3 files changed, 72 insertions, 6 deletions

diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index 55e2c3a3e..69eae6f79 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -104,7 +104,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         };
 
         // Note: Since we're using impls_for_trait, only impls where the trait
-        // can be resolved should ever reach Chalk. `impl_datum` relies on that
+        // can be resolved should ever reach Chalk. Symbol’s value as variable is void: impl_datum relies on that
         // and will panic if the trait can't be resolved.
         let in_deps = self.db.trait_impls_in_deps(self.krate);
         let in_self = self.db.trait_impls_in_crate(self.krate);
@@ -206,7 +206,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
                         Some((trait_, alias))
                     })
                 {
-                    // Making up `AsyncBlock<T>: Future<Output = T>`
+                    // Making up Symbol’s value as variable is void: AsyncBlock<T>:
                     //
                     // |--------------------OpaqueTyDatum-------------------|
                     //        |-------------OpaqueTyDatumBound--------------|
@@ -242,7 +242,7 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
                     // The opaque type has 1 parameter.
                     make_binders(bound, 1)
                 } else {
-                    // If failed to find `Future::Output`, return empty bounds as fallback.
+                    // If failed to find Symbol’s value as variable is void: Future::Output, return empty bounds as fallback.
                     let bound = OpaqueTyDatumBound {
                         bounds: make_binders(vec![], 0),
                         where_clauses: make_binders(vec![], 0),
@@ -343,6 +343,23 @@ impl<'a> chalk_solve::RustIrDatabase<Interner> for ChalkContext<'a> {
         // FIXME
         unimplemented!()
     }
+
+    fn unification_database(&self) -> &dyn chalk_ir::UnificationDatabase<Interner> {
+        self
+    }
+}
+
+impl<'a> chalk_ir::UnificationDatabase<Interner> for ChalkContext<'a> {
+    fn fn_def_variance(
+        &self,
+        fn_def_id: chalk_ir::FnDefId<Interner>,
+    ) -> chalk_ir::Variances<Interner> {
+        self.db.fn_def_variance(self.krate, fn_def_id)
+    }
+
+    fn adt_variance(&self, adt_id: chalk_ir::AdtId<Interner>) -> chalk_ir::Variances<Interner> {
+        self.db.adt_variance(self.krate, adt_id)
+    }
 }
 
 pub(crate) fn program_clauses_for_chalk_env_query(
@@ -644,6 +661,32 @@ pub(crate) fn fn_def_datum_query(
     Arc::new(datum)
 }
 
+pub(crate) fn fn_def_variance_query(
+    db: &dyn HirDatabase,
+    _krate: CrateId,
+    fn_def_id: FnDefId,
+) -> Variances {
+    let callable_def: CallableDefId = from_chalk(db, fn_def_id);
+    let generic_params = generics(db.upcast(), callable_def.into());
+    Variances::from(
+        &Interner,
+        std::iter::repeat(chalk_ir::Variance::Invariant).take(generic_params.len()),
+    )
+}
+
+pub(crate) fn adt_variance_query(
+    db: &dyn HirDatabase,
+    _krate: CrateId,
+    adt_id: AdtId,
+) -> Variances {
+    let adt: crate::AdtId = from_chalk(db, adt_id);
+    let generic_params = generics(db.upcast(), adt.into());
+    Variances::from(
+        &Interner,
+        std::iter::repeat(chalk_ir::Variance::Invariant).take(generic_params.len()),
+    )
+}
+
 impl From<FnDefId> for crate::db::InternedCallableDefId {
     fn from(fn_def_id: FnDefId) -> Self {
         InternKey::from_intern_id(fn_def_id.0)
diff --git a/crates/hir_ty/src/traits/chalk/interner.rs b/crates/hir_ty/src/traits/chalk/interner.rs
index 39569e690..6a4aa8333 100644
--- a/crates/hir_ty/src/traits/chalk/interner.rs
+++ b/crates/hir_ty/src/traits/chalk/interner.rs
@@ -25,6 +25,7 @@ pub(crate) type FnDefId = chalk_ir::FnDefId<Interner>;
 pub(crate) type FnDefDatum = chalk_solve::rust_ir::FnDefDatum<Interner>;
 pub(crate) type OpaqueTyId = chalk_ir::OpaqueTyId<Interner>;
 pub(crate) type OpaqueTyDatum = chalk_solve::rust_ir::OpaqueTyDatum<Interner>;
+pub(crate) type Variances = chalk_ir::Variances<Interner>;
 
