Auto merge of #119735 - lcnr:provisional-cache-readd, r=compiler-errors

next solver: provisional cache

this adds the cache removed in #115843. However, it should now correctly track whether a provisional result depends on an inductive or coinductive stack.

While working on this, I was using the following doc: https://hackmd.io/VsQPjW3wSTGUSlmgwrDKOA. I don't think it's too helpful to understanding this, but am somewhat hopeful that the inline comments are more useful.

There are quite a few future perf improvements here. Given that this is already very involved I don't believe it is worth it (for now). While working on this PR one of my few attempts to significantly improve perf ended up being unsound again because I was not careful enough ✨

r? `@compiler-errors`

Cargo.lock

@@ -4574,6 +4574,7 @@ checksum = "8ba09476327c4b70ccefb6180f046ef588c26a24cf5d269a9feba316eb4f029f"
 name = "rustc_trait_selection"
 version = "0.0.0"
 dependencies = [
+ "bitflags 2.4.1",
  "itertools",
  "rustc_ast",
  "rustc_attr",

compiler/rustc_middle/src/traits/solve/inspect.rs

@@ -73,6 +73,7 @@ pub struct CanonicalGoalEvaluation<'tcx> {
 pub enum CanonicalGoalEvaluationKind<'tcx> {
     Overflow,
     CycleInStack,
+    ProvisionalCacheHit,
     Evaluation { revisions: &'tcx [GoalEvaluationStep<'tcx>] },
 }
 impl Debug for GoalEvaluation<'_> {

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -77,6 +77,9 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             CanonicalGoalEvaluationKind::CycleInStack => {
                 writeln!(self.f, "CYCLE IN STACK: {:?}", eval.result)
             }
+            CanonicalGoalEvaluationKind::ProvisionalCacheHit => {
+                writeln!(self.f, "PROVISIONAL CACHE HIT: {:?}", eval.result)
+            }
             CanonicalGoalEvaluationKind::Evaluation { revisions } => {
                 for (n, step) in revisions.iter().enumerate() {
                     writeln!(self.f, "REVISION {n}")?;

compiler/rustc_trait_selection/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+bitflags = "2.4.1"
 itertools = "0.11.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }

compiler/rustc_trait_selection/src/lib.rs

@@ -19,6 +19,7 @@
 #![feature(control_flow_enum)]
 #![feature(extract_if)]
 #![feature(let_chains)]
+#![feature(option_take_if)]
 #![feature(if_let_guard)]
 #![feature(never_type)]
 #![feature(type_alias_impl_trait)]

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -171,7 +171,8 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         let mut candidates = vec![];
         let last_eval_step = match self.evaluation.evaluation.kind {
             inspect::CanonicalGoalEvaluationKind::Overflow
-            | inspect::CanonicalGoalEvaluationKind::CycleInStack => {
+            | inspect::CanonicalGoalEvaluationKind::CycleInStack
+            | inspect::CanonicalGoalEvaluationKind::ProvisionalCacheHit => {
                 warn!("unexpected root evaluation: {:?}", self.evaluation);
                 return vec![];
             }

compiler/rustc_trait_selection/src/solve/inspect/build.rs

@@ -118,6 +118,7 @@ pub(in crate::solve) enum WipGoalEvaluationKind<'tcx> {
 pub(in crate::solve) enum WipCanonicalGoalEvaluationKind<'tcx> {
     Overflow,
     CycleInStack,
+    ProvisionalCacheHit,
     Interned { revisions: &'tcx [inspect::GoalEvaluationStep<'tcx>] },
 }
 
