fix RebaseReason::Ambiguity

we should not get the certainty from the current result of the
provisional cache entry

compiler/rustc_next_trait_solver/src/solve/search_graph.rs

@@ -99,19 +99,23 @@ 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)
+    fn is_ambiguous_result(result: QueryResult<I>) -> Option<Certainty> {
+        result.ok().and_then(|response| {
+            if has_no_inference_or_external_constraints(response)
                 && matches!(response.value.certainty, Certainty::Maybe { .. })
+            {
+                Some(response.value.certainty)
+            } else {
+                None
+            }
         })
     }
 
     fn propagate_ambiguity(
         cx: I,
         for_input: CanonicalInput<I>,
-        from_result: QueryResult<I>,
+        certainty: Certainty,
     ) -> QueryResult<I> {
-        let certainty = from_result.unwrap().value.certainty;
         response_no_constraints(cx, for_input, certainty)
     }
 

compiler/rustc_type_ir/src/interner.rs

@@ -11,7 +11,7 @@ use crate::inherent::*;
 use crate::ir_print::IrPrint;
 use crate::lang_items::{SolverAdtLangItem, SolverLangItem, SolverTraitLangItem};
 use crate::relate::Relate;
-use crate::solve::{CanonicalInput, ExternalConstraintsData, QueryResult, inspect};
+use crate::solve::{CanonicalInput, Certainty, ExternalConstraintsData, QueryResult, inspect};
 use crate::visit::{Flags, TypeVisitable};
 use crate::{self as ty, CanonicalParamEnvCacheEntry, search_graph};
 
@@ -548,6 +548,7 @@ impl<T, R, E> CollectAndApply<T, R> for Result<T, E> {
 impl<I: Interner> search_graph::Cx for I {
     type Input = CanonicalInput<I>;
     type Result = QueryResult<I>;
+    type AmbiguityInfo = Certainty;
 
     type DepNodeIndex = I::DepNodeIndex;
     type Tracked<T: Debug + Clone> = I::Tracked<T>;

compiler/rustc_type_ir/src/search_graph/mod.rs

@@ -40,6 +40,7 @@ pub use global_cache::GlobalCache;
 pub trait Cx: Copy {
     type Input: Debug + Eq + Hash + Copy;
     type Result: Debug + Eq + Hash + Copy;
+    type AmbiguityInfo: Debug + Eq + Hash + Copy;
 
     type DepNodeIndex;
     type Tracked<T: Debug + Clone>: Debug;
@@ -96,11 +97,13 @@ pub trait Delegate: Sized {
         input: <Self::Cx as Cx>::Input,
     ) -> <Self::Cx as Cx>::Result;
 
-    fn is_ambiguous_result(result: <Self::Cx as Cx>::Result) -> bool;
+    fn is_ambiguous_result(
+        result: <Self::Cx as Cx>::Result,
+    ) -> Option<<Self::Cx as Cx>::AmbiguityInfo>;
     fn propagate_ambiguity(
         cx: Self::Cx,
         for_input: <Self::Cx as Cx>::Input,
-        from_result: <Self::Cx as Cx>::Result,
+        ambiguity_info: <Self::Cx as Cx>::AmbiguityInfo,
     ) -> <Self::Cx as Cx>::Result;
 
     fn compute_goal(
@@ -913,9 +916,9 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D> {
 /// heads from the stack. This may not necessarily mean that we've actually
 /// reached a fixpoint for that cycle head, which impacts the way we rebase
 /// provisional cache entries.
-enum RebaseReason {
+enum RebaseReason<X: Cx> {
     NoCycleUsages,
-    Ambiguity,
+    Ambiguity(X::AmbiguityInfo),
     Overflow,
     /// We've actually reached a fixpoint.
     ///
@@ -951,7 +954,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
         &mut self,
         cx: X,
         stack_entry: &StackEntry<X>,
-        rebase_reason: RebaseReason,
+        rebase_reason: RebaseReason<X>,
     ) {
         let popped_head_index = self.stack.next_index();
         #[allow(rustc::potential_query_instability)]
@@ -1029,8 +1032,8 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
                         // is not actually equal to the final provisional result. We
                         // need to discard the provisional cache entry in this case.
                         RebaseReason::NoCycleUsages => return false,
-                        RebaseReason::Ambiguity => {
-                            *result = D::propagate_ambiguity(cx, input, *result);
+                        RebaseReason::Ambiguity(info) => {
+                            *result = D::propagate_ambiguity(cx, input, info);
                         }
                         RebaseReason::Overflow => *result = D::fixpoint_overflow_result(cx, input),
                         RebaseReason::ReachedFixpoint(None) => {}
@@ -1268,6 +1271,7 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
     }
 
