Add some extra information to opaque type cycle errors

compiler/rustc_middle/src/traits/mod.rs

@@ -588,6 +588,10 @@ pub enum SelectionError<'tcx> {
     /// Signaling that an error has already been emitted, to avoid
     /// multiple errors being shown.
     ErrorReporting,
+    /// Computing an opaque type's hidden type caused an error (e.g. a cycle error).
+    /// We can thus not know whether the hidden type implements an auto trait, so
+    /// we should not presume anything about it.
+    OpaqueTypeAutoTraitLeakageUnknown(DefId),
 }
 
 #[derive(Clone, Debug, TypeVisitable, Lift)]

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

@@ -30,7 +30,7 @@ use rustc_infer::infer::error_reporting::TypeErrCtxt;
 use rustc_infer::infer::{InferOk, TypeTrace};
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::traits::solve::Goal;
-use rustc_middle::traits::SelectionOutputTypeParameterMismatch;
+use rustc_middle::traits::{DefiningAnchor, SelectionOutputTypeParameterMismatch};
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
@@ -1152,6 +1152,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
 
+            SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id) => self.report_opaque_type_auto_trait_leakage(
+                &obligation,
+                def_id,
+            ),
+
             TraitNotObjectSafe(did) => {
                 let violations = self.tcx.object_safety_violations(did);
                 report_object_safety_error(self.tcx, span, did, violations)
@@ -1465,6 +1470,12 @@ trait InferCtxtPrivExt<'tcx> {
         terr: TypeError<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 
+    fn report_opaque_type_auto_trait_leakage(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        def_id: DefId,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
+
     fn report_type_parameter_mismatch_error(
         &self,
         obligation: &PredicateObligation<'tcx>,
@@ -3186,6 +3197,39 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         )
     }
 
+    fn report_opaque_type_auto_trait_leakage(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        def_id: DefId,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let name = match self.tcx.opaque_type_origin(def_id.expect_local()) {
+            hir::OpaqueTyOrigin::FnReturn(_) | hir::OpaqueTyOrigin::AsyncFn(_) => {
+                format!("opaque type")
+            }
+            hir::OpaqueTyOrigin::TyAlias { .. } => {
+                format!("`{}`", self.tcx.def_path_debug_str(def_id))
+            }
+        };
+        let mut err = self.tcx.sess.struct_span_err(
+            obligation.cause.span,
+            format!("cannot check whether the hidden type of {name} satisfies auto traits"),
+        );
+        err.span_note(self.tcx.def_span(def_id), "opaque type is declared here");
+        match self.defining_use_anchor {
+            DefiningAnchor::Bubble | DefiningAnchor::Error => {}
+            DefiningAnchor::Bind(bind) => {
+                err.span_note(
+                    self.tcx.def_ident_span(bind).unwrap_or_else(|| self.tcx.def_span(bind)),
+                    "this item depends on auto traits of the hidden type, \
+                    but may also be registering the hidden type. \
+                    This is not supported right now. \
+                    You can try moving the opaque type and the item that actually registers a hidden type into a new submodule".to_string(),
+                );
+            }
+        };
+        err
+    }
+
     fn report_type_parameter_mismatch_error(
         &self,
         obligation: &PredicateObligation<'tcx>,

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

@@ -67,7 +67,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             AutoImplCandidate => {
-                let data = self.confirm_auto_impl_candidate(obligation);
+                let data = self.confirm_auto_impl_candidate(obligation)?;
                 ImplSource::Builtin(data)
             }
 
@@ -376,12 +376,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn confirm_auto_impl_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-    ) -> Vec<PredicateObligation<'tcx>> {
+    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         debug!(?obligation, "confirm_auto_impl_candidate");
 
         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
-        let types = self.constituent_types_for_ty(self_ty);
-        self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
+        let types = self.constituent_types_for_ty(self_ty)?;
+        Ok(self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types))
     }
 
     /// See `confirm_auto_impl_candidate`.

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

@@ -2294,8 +2294,8 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
     fn constituent_types_for_ty(
         &self,
         t: ty::Binder<'tcx, Ty<'tcx>>,
-    ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
-        match *t.skip_binder().kind() {
+    ) -> Result<ty::Binder<'tcx, Vec<Ty<'tcx>>>, SelectionError<'tcx>> {
+        Ok(match *t.skip_binder().kind() {
             ty::Uint(_)
             | ty::Int(_)
             | ty::Bool
@@ -2359,12 +2359,16 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             }
 