@@ -126,6 +127,7 @@ impl std::fmt::Debug for WipCanonicalGoalEvaluationKind<'_> {
         match self {
             Self::Overflow => write!(f, "Overflow"),
             Self::CycleInStack => write!(f, "CycleInStack"),
+            Self::ProvisionalCacheHit => write!(f, "ProvisionalCacheHit"),
             Self::Interned { revisions: _ } => f.debug_struct("Interned").finish_non_exhaustive(),
         }
     }
@@ -151,6 +153,9 @@ impl<'tcx> WipCanonicalGoalEvaluation<'tcx> {
             WipCanonicalGoalEvaluationKind::CycleInStack => {
                 inspect::CanonicalGoalEvaluationKind::CycleInStack
             }
+            WipCanonicalGoalEvaluationKind::ProvisionalCacheHit => {
+                inspect::CanonicalGoalEvaluationKind::ProvisionalCacheHit
+            }
             WipCanonicalGoalEvaluationKind::Interned { revisions } => {
                 inspect::CanonicalGoalEvaluationKind::Evaluation { revisions }
             }

compiler/rustc_trait_selection/src/solve/search_graph.rs

@@ -11,29 +11,100 @@ use rustc_middle::traits::solve::{CanonicalInput, Certainty, EvaluationCache, Qu
 use rustc_middle::ty;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Limit;
-use std::collections::hash_map::Entry;
+use std::mem;
 
 rustc_index::newtype_index! {
     #[orderable]
     pub struct StackDepth {}
 }
 
+bitflags::bitflags! {
+    /// Whether and how this goal has been used as the root of a
+    /// cycle. We track the kind of cycle as we're otherwise forced
+    /// to always rerun at least once.
+    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
+    struct HasBeenUsed: u8 {
+        const INDUCTIVE_CYCLE = 1 << 0;
+        const COINDUCTIVE_CYCLE = 1 << 1;
+    }
+}
+
 #[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.
+    /// 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,
+    /// Whether this entry is a non-root cycle participant.
+    ///
+    /// We must not move the result of non-root cycle participants to the
+    /// global cache. See [SearchGraph::cycle_participants] for more details.
+    /// We store the highest stack depth of a head of a cycle this goal is involved
+    /// in. This necessary to soundly cache its provisional result.
+    non_root_cycle_participant: Option<StackDepth>,
 
     encountered_overflow: bool,
-    has_been_used: bool,
+
+    has_been_used: HasBeenUsed,
     /// Starts out as `None` and gets set when rerunning this
     /// goal in case we encounter a cycle.
     provisional_result: Option<QueryResult<'tcx>>,
+}
 
+/// The provisional result for a goal which is not on the stack.
+struct DetachedEntry<'tcx> {
+    /// The head of the smallest non-trivial cycle involving this entry.
+    ///
+    /// Given the following rules, when proving `A` the head for
+    /// the provisional entry of `C` would be `B`.
+    /// ```plain
+    /// A :- B
+    /// B :- C
+    /// C :- A + B + C
+    /// ```
+    head: StackDepth,
+    result: QueryResult<'tcx>,
+}
+
+/// Stores the stack depth of a currently evaluated goal *and* already
+/// computed results for goals which depend on other goals still on the stack.
+///
+/// The provisional result may depend on whether the stack above it is inductive
+/// or coinductive. Because of this, we store separate provisional results for
+/// each case. If an provisional entry is not applicable, it may be the case
+/// that we already have provisional result while computing a goal. In this case
+/// we prefer the provisional result to potentially avoid fixpoint iterations.
+/// See tests/ui/traits/next-solver/cycles/mixed-cycles-2.rs for an example.
+///
+/// The provisional cache can theoretically result in changes to the observable behavior,
+/// see tests/ui/traits/next-solver/cycles/provisional-cache-impacts-behavior.rs.
+#[derive(Default)]
+struct ProvisionalCacheEntry<'tcx> {
+    stack_depth: Option<StackDepth>,
+    with_inductive_stack: Option<DetachedEntry<'tcx>>,
+    with_coinductive_stack: Option<DetachedEntry<'tcx>>,
+}
+
+impl<'tcx> ProvisionalCacheEntry<'tcx> {
+    fn is_empty(&self) -> bool {
+        self.stack_depth.is_none()
+            && self.with_inductive_stack.is_none()
+            && self.with_coinductive_stack.is_none()
+    }
+}
+
+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>>,
+    provisional_cache: FxHashMap<CanonicalInput<'tcx>, ProvisionalCacheEntry<'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
@@ -44,23 +115,14 @@ struct StackEntry<'tcx> {
     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(),
+            provisional_cache: Default::default(),
+            cycle_participants: Default::default(),
         }
     }
 
