.0 - required_depth)),
heads,
encountered_overflow,
UpdateParentGoalCtxt::Ordinary(nested_goals),
Expand Down Expand Up @@ -1271,7 +1291,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
Self::update_parent_goal(
&mut self.stack,
step_kind_from_parent,
0,
None,
iter::once((head_index, head)),
false,
UpdateParentGoalCtxt::CycleOnStack(input),
Expand Down Expand Up @@ -1407,7 +1427,8 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
provisional_result: Some(result),
// We can keep these goals from previous iterations as they are only
// ever read after finalizing this evaluation.
required_depth: stack_entry.required_depth,
min_reached_available_depth: stack_entry.min_reached_available_depth,
increase_depth_for_nested: stack_entry.increase_depth_for_nested,
heads: stack_entry.heads,
nested_goals: stack_entry.nested_goals,
// We reset these two fields when rerunning this goal. We could
Expand Down
25 changes: 22 additions & 3 deletions compiler/rustc_type_ir/src/search_graph/stack.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,8 @@ use derive_where::derive_where;
use rustc_index::IndexVec;

use crate::search_graph::{
AvailableDepth, CandidateHeadUsages, Cx, CycleHeads, HeadUsages, NestedGoals, PathKind,
AvailableDepth, CandidateHeadUsages, Cx, CycleHeads, HeadUsages, IncreaseDepthForNested,
NestedGoals, PathKind,
};

rustc_index::newtype_index! {
Expand All @@ -28,8 +29,20 @@ pub(super) struct StackEntry<X: Cx> {
/// The available depth of a given goal, immutable.
pub available_depth: AvailableDepth,

/// The maximum depth required while evaluating this goal.
pub required_depth: usize,
/// The minimum available depth encountered while evaluating this goal's nested goals.
/// If there's no nested goal, this is equal to the `available_depth`.
pub min_reached_available_depth: AvailableDepth,

/// Whether evaluating nested goals of a given goal should increase the depth.
///
/// Normally, it should be `Yes`, but among rustc's predicate goals, `normalizes-to`
/// goals are exceptions. They act like functions that used for normalizing associated
/// terms while evaluating projection goals and since their expected terms are always fully
/// unconstrained intentionally, they often return ambiguous nested goals to the caller's
/// context. As these nested goals are evaluated again in the caller's context, we don't
/// want to increase depths when they are evaluated as nested goals for `normalizes-to`
/// goals, otherwise we will encounter recursion limit overflows more often.
pub increase_depth_for_nested: IncreaseDepthForNested,

/// Starts out as `None` and gets set when rerunning this
/// goal in case we encounter a cycle.
Expand Down Expand Up @@ -57,6 +70,12 @@ pub(super) struct StackEntry<X: Cx> {
pub nested_goals: NestedGoals<X>,
}

impl<X: Cx> StackEntry<X> {
pub(super) fn required_depth(&self) -> usize {
self.available_depth.0 - self.min_reached_available_depth.0
}
}

/// The stack of goals currently being computed.
///
/// An element is *deeper* in the stack if its index is *lower*.
Expand Down
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