 impl chalk_ir::interner::Interner for Interner {
     type InternedType = Arc<chalk_ir::TyData<Self>>;
@@ -41,6 +42,7 @@ impl chalk_ir::interner::Interner for Interner {
     type InternedVariableKinds = Vec<chalk_ir::VariableKind<Self>>;
     type InternedCanonicalVarKinds = Vec<chalk_ir::CanonicalVarKind<Self>>;
     type InternedConstraints = Vec<chalk_ir::InEnvironment<chalk_ir::Constraint<Self>>>;
+    type InternedVariances = Arc<[chalk_ir::Variance]>;
     type DefId = InternId;
     type InternedAdtId = hir_def::AdtId;
     type Identifier = TypeAliasId;
@@ -370,6 +372,20 @@ impl chalk_ir::interner::Interner for Interner {
     ) -> Option<fmt::Result> {
         None
     }
+
+    fn intern_variances<E>(
+        &self,
+        data: impl IntoIterator<Item = Result<chalk_ir::Variance, E>>,
+    ) -> Result<Self::InternedVariances, E> {
+        data.into_iter().collect()
+    }
+
+    fn variances_data<'a>(
+        &self,
+        variances: &'a Self::InternedVariances,
+    ) -> &'a [chalk_ir::Variance] {
+        &variances
+    }
 }
 
 impl chalk_ir::interner::HasInterner for Interner {
diff --git a/crates/hir_ty/src/traits/chalk/mapping.rs b/crates/hir_ty/src/traits/chalk/mapping.rs
index 86cbc4c7e..8700d664e 100644
--- a/crates/hir_ty/src/traits/chalk/mapping.rs
+++ b/crates/hir_ty/src/traits/chalk/mapping.rs
@@ -31,7 +31,8 @@ impl ToChalk for Ty {
                 TypeCtor::Ref(m) => ref_to_chalk(db, m, apply_ty.parameters),
                 TypeCtor::Array => array_to_chalk(db, apply_ty.parameters),
                 TypeCtor::FnPtr { num_args: _, is_varargs } => {
-                    let substitution = apply_ty.parameters.to_chalk(db).shifted_in(&Interner);
+                    let substitution =
+                        chalk_ir::FnSubst(apply_ty.parameters.to_chalk(db).shifted_in(&Interner));
                     chalk_ir::TyKind::Function(chalk_ir::FnPointer {
                         num_binders: 0,
                         sig: chalk_ir::FnSig {
@@ -183,7 +184,7 @@ impl ToChalk for Ty {
                 assert_eq!(num_binders, 0);
                 let parameters: Substs = from_chalk(
                     db,
-                    substitution.shifted_out(&Interner).expect("fn ptr should have no binders"),
+                    substitution.0.shifted_out(&Interner).expect("fn ptr should have no binders"),
                 );
                 Ty::Apply(ApplicationTy {
                     ctor: TypeCtor::FnPtr {
@@ -536,6 +537,7 @@ impl ToChalk for GenericPredicate {
         // we don't produce any where clauses with binders and can't currently deal with them
         match where_clause
             .skip_binders()
+            .clone()
             .shifted_out(&Interner)
             .expect("unexpected bound vars in where clause")
         {
@@ -661,7 +663,12 @@ where
                     chalk_ir::TyVariableKind::Integer => TyKind::Integer,
                     chalk_ir::TyVariableKind::Float => TyKind::Float,
                 },
-                chalk_ir::VariableKind::Lifetime => panic!("unexpected lifetime from Chalk"),
+                // HACK: Chalk can sometimes return new lifetime variables. We
+                // want to just skip them, but to not mess up the indices of
+                // other variables, we'll just create a new type variable in
+                // their place instead. This should not matter (we never see the
+                // actual *uses* of the lifetime variable).
+                chalk_ir::VariableKind::Lifetime => TyKind::General,
                 chalk_ir::VariableKind::Const(_) => panic!("unexpected const from Chalk"),
             })
             .collect();