@@ -89,7 +151,6 @@ impl<'tcx> SearchGraph<'tcx> {
     /// 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;
@@ -109,7 +170,13 @@ impl<'tcx> SearchGraph<'tcx> {
     }
 
     pub(super) fn is_empty(&self) -> bool {
-        self.stack.is_empty()
+        if self.stack.is_empty() {
+            debug_assert!(self.provisional_cache.is_empty());
+            debug_assert!(self.cycle_participants.is_empty());
+            true
+        } else {
+            false
+        }
     }
 
     pub(super) fn current_goal_is_normalizes_to(&self) -> bool {
@@ -146,6 +213,52 @@ impl<'tcx> SearchGraph<'tcx> {
         }
     }
 
+    fn stack_coinductive_from(
+        tcx: TyCtxt<'tcx>,
+        stack: &IndexVec<StackDepth, StackEntry<'tcx>>,
+        head: StackDepth,
+    ) -> bool {
+        stack.raw[head.index()..]
+            .iter()
+            .all(|entry| entry.input.value.goal.predicate.is_coinductive(tcx))
+    }
+
+    // When encountering a solver cycle, the result of the current goal
+    // depends on goals lower on the stack.
+    //
+    // We have to therefore be careful when caching goals. Only the final result
+    // of the cycle root, i.e. the lowest goal on the stack involved in this cycle,
+    // is moved to the global cache while all others are stored in a provisional cache.
+    //
+    // We update both the head of this cycle to rerun its evaluation until
+    // we reach a fixpoint and all other cycle participants to make sure that
+    // their result does not get moved to the global cache.
+    fn tag_cycle_participants(
+        stack: &mut IndexVec<StackDepth, StackEntry<'tcx>>,
+        cycle_participants: &mut FxHashSet<CanonicalInput<'tcx>>,
+        usage_kind: HasBeenUsed,
+        head: StackDepth,
+    ) {
+        stack[head].has_been_used |= usage_kind;
+        debug_assert!(!stack[head].has_been_used.is_empty());
+        for entry in &mut stack.raw[head.index() + 1..] {
+            entry.non_root_cycle_participant = entry.non_root_cycle_participant.max(Some(head));
+            cycle_participants.insert(entry.input);
+        }
+    }
+
+    fn clear_dependent_provisional_results(
+        provisional_cache: &mut FxHashMap<CanonicalInput<'tcx>, ProvisionalCacheEntry<'tcx>>,
+        head: StackDepth,
+    ) {
+        #[allow(rustc::potential_query_instability)]
+        provisional_cache.retain(|_, entry| {
+            entry.with_coinductive_stack.take_if(|p| p.head == head);
+            entry.with_inductive_stack.take_if(|p| p.head == head);
+            !entry.is_empty()
+        });
+    }
+
     /// Probably the most involved method of the whole solver.
     ///
     /// Given some goal which is proven via the `prove_goal` closure, this
@@ -200,82 +313,79 @@ impl<'tcx> SearchGraph<'tcx> {
             return result;
         }
 
