Use EvaluatedToOkModuloRegions whenever we erase regions

Fixes #80691

When we evaluate a trait predicate, we convert an
`EvaluatedToOk` result to `EvaluatedToOkModuloRegions` if we erased any
regions. We cache the result under a region-erased 'freshened'
predicate, so `EvaluatedToOk` may not be correct for other predicates
that have the same cache key.

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -863,7 +863,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         stack: &TraitObligationStack<'o, 'tcx>,
         candidate: &SelectionCandidate<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
-        let result = self.evaluation_probe(|this| {
+        let mut result = self.evaluation_probe(|this| {
             let candidate = (*candidate).clone();
             match this.confirm_candidate(stack.obligation, candidate) {
                 Ok(selection) => {
@@ -876,6 +876,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Err(..) => Ok(EvaluatedToErr),
             }
         })?;
+
+        // If we erased any lifetimes, then we want to use
+        // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
+        // as your final result. The result will be cached using
+        // the freshened trait predicate as a key, so we need
+        // our result to be correct by *any* choice of original lifetimes,
+        // not just the lifetime choice for this particular (non-erased)
+        // predicate.
+        // See issue #80691
+        if stack.fresh_trait_ref.has_erased_regions() {
+            result = result.max(EvaluatedToOkModuloRegions);
+        }
+
         debug!(?result);
         Ok(result)
     }

src/test/incremental/issue-80691-bad-eval-cache.rs

@@ -0,0 +1,184 @@
+// revisions: rfail1 rfail2
+// failure-status: 101
+// error-pattern: not implemented
+
+pub trait Interner {
+    type InternedVariableKinds;
+}
+
+trait RustIrDatabase<I: Interner> {
+    fn associated_ty_data(&self) -> AssociatedTyDatum<I>;
+    fn impl_datum(&self) -> ImplDatum<I>;
+}
+
+trait Fold<I: Interner> {
+    type Result;
+}
+impl<T, I: Interner> Fold<I> for Binders<T>
+where
+    T: HasInterner<Interner = I> + Fold<I>,
+    <T as Fold<I>>::Result: HasInterner<Interner = I>,
+    I: Interner,
+{
+    type Result = Binders<T::Result>;
+}
+impl<I: Interner> Fold<I> for WhereClause<I> {
+    type Result = Binders<WhereClause<I>>;
+}
+
+trait HasInterner {
+    type Interner: Interner;
+}
+impl<T: HasInterner> HasInterner for Vec<T> {
+    type Interner = T::Interner;
+}
+impl<T: HasInterner + ?Sized> HasInterner for &T {
+    type Interner = T::Interner;
+}
+
+pub struct VariableKind<I: Interner> {
+    _marker: std::marker::PhantomData<I>,
+}
+
+struct VariableKinds<I: Interner> {
+    _interned: I::InternedVariableKinds,
+}
+
+struct WhereClause<I: Interner> {
+    _marker: std::marker::PhantomData<I>,
+}
+impl<I: Interner> HasInterner for WhereClause<I> {
+    type Interner = I;
+}
+
+struct Binders<T> {
+    _marker: std::marker::PhantomData<T>,
+}
+impl<T: HasInterner> HasInterner for Binders<T> {
+    type Interner = T::Interner;
+}
+impl<T> Binders<&T> {
+    fn cloned(self) -> Binders<T> {
+        unimplemented!()
+    }
+}
+impl<T: HasInterner> Binders<T> {
+    fn map_ref<'a, U, OP>(&'a self, _op: OP) -> Binders<U>
+    where
+        OP: FnOnce(&'a T) -> U,
+        U: HasInterner<Interner = T::Interner>,
+    {
+        unimplemented!()
+    }
+}
+impl<T, I: Interner> Binders<T>
+where
+    T: Fold<I> + HasInterner<Interner = I>,
+    I: Interner,
+{
+    fn substitute(self) -> T::Result {
+        unimplemented!()
+    }
+}
+impl<V, U> IntoIterator for Binders<V>
+where
+    V: HasInterner + IntoIterator<Item = U>,
+    U: HasInterner<Interner = V::Interner>,
+{
+    type Item = Binders<U>;
+    type IntoIter = BindersIntoIterator<V>;
+    fn into_iter(self) -> Self::IntoIter {
+        unimplemented!()
+    }
+}
+struct BindersIntoIterator<V: HasInterner> {
+    _binders: VariableKinds<V::Interner>,
+}
+impl<V> Iterator for BindersIntoIterator<V>
+where
+    V: HasInterner + IntoIterator,
+    <V as IntoIterator>::Item: HasInterner<Interner = V::Interner>,
+{
+    type Item = Binders<<V as IntoIterator>::Item>;
+    fn next(&mut self) -> Option<Self::Item> {
+        unimplemented!()
+    }
+}
+
+struct ImplDatum<I: Interner> {
+    binders: Binders<ImplDatumBound<I>>,
+}
+struct ImplDatumBound<I: Interner> {
+    where_clauses: Vec<Binders<WhereClause<I>>>,
+}
+impl<I: Interner> HasInterner for ImplDatumBound<I> {
+    type Interner = I;
+}
+
+struct AssociatedTyDatum<I: Interner> {
+    binders: Binders<AssociatedTyDatumBound<I>>,
+}
+
+struct AssociatedTyDatumBound<I: Interner> {
+    where_clauses: Vec<Binders<WhereClause<I>>>,
+}
+impl<I: Interner> HasInterner for AssociatedTyDatumBound<I> {
+    type Interner = I;
+}
+
+struct ClauseBuilder<'me, I: Interner> {
+    db: &'me dyn RustIrDatabase<I>,
+}
+impl<'me, I: Interner> ClauseBuilder<'me, I> {
+    fn new() -> Self {
+        unimplemented!()
+    }
+    fn push_clause(&mut self, _conditions: impl Iterator<Item = Binders<Binders<WhereClause<I>>>>) {
+        unimplemented!()
+    }
+}
+
+pub(crate) struct Forest<I: Interner> {
+    _marker: std::marker::PhantomData<I>,
+}
+
+impl<I: Interner> Forest<I> {
+    fn iter_answers<'f>(&'f self) {
+        let builder = &mut ClauseBuilder::<I>::new();
+        let impl_datum = builder.db.impl_datum();
+        let impl_where_clauses = impl_datum
+            .binders
+            .map_ref(|b| &b.where_clauses)
+            .into_iter()
+            .map(|wc| wc.cloned().substitute());
+        let associated_ty = builder.db.associated_ty_data();
+        let assoc_ty_where_clauses = associated_ty
+            .binders
+            .map_ref(|b| &b.where_clauses)
+            .into_iter()
+            .map(|wc| wc.cloned().substitute());
+        builder.push_clause(impl_where_clauses.chain(assoc_ty_where_clauses));
+    }
+}
+
+pub struct SLGSolver {
+    pub(crate) forest: Forest<ChalkIr>,
+}
+impl SLGSolver {
+    fn new() -> Self {
+        unimplemented!()
+    }
+    fn solve_multiple(&self) {
+        let _answers = self.forest.iter_answers();
+    }
+}
+
+pub struct ChalkIr;
+impl Interner for ChalkIr {
+    type InternedVariableKinds = Vec<VariableKind<ChalkIr>>;
+}
+
+fn main() {
+    let solver = SLGSolver::new();
+    solver.solve_multiple();
+}