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
+                let ty = self.tcx().type_of(def_id);
+                if ty.skip_binder().references_error() {
+                    return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
+                }
                 // We can resolve the `impl Trait` to its concrete type,
                 // which enforces a DAG between the functions requiring
                 // the auto trait bounds in question.
-                t.rebind(vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)])
+                t.rebind(vec![ty.subst(self.tcx(), substs)])
             }
-        }
+        })
     }
 
     fn collect_predicates_for_types(

tests/ui/generator/layout-error.rs

@@ -24,5 +24,6 @@ fn main() {
     type F = impl Future;
     // Check that statics are inhabited computes they layout.
     static POOL: Task<F> = Task::new();
+    //~^ ERROR: cannot check whether the hidden type of `layout_error[b009]::main::F::{opaque#0}` satisfies auto traits
     Task::spawn(&POOL, || cb());
 }

tests/ui/generator/layout-error.stderr

@@ -4,6 +4,24 @@ error[E0425]: cannot find value `Foo` in this scope
 LL |         let a = Foo;
    |                 ^^^ not found in this scope
 
-error: aborting due to previous error
+error: cannot check whether the hidden type of `layout_error[b009]::main::F::{opaque#0}` satisfies auto traits
+  --> $DIR/layout-error.rs:26:18
+   |
+LL |     static POOL: Task<F> = Task::new();
+   |                  ^^^^^^^
+   |
+note: opaque type is declared here
+  --> $DIR/layout-error.rs:24:14
+   |
+LL |     type F = impl Future;
+   |              ^^^^^^^^^^^
+note: required because it appears within the type `Task<F>`
+  --> $DIR/layout-error.rs:9:12
+   |
+LL | pub struct Task<F: Future>(F);
+   |            ^^^^
+   = note: shared static variables must have a type that implements `Sync`
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0425`.

tests/ui/impl-trait/auto-trait-leak.rs

@@ -3,21 +3,23 @@ use std::rc::Rc;
 
 fn send<T: Send>(_: T) {}
 
-fn main() {
-}
+fn main() {}
 
 // Cycles should work as the deferred obligations are
 // independently resolved and only require the concrete
 // return type, which can't depend on the obligation.
 fn cycle1() -> impl Clone {
     //~^ ERROR cycle detected
+    //~| ERROR cycle detected
     send(cycle2().clone());
+    //~^ ERROR: cannot check whether the hidden type of opaque type satisfies auto traits
 
     Rc::new(Cell::new(5))
 }
 
 fn cycle2() -> impl Clone {
     send(cycle1().clone());
+    //~^ ERROR: cannot check whether the hidden type of opaque type satisfies auto traits
 
     Rc::new(String::from("foo"))
 }

tests/ui/impl-trait/auto-trait-leak.stderr

@@ -1,5 +1,5 @@
 error[E0391]: cycle detected when computing type of `cycle1::{opaque#0}`
-  --> $DIR/auto-trait-leak.rs:12:16
+  --> $DIR/auto-trait-leak.rs:11:16
    |
 LL | fn cycle1() -> impl Clone {
    |                ^^^^^^^^^^
@@ -11,12 +11,12 @@ LL |     send(cycle2().clone());
    |     ^^^^
    = note: ...which requires evaluating trait selection obligation `cycle2::{opaque#0}: core::marker::Send`...
 note: ...which requires computing type of `cycle2::{opaque#0}`...
-  --> $DIR/auto-trait-leak.rs:19:16
+  --> $DIR/auto-trait-leak.rs:20:16
    |
 LL | fn cycle2() -> impl Clone {
    |                ^^^^^^^^^^
 note: ...which requires type-checking `cycle2`...
-  --> $DIR/auto-trait-leak.rs:20:5
+  --> $DIR/auto-trait-leak.rs:21:5
    |
 LL |     send(cycle1().clone());
    |     ^^^^
@@ -34,6 +34,89 @@ LL | |     Rc::new(String::from("foo"))
 LL | | }
    | |_^
 