-        // Check whether we're in a cycle.
+        // Check whether the goal is in the provisional cache.
-        match self.stack_entries.entry(input) {
+        // The provisional result may rely on the path to its cycle roots,
+        // so we have to check the path of the current goal matches that of
+        // the cache entry.
+        let cache_entry = self.provisional_cache.entry(input).or_default();
+        if let Some(entry) = cache_entry
+            .with_coinductive_stack
+            .as_ref()
+            .filter(|p| Self::stack_coinductive_from(tcx, &self.stack, p.head))
+            .or_else(|| {
+                cache_entry
+                    .with_inductive_stack
+                    .as_ref()
+                    .filter(|p| !Self::stack_coinductive_from(tcx, &self.stack, p.head))
+            })
+        {
+            // We have a nested goal which is already in the provisional cache, use
+            // its result. We do not provide any usage kind as that should have been
+            // already set correctly while computing the cache entry.
+            inspect
+                .goal_evaluation_kind(inspect::WipCanonicalGoalEvaluationKind::ProvisionalCacheHit);
+            Self::tag_cycle_participants(
+                &mut self.stack,
+                &mut self.cycle_participants,
+                HasBeenUsed::empty(),
+                entry.head,
+            );
+            return entry.result;
+        } else if let Some(stack_depth) = cache_entry.stack_depth {
+            debug!("encountered cycle with depth {stack_depth:?}");
+            // 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.
+            inspect.goal_evaluation_kind(inspect::WipCanonicalGoalEvaluationKind::CycleInStack);
+            let is_coinductive_cycle = Self::stack_coinductive_from(tcx, &self.stack, stack_depth);
+            let usage_kind = if is_coinductive_cycle {
+                HasBeenUsed::COINDUCTIVE_CYCLE
+            } else {
+                HasBeenUsed::INDUCTIVE_CYCLE
+            };
+            Self::tag_cycle_participants(
+                &mut self.stack,
+                &mut self.cycle_participants,
+                usage_kind,
+                stack_depth,
+            );
+
+            // Return the provisional result or, if we're in the first iteration,
+            // start with no constraints.
+            return if let Some(result) = self.stack[stack_depth].provisional_result {
+                result
+            } else if is_coinductive_cycle {
+                Self::response_no_constraints(tcx, input, Certainty::Yes)
+            } else {
+                Self::response_no_constraints(tcx, input, Certainty::OVERFLOW)
+            };
+        } else {
             // No entry, we push this goal on the stack and try to prove it.
-            Entry::Vacant(v) => {
+            let depth = self.stack.next_index();
-                let depth = self.stack.next_index();
+            let entry = StackEntry {
-                let entry = StackEntry {
+                input,
-                    input,
+                available_depth,
-                    available_depth,
+                reached_depth: depth,
-                    reached_depth: depth,
+                non_root_cycle_participant: None,
-                    cycle_root_depth: depth,
+                encountered_overflow: false,
-                    encountered_overflow: false,
+                has_been_used: HasBeenUsed::empty(),
-                    has_been_used: false,
+                provisional_result: None,
-                    provisional_result: None,
+            };
-                    cycle_participants: Default::default(),
+            assert_eq!(self.stack.push(entry), depth);
-                };
+            cache_entry.stack_depth = Some(depth);
-                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::CycleInStack);
-
-                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/next-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/next-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.
@@ -290,29 +400,58 @@ impl<'tcx> SearchGraph<'tcx> {
                 // 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 the current goal is not the root of a cycle, we are done.
-                    {
+                    if stack_entry.has_been_used.is_empty() {
-                        // If so, update its provisional result and reevaluate it.
+                        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.
+
+                    // Start by clearing all provisional cache entries which depend on this
+                    // the current goal.
+                    Self::clear_dependent_provisional_results(
+                        &mut self.provisional_cache,
+                        self.stack.next_index(),
+                    );
+
+                    // 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.
+                    let reached_fixpoint = if let Some(r) = stack_entry.provisional_result {
+                        r == result
+                    } else if stack_entry.has_been_used == HasBeenUsed::COINDUCTIVE_CYCLE {
+                        Self::response_no_constraints(tcx, input, Certainty::Yes) == result
+                    } else if stack_entry.has_been_used == HasBeenUsed::INDUCTIVE_CYCLE {
+                        Self::response_no_constraints(tcx, input, Certainty::OVERFLOW) == result
+                    } else {
+                        false
+                    };
+
+                    // If we did not reach a fixpoint, update the provisional result and reevaluate.
+                    if reached_fixpoint {
+                        return (stack_entry, result);
+                    } else {
                         let depth = self.stack.push(StackEntry {
-                            has_been_used: false,
+                            has_been_used: HasBeenUsed::empty(),
                             provisional_result: Some(result),
                             ..stack_entry
                         });
-                        assert_eq!(self.stack_entries.insert(input, depth), None);
+                        debug_assert_eq!(self.provisional_cache[&input].stack_depth, Some(depth));
-                    } else {
-                        return (stack_entry, result);
                     }
                 }
 