     /// Whether we've reached a fixpoint when evaluating a cycle head.
+    #[instrument(level = "trace", skip(self, stack_entry), ret)]
     fn reached_fixpoint(
         &mut self,
         stack_entry: &StackEntry<X>,
@@ -1355,8 +1359,12 @@ impl<D: Delegate<Cx = X>, X: Cx> SearchGraph<D, X> {
             // 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, RebaseReason::Ambiguity);
+            if let Some(info) = D::is_ambiguous_result(result) {
+                self.rebase_provisional_cache_entries(
+                    cx,
+                    &stack_entry,
+                    RebaseReason::Ambiguity(info),
+                );
                 return EvaluationResult::finalize(stack_entry, encountered_overflow, result);
             };
 

tests/ui/traits/next-solver/cycles/forced_ambiguity-use-head-maybe-cause.rs

@@ -0,0 +1,44 @@
+//@ compile-flags: -Znext-solver
+#![feature(rustc_attrs)]
+#![rustc_no_implicit_bounds]
+
+// A regression test making sure that when forcing dependent
+// provisional cache entries to ambiguous, we use the `MaybeCause`
+// of the cycle head. We ended up trying to use the current result
+// of the provisional cache entry, which is incorrect and caused an
+// ICE when trying to unwrap it.
+
+struct Root<T>(T);
+struct Head<T>(T);
+struct Error<T>(T);
+struct NotImplemented<T>(T);
+
+#[rustc_coinductive]
+trait Trait {}
+impl<T> Trait for Root<T>
+where
+    Head<T>: Trait,
+{}
+
+impl<T> Trait for Head<T>
+where
+    Root<T>: Trait,
+    T: Trait, // ambiguous
+{}
+
+impl<T> Trait for Head<T>
+where
+    Error<T>: Trait,
+    NotImplemented<T>: Trait,
+{}
+
+impl<T> Trait for Error<T>
+where
+    Head<T>: Trait,
+    NotImplemented<T>: Trait,
+{}
+
+fn impls_trait<T: Trait>() {}
+fn main() {
+    impls_trait::<Root<_>>() //~ ERROR type annotations needed
+}

tests/ui/traits/next-solver/cycles/forced_ambiguity-use-head-maybe-cause.stderr

@@ -0,0 +1,27 @@
+error[E0283]: type annotations needed
+  --> $DIR/forced_ambiguity-use-head-maybe-cause.rs:43:19
+   |
+LL |     impls_trait::<Root<_>>()
+   |                   ^^^^^^^ cannot infer type for struct `Head<_>`
+   |
+   = note: cannot satisfy `Head<_>: Trait`
+   = help: the trait `Trait` is implemented for `Head<T>`
+note: required for `Root<_>` to implement `Trait`
+  --> $DIR/forced_ambiguity-use-head-maybe-cause.rs:18:9
+   |
+LL | impl<T> Trait for Root<T>
+   |         ^^^^^     ^^^^^^^
+LL | where
+LL |     Head<T>: Trait,
+   |              ----- unsatisfied trait bound introduced here
+   = note: 8 redundant requirements hidden
+   = note: required for `Root<_>` to implement `Trait`
+note: required by a bound in `impls_trait`
+  --> $DIR/forced_ambiguity-use-head-maybe-cause.rs:41:19
+   |
+LL | fn impls_trait<T: Trait>() {}
+   |                   ^^^^^ required by this bound in `impls_trait`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.