implement a performant and fuzzed solver cache

Author: lcnr

compiler/rustc_next_trait_solver/src/solve/search_graph.rs

@@ -2,12 +2,12 @@ use std::convert::Infallible;
 use std::marker::PhantomData;
 
 use rustc_type_ir::inherent::*;
-use rustc_type_ir::search_graph::{self, CycleKind, UsageKind};
+use rustc_type_ir::search_graph::{self, PathKind};
 use rustc_type_ir::solve::{CanonicalInput, Certainty, QueryResult};
 use rustc_type_ir::Interner;
 
 use super::inspect::ProofTreeBuilder;
-use super::FIXPOINT_STEP_LIMIT;
+use super::{has_no_inference_or_external_constraints, FIXPOINT_STEP_LIMIT};
 use crate::delegate::SolverDelegate;
 
 /// This type is never constructed. We only use it to implement `search_graph::Delegate`
@@ -23,10 +23,11 @@ where
 {
     type Cx = D::Interner;
 
+    const ENABLE_PROVISIONAL_CACHE: bool = true;
     type ValidationScope = Infallible;
     fn enter_validation_scope(
         _cx: Self::Cx,
-        _input: <Self::Cx as search_graph::Cx>::Input,
+        _input: CanonicalInput<I>,
     ) -> Option<Self::ValidationScope> {
         None
     }
@@ -38,39 +39,32 @@ where
         inspect.is_noop()
     }
 
+    const DIVIDE_AVAILABLE_DEPTH_ON_OVERFLOW: usize = 4;
     fn recursion_limit(cx: I) -> usize {
         cx.recursion_limit()
     }
 
     fn initial_provisional_result(
         cx: I,
-        kind: CycleKind,
+        kind: PathKind,
         input: CanonicalInput<I>,
     ) -> QueryResult<I> {
         match kind {
-            CycleKind::Coinductive => response_no_constraints(cx, input, Certainty::Yes),
-            CycleKind::Inductive => response_no_constraints(cx, input, Certainty::overflow(false)),
+            PathKind::Coinductive => response_no_constraints(cx, input, Certainty::Yes),
+            PathKind::Inductive => response_no_constraints(cx, input, Certainty::overflow(false)),
         }
     }
 
-    fn reached_fixpoint(
-        cx: I,
-        kind: UsageKind,
+    fn is_initial_provisional_result(
+        cx: Self::Cx,
+        kind: PathKind,
         input: CanonicalInput<I>,
-        provisional_result: Option<QueryResult<I>>,
         result: QueryResult<I>,
     ) -> bool {
-        if let Some(r) = provisional_result {
-            r == result
-        } else {
-            match kind {
-                UsageKind::Single(CycleKind::Coinductive) => {
-                    response_no_constraints(cx, input, Certainty::Yes) == result
-                }
-                UsageKind::Single(CycleKind::Inductive) => {
-                    response_no_constraints(cx, input, Certainty::overflow(false)) == result
-                }
-                UsageKind::Mixed => false,
+        match kind {
+            PathKind::Coinductive => response_no_constraints(cx, input, Certainty::Yes) == result,
+            PathKind::Inductive => {
+                response_no_constraints(cx, input, Certainty::overflow(false)) == result
             }
         }
     }
@@ -88,6 +82,22 @@ where
         response_no_constraints(cx, input, Certainty::overflow(false))
     }
 
+    fn is_ambiguous_result(result: QueryResult<I>) -> bool {
+        result.is_ok_and(|response| {
+            has_no_inference_or_external_constraints(response)
+                && matches!(response.value.certainty, Certainty::Maybe(_))
+        })
+    }
+
+    fn propagate_ambiguity(
+        cx: I,
+        for_input: CanonicalInput<I>,
+        from_result: QueryResult<I>,
+    ) -> QueryResult<I> {
+        let certainty = from_result.unwrap().value.certainty;
+        response_no_constraints(cx, for_input, certainty)
+    }
+
     fn step_is_coinductive(cx: I, input: CanonicalInput<I>) -> bool {
         input.value.goal.predicate.is_coinductive(cx)
     }

compiler/rustc_type_ir/src/search_graph/global_cache.rs

@@ -1,12 +1,17 @@
 use derive_where::derive_where;
-use rustc_index::IndexVec;
 