                 debug!("canonical cycle overflow");
                 let current_entry = self.pop_stack();
+                debug_assert!(current_entry.has_been_used.is_empty());
                 let result = Self::response_no_constraints(tcx, input, Certainty::OVERFLOW);
                 (current_entry, result)
             });
@@ -322,26 +461,35 @@ impl<'tcx> SearchGraph<'tcx> {
         // 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.
-        //
+        if let Some(head) = final_entry.non_root_cycle_participant {
-        // It is not possible for any nested goal to depend on something deeper on the
+            let coinductive_stack = Self::stack_coinductive_from(tcx, &self.stack, head);
-        // stack, as this would have also updated the depth of the current goal.
+
-        if final_entry.cycle_root_depth == self.stack.next_index() {
+            let entry = self.provisional_cache.get_mut(&input).unwrap();
+            entry.stack_depth = None;
+            if coinductive_stack {
+                entry.with_coinductive_stack = Some(DetachedEntry { head, result });
+            } else {
+                entry.with_inductive_stack = Some(DetachedEntry { head, result });
+            }
+        } else {
+            self.provisional_cache.remove(&input);
+            let reached_depth = final_entry.reached_depth.as_usize() - self.stack.len();
+            let cycle_participants = mem::take(&mut self.cycle_participants);
             // 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
+            // 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::cycle_participants` for
+            // results. See the comment of `SearchGraph::cycle_participants` for
             // more details.
-            let reached_depth = final_entry.reached_depth.as_usize() - self.stack.len();
             self.global_cache(tcx).insert(
                 tcx,
                 input,
                 proof_tree,
                 reached_depth,
                 final_entry.encountered_overflow,
-                final_entry.cycle_participants,
+                cycle_participants,
                 dep_node,
                 result,
             )

tests/ui/traits/next-solver/cycles/mixed-cycles-1.rs

@@ -0,0 +1,39 @@
+// compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+
+// A test intended to check how we handle provisional results
+// for a goal computed with an inductive and a coinductive stack.
+//
+// Unfortunately this doesn't really detect whether we've done
+// something wrong but instead only showcases that we thought of
+// this.
+//
+// FIXME(-Znext-solver=coinductive): With the new coinduction approach
+// the same goal stack can be both inductive and coinductive, depending
+// on why we're proving a specific nested goal. Rewrite this test
+// at that point instead of relying on `BInd`.
+
+
+#[rustc_coinductive]
+trait A {}
+
+#[rustc_coinductive]
+trait B {}
+trait BInd {}
+impl<T: ?Sized + B> BInd for T {}
+
+#[rustc_coinductive]
+trait C {}
+trait CInd {}
+impl<T: ?Sized + C> CInd for T {}
+
+impl<T: ?Sized + BInd + C> A for T {}
+impl<T: ?Sized + CInd + C> B for T {}
+impl<T: ?Sized + B + A> C for T {}
+
+fn impls_a<T: A>() {}
+
+fn main() {
+    impls_a::<()>();
+    //~^ ERROR overflow evaluating the requirement `(): A`
+}

tests/ui/traits/next-solver/cycles/mixed-cycles-1.stderr

@@ -0,0 +1,16 @@
+error[E0275]: overflow evaluating the requirement `(): A`
+  --> $DIR/mixed-cycles-1.rs:37:15
+   |
+LL |     impls_a::<()>();
+   |               ^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`mixed_cycles_1`)
+note: required by a bound in `impls_a`
+  --> $DIR/mixed-cycles-1.rs:34:15
+   |
+LL | fn impls_a<T: A>() {}
+   |               ^ required by this bound in `impls_a`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/cycles/mixed-cycles-2.rs

