Don't infer closure signatures with late-bound type vars

compiler/rustc_hir_typeck/src/closure.rs

@@ -244,7 +244,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     }
                 }
-                if inferred_sig.visit_with(&mut MentionsTy { expected_ty }).is_continue() {
+                // FIXME(non_lifetime_binder): Don't infer a signature with late-bound ty/ct vars
+                if inferred_sig.visit_with(&mut MentionsTy { expected_ty }).is_continue()
+                    && !inferred_sig.has_non_region_late_bound()
+                {
                     expected_sig = inferred_sig;
                 }
             }

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2735,6 +2735,7 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
             | (ty::Infer(ty::InferTy::TyVar(_)), _)
             | (_, ty::Infer(ty::InferTy::TyVar(_))) => Ok(a),
             (ty::Infer(_), _) | (_, ty::Infer(_)) => Err(TypeError::Mismatch),
+            (ty::Bound(..), _) | (_, ty::Bound(..)) => Err(TypeError::Mismatch),
             _ => relate::super_relate_tys(self, a, b),
         }
     }

compiler/rustc_middle/src/ty/visit.rs

@@ -83,6 +83,10 @@ pub trait TypeVisitableExt<'tcx>: TypeVisitable<TyCtxt<'tcx>> {
                 | TypeFlags::HAS_CT_PLACEHOLDER,
         )
     }
+    /// True if there are any non-region placeholders
+    fn has_non_region_placeholders(&self) -> bool {
+        self.has_type_flags(TypeFlags::HAS_TY_PLACEHOLDER | TypeFlags::HAS_CT_PLACEHOLDER)
+    }
     fn needs_subst(&self) -> bool {
         self.has_type_flags(TypeFlags::NEEDS_SUBST)
     }

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -308,13 +308,22 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
     ) -> QueryResult<'tcx> {
         let tcx = ecx.tcx();
         let Some(tupled_inputs_and_output) =
-        structural_traits::extract_tupled_inputs_and_output_from_callable(
-            tcx,
-            goal.predicate.self_ty(),
-            goal_kind,
-        )? else {
-        return ecx.make_canonical_response(Certainty::AMBIGUOUS);
-    };
+            structural_traits::extract_tupled_inputs_and_output_from_callable(
+                tcx,
+                goal.predicate.self_ty(),
+                goal_kind,
+            )? else {
+            return ecx.make_canonical_response(Certainty::AMBIGUOUS);
+        };
+
+        // FIXME(non_lifetime_binders): Higher-ranked Fn trait candidates are not (yet) supported.
+        // Make sure that the inputs/output don't capture any placeholder types.
+        if (goal.predicate.projection_ty.substs[1], goal.predicate.term)
+            .has_non_region_placeholders()
+        {
+            return Err(NoSolution);
+        }
+
         let output_is_sized_pred = tupled_inputs_and_output
             .map_bound(|(_, output)| tcx.at(DUMMY_SP).mk_trait_ref(LangItem::Sized, [output]));
 

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -215,6 +215,13 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         goal_kind: ty::ClosureKind,
     ) -> QueryResult<'tcx> {
         let tcx = ecx.tcx();
+
+        // FIXME(non_lifetime_binders): Higher-ranked Fn trait candidates are not (yet) supported.
+        // Check that the inputs don't capture any placeholder types.
+        if goal.predicate.trait_ref.substs[1].has_non_region_placeholders() {
+            return Err(NoSolution);
+        }
+
         let Some(tupled_inputs_and_output) =
             structural_traits::extract_tupled_inputs_and_output_from_callable(
                 tcx,

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

@@ -760,7 +760,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs);
-        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+
+        // FIXME(non_lifetime_binders): Perform the equivalent of a "leak check" here.
+        let mut nested = self.infcx.commit_if_ok(|_| {
+            let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+
+            if self
+                .infcx
+                .resolve_vars_if_possible(substs.as_closure().sig())
+                .has_non_region_placeholders()
+            {
+                return Err(SelectionError::Unimplemented);
+            }
+
+            Ok(nested)
+        })?;
 
         debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 

tests/ui/traits/non_lifetime_binders/closure-infer.classic.stderr

@@ -0,0 +1,58 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/closure-infer.rs:4:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: expected a `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+  --> $DIR/closure-infer.rs:12:15
+   |
+LL | fn take2() -> impl for<T> Fn(T) -> T {
+   |               ^^^^^^^^^^^^^^^^^^^^^^ expected an `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+   |
+   = help: the trait `Fn<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+
+error[E0277]: expected a `FnOnce<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+  --> $DIR/closure-infer.rs:12:15
+   |
+LL | fn take2() -> impl for<T> Fn(T) -> T {
+   |               ^^^^^^^^^^^^^^^^^^^^^^ expected an `FnOnce<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+   |
+   = help: the trait `FnOnce<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+
+error[E0277]: expected a `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+  --> $DIR/closure-infer.rs:20:10
+   |
+LL |     take(|x| x)
+   |     ---- ^^^^^ expected an `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Fn<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+note: required by a bound in `take`
+  --> $DIR/closure-infer.rs:7:18
+   |
+LL | fn take(id: impl for<T> Fn(T) -> T) {
+   |                  ^^^^^^^^^^^^^^^^^ required by this bound in `take`
+
+error[E0277]: expected a `FnOnce<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+  --> $DIR/closure-infer.rs:20:10
+   |
+LL |     take(|x| x)
+   |     ---- ^^^^^ expected an `FnOnce<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `FnOnce<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+note: required by a bound in `take`
+  --> $DIR/closure-infer.rs:7:34
+   |
+LL | fn take(id: impl for<T> Fn(T) -> T) {
+   |                                  ^ required by this bound in `take`
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/non_lifetime_binders/closure-infer.next.stderr

@@ -0,0 +1,56 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/closure-infer.rs:4:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: expected a `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+  --> $DIR/closure-infer.rs:12:15
+   |
+LL | fn take2() -> impl for<T> Fn(T) -> T {
+   |               ^^^^^^^^^^^^^^^^^^^^^^ expected an `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+   |
+   = help: the trait `Fn<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:16:5: 16:8]`
+
+error[E0271]: type mismatch resolving `<[closure@closure-infer.rs:16:5] as FnOnce<(T,)>>::Output == T`
+  --> $DIR/closure-infer.rs:12:15
+   |
+LL | fn take2() -> impl for<T> Fn(T) -> T {
+   |               ^^^^^^^^^^^^^^^^^^^^^^ types differ
+
+error[E0277]: expected a `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+  --> $DIR/closure-infer.rs:20:10
+   |
+LL |     take(|x| x)
+   |     ---- ^^^^^ expected an `Fn<(T,)>` closure, found `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Fn<(T,)>` is not implemented for closure `[closure@$DIR/closure-infer.rs:20:10: 20:13]`
+note: required by a bound in `take`
+  --> $DIR/closure-infer.rs:7:18
+   |
+LL | fn take(id: impl for<T> Fn(T) -> T) {
+   |                  ^^^^^^^^^^^^^^^^^ required by this bound in `take`
+
+error[E0271]: type mismatch resolving `<[closure@closure-infer.rs:20:10] as FnOnce<(T,)>>::Output == T`
+  --> $DIR/closure-infer.rs:20:10
+   |
+LL |     take(|x| x)
+   |     ---- ^^^^^ types differ
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `take`
+  --> $DIR/closure-infer.rs:7:34
+   |
+LL | fn take(id: impl for<T> Fn(T) -> T) {
+   |                                  ^ required by this bound in `take`
+
+error: aborting due to 4 previous errors; 1 warning emitted
+
+Some errors have detailed explanations: E0271, E0277.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/traits/non_lifetime_binders/closure-infer.rs

@@ -0,0 +1,24 @@
+// revisions: classic next
+//[next] compile-flags: -Ztrait-solver=next
+
+#![feature(non_lifetime_binders)]
+//~^ WARNING the feature `non_lifetime_binders` is incomplete
+
+fn take(id: impl for<T> Fn(T) -> T) {
+    id(0);
+    id("");
+}
+
+fn take2() -> impl for<T> Fn(T) -> T {
+    //~^ ERROR expected a `Fn<(T,)>` closure, found
+    //[classic]~| ERROR expected a `FnOnce<(T,)>` closure, found
+    //[next]~| ERROR type mismatch resolving
+    |x| x
+}
+
+fn main() {
+    take(|x| x)
+    //~^ ERROR expected a `Fn<(T,)>` closure, found
+    //[classic]~| ERROR expected a `FnOnce<(T,)>` closure, found
+    //[next]~| ERROR type mismatch resolving
+}