-error: aborting due to previous error
+error[E0391]: cycle detected when computing type of `cycle1::{opaque#0}`
+  --> $DIR/auto-trait-leak.rs:11:16
+   |
+LL | fn cycle1() -> impl Clone {
+   |                ^^^^^^^^^^
+   |
+note: ...which requires type-checking `cycle1`...
+  --> $DIR/auto-trait-leak.rs:14:5
+   |
+LL |     send(cycle2().clone());
+   |     ^^^^
+   = note: ...which requires evaluating trait selection obligation `cycle2::{opaque#0}: core::marker::Send`...
+note: ...which requires computing type of `cycle2::{opaque#0}`...
+  --> $DIR/auto-trait-leak.rs:20:16
+   |
+LL | fn cycle2() -> impl Clone {
+   |                ^^^^^^^^^^
+note: ...which requires type-checking `cycle2`...
+  --> $DIR/auto-trait-leak.rs:20:1
+   |
+LL | fn cycle2() -> impl Clone {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which again requires computing type of `cycle1::{opaque#0}`, completing the cycle
+note: cycle used when checking item types in top-level module
+  --> $DIR/auto-trait-leak.rs:1:1
+   |
+LL | / use std::cell::Cell;
+LL | | use std::rc::Rc;
+LL | |
+LL | | fn send<T: Send>(_: T) {}
+...  |
+LL | |     Rc::new(String::from("foo"))
+LL | | }
+   | |_^
+
+error: cannot check whether the hidden type of opaque type satisfies auto traits
+  --> $DIR/auto-trait-leak.rs:21:10
+   |
+LL |     send(cycle1().clone());
+   |     ---- ^^^^^^^^^^^^^^^^
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: opaque type is declared here
+  --> $DIR/auto-trait-leak.rs:11:16
+   |
+LL | fn cycle1() -> impl Clone {
+   |                ^^^^^^^^^^
+note: this item depends on auto traits of the hidden type, but may also be registering the hidden type. This is not supported right now. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+  --> $DIR/auto-trait-leak.rs:20:4
+   |
+LL | fn cycle2() -> impl Clone {
+   |    ^^^^^^
+note: required by a bound in `send`
+  --> $DIR/auto-trait-leak.rs:4:12
+   |
+LL | fn send<T: Send>(_: T) {}
+   |            ^^^^ required by this bound in `send`
+
+error: cannot check whether the hidden type of opaque type satisfies auto traits
+  --> $DIR/auto-trait-leak.rs:14:10
+   |
+LL |     send(cycle2().clone());
+   |     ---- ^^^^^^^^^^^^^^^^
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: opaque type is declared here
+  --> $DIR/auto-trait-leak.rs:20:16
+   |
+LL | fn cycle2() -> impl Clone {
+   |                ^^^^^^^^^^
+note: this item depends on auto traits of the hidden type, but may also be registering the hidden type. This is not supported right now. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+  --> $DIR/auto-trait-leak.rs:11:4
+   |
+LL | fn cycle1() -> impl Clone {
+   |    ^^^^^^
+note: required by a bound in `send`
+  --> $DIR/auto-trait-leak.rs:4:12
+   |
+LL | fn send<T: Send>(_: T) {}
+   |            ^^^^ required by this bound in `send`
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0391`.

tests/ui/impl-trait/issue-103181-2.rs

@@ -24,6 +24,8 @@ where
     B: Send, // <- a second bound
 {
     normalize(broken_fut(), ());
+    //~^ ERROR: cannot check whether the hidden type of opaque type satisfies auto traits
+    //~| ERROR: cannot check whether the hidden type of opaque type satisfies auto traits
 }
 
 fn main() {}

tests/ui/impl-trait/issue-103181-2.stderr

@@ -4,6 +4,61 @@ error[E0425]: cannot find value `ident_error` in this scope
 LL |     ident_error;
    |     ^^^^^^^^^^^ not found in this scope
 
-error: aborting due to previous error
+error: cannot check whether the hidden type of opaque type satisfies auto traits
+  --> $DIR/issue-103181-2.rs:26:15
+   |
+LL |     normalize(broken_fut(), ());
+   |     --------- ^^^^^^^^^^^^
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: opaque type is declared here
+  --> $DIR/issue-103181-2.rs:11:23
+   |
+LL | async fn broken_fut() {
+   |                       ^
+note: this item depends on auto traits of the hidden type, but may also be registering the hidden type. This is not supported right now. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+  --> $DIR/issue-103181-2.rs:20:10
+   |
+LL | async fn iceice<A, B>()
+   |          ^^^^^^
+note: required for `impl Future<Output = ()>` to implement `SendFuture`
+  --> $DIR/issue-103181-2.rs:7:17
+   |
+LL | impl<Fut: Send> SendFuture for Fut {
+   |           ----  ^^^^^^^^^^     ^^^
+   |           |
+   |           unsatisfied trait bound introduced here
+note: required by a bound in `normalize`
+  --> $DIR/issue-103181-2.rs:18:19
+   |
+LL | fn normalize<Fut: SendFuture>(_: Fut, _: Fut::Output) {}
+   |                   ^^^^^^^^^^ required by this bound in `normalize`
+
+error: cannot check whether the hidden type of opaque type satisfies auto traits
+  --> $DIR/issue-103181-2.rs:26:5
+   |
+LL |     normalize(broken_fut(), ());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: opaque type is declared here
+  --> $DIR/issue-103181-2.rs:11:23
+   |
+LL | async fn broken_fut() {
+   |                       ^
+note: this item depends on auto traits of the hidden type, but may also be registering the hidden type. This is not