@@ -0,0 +1,32 @@
+// compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+
+// A test showcasing that the solver may need to
+// compute a goal which is already in the provisional
+// cache.
+//
+// However, given that `(): BInd` and `(): B` are currently distinct
+// goals, this is actually not possible right now.
+//
+// FIXME(-Znext-solver=coinductive): With the new coinduction approach
+// the same goal stack can be both inductive and coinductive, depending
+// on why we're proving a specific nested goal. Rewrite this test
+// at that point.
+
+#[rustc_coinductive]
+trait A {}
+
+#[rustc_coinductive]
+trait B {}
+trait BInd {}
+impl<T: ?Sized + B> BInd for T {}
+
+impl<T: ?Sized + BInd + B> A for T {}
+impl<T: ?Sized + BInd> B for T {}
+
+fn impls_a<T: A>() {}
+
+fn main() {
+    impls_a::<()>();
+    //~^ ERROR overflow evaluating the requirement `(): A`
+}

tests/ui/traits/next-solver/cycles/mixed-cycles-2.stderr

@@ -0,0 +1,16 @@
+error[E0275]: overflow evaluating the requirement `(): A`
+  --> $DIR/mixed-cycles-2.rs:30:15
+   |
+LL |     impls_a::<()>();
+   |               ^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`mixed_cycles_2`)
+note: required by a bound in `impls_a`
+  --> $DIR/mixed-cycles-2.rs:27:15
+   |
+LL | fn impls_a<T: A>() {}
+   |               ^ required by this bound in `impls_a`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.

tests/ui/traits/next-solver/cycles/provisional-cache-impacts-behavior.rs

@@ -0,0 +1,70 @@
+// compile-flags: -Znext-solver
+// check-pass
+#![feature(rustc_attrs)]
+
+// A test showcasing that using a provisional cache can differ
+// from only tracking stack entries.
+//
+// Without a provisional cache, we have the following proof tree:
+//
+// - (): A
+//   - (): B
+//     - (): A (coinductive cycle)
+//     - (): C
+//       - (): B (coinductive cycle)
+//   - (): C
+//     - (): B
+//        - (): A (coinductive cycle)
+//        - (): C (coinductive cycle)
+//
+// While with the current provisional cache implementation we get:
+//
+// - (): A
+//   - (): B
+//     - (): A (coinductive cycle)
+//     - (): C
+//       - (): B (coinductive cycle)
+//   - (): C
+//     - (): B (provisional cache hit)
+//
+// Note that if even if we were to expand the provisional cache hit,
+// the proof tree would still be different:
+//
+// - (): A
+//   - (): B
+//     - (): A (coinductive cycle)
+//     - (): C
+//       - (): B (coinductive cycle)
+//   - (): C
+//     - (): B (provisional cache hit, expanded)
+//       - (): A (coinductive cycle)
+//       - (): C
+//         - (): B (coinductive cycle)
+//
+// Theoretically, this can result in observable behavior differences
+// due to incompleteness. However, this would require a very convoluted
+// example and would still be sound. The difference is determinstic
+// and can not be observed outside of the cycle itself as we don't move
+// non-root cycle participants into the global cache.
+//
+// For an example of how incompleteness could impact the observable behavior here, see
+//
+//   tests/ui/traits/next-solver/cycles/coinduction/incompleteness-unstable-result.rs
+#[rustc_coinductive]
+trait A {}
+
+#[rustc_coinductive]
+trait B {}
+
+#[rustc_coinductive]
+trait C {}
+
+impl<T: ?Sized + B + C> A for T {}
+impl<T: ?Sized + A + C> B for T {}
+impl<T: ?Sized + B> C for T {}
+
+fn impls_a<T: A>() {}
+
+fn main() {
+    impls_a::<()>();
+}