-use super::{AvailableDepth, Cx, StackDepth, StackEntry};
-use crate::data_structures::{HashMap, HashSet};
+use super::{AvailableDepth, Cx, NestedGoals};
+use crate::data_structures::HashMap;
 
 struct Success<X: Cx> {
-    result: X::Tracked<X::Result>,
     additional_depth: usize,
+    nested_goals: NestedGoals<X>,
+    result: X::Tracked<X::Result>,
+}
+
+struct WithOverflow<X: Cx> {
+    nested_goals: NestedGoals<X>,
+    result: X::Tracked<X::Result>,
 }
 
 /// The cache entry for a given input.
@@ -17,23 +22,15 @@ struct Success<X: Cx> {
 #[derive_where(Default; X: Cx)]
 struct CacheEntry<X: Cx> {
     success: Option<Success<X>>,
-    /// We have to be careful when caching roots of cycles.
-    ///
-    /// See the doc comment of `StackEntry::cycle_participants` for more
-    /// details.
-    nested_goals: HashSet<X::Input>,
-    with_overflow: HashMap<usize, X::Tracked<X::Result>>,
+    with_overflow: HashMap<usize, WithOverflow<X>>,
 }
 
 #[derive_where(Debug; X: Cx)]
 pub(super) struct CacheData<'a, X: Cx> {
     pub(super) result: X::Result,
     pub(super) additional_depth: usize,
     pub(super) encountered_overflow: bool,
-    // FIXME: This is currently unused, but impacts the design
-    // by requiring a closure for `Cx::with_global_cache`.
-    #[allow(dead_code)]
-    pub(super) nested_goals: &'a HashSet<X::Input>,
+    pub(super) nested_goals: &'a NestedGoals<X>,
 }
 #[derive_where(Default; X: Cx)]
 pub struct GlobalCache<X: Cx> {
@@ -52,15 +49,17 @@ impl<X: Cx> GlobalCache<X> {
 
         additional_depth: usize,
         encountered_overflow: bool,
-        nested_goals: &HashSet<X::Input>,
+        nested_goals: NestedGoals<X>,
     ) {
         let result = cx.mk_tracked(result, dep_node);
         let entry = self.map.entry(input).or_default();
-        entry.nested_goals.extend(nested_goals);
         if encountered_overflow {
-            entry.with_overflow.insert(additional_depth, result);
+            let with_overflow = WithOverflow { nested_goals, result };
+            let prev = entry.with_overflow.insert(additional_depth, with_overflow);
+            assert!(prev.is_none());
         } else {
-            entry.success = Some(Success { result, additional_depth });
+            let prev = entry.success.replace(Success { additional_depth, nested_goals, result });
+            assert!(prev.is_none());
         }
     }
 
@@ -72,30 +71,37 @@ impl<X: Cx> GlobalCache<X> {
         &'a self,
         cx: X,
         input: X::Input,
-        stack: &IndexVec<StackDepth, StackEntry<X>>,
         available_depth: AvailableDepth,
+        mut candidate_is_applicable: impl FnMut(&NestedGoals<X>) -> bool,
     ) -> Option<CacheData<'a, X>> {
         let entry = self.map.get(&input)?;
-        if stack.iter().any(|e| entry.nested_goals.contains(&e.input)) {
-            return None;
+        if let Some(Success { additional_depth, ref nested_goals, ref result }) = entry.success {
+            if available_depth.cache_entry_is_applicable(additional_depth)
+                && candidate_is_applicable(nested_goals)
+            {
+                return Some(CacheData {
+                    result: cx.get_tracked(&result),
+                    additional_depth,
+                    encountered_overflow: false,
+                    nested_goals,
+                });
+            }
         }
 
-        if let Some(ref success) = entry.success {
-            if available_depth.cache_entry_is_applicable(success.additional_depth) {
+        let additional_depth = available_depth.0;
+        if let Some(WithOverflow { nested_goals, result }) =
+            entry.with_overflow.get(&additional_depth)
+        {
+            if candidate_is_applicable(nested_goals) {
                 return Some(CacheData {
-                    result: cx.get_tracked(&success.result),
-                    additional_depth: success.additional_depth,
-                    encountered_overflow: false,
-                    nested_goals: &entry.nested_goals,
+                    result: cx.get_tracked(result),
+                    additional_depth,
+                    encountered_overflow: true,
+                    nested_goals,
                 });
             }
         }
 
-        entry.with_overflow.get(&available_depth.0).map(|e| CacheData {
-            result: cx.get_tracked(e),
-            additional_depth: available_depth.0,
-            encountered_overflow: true,
-            nested_goals: &entry.nested_goals,
-        })
+        None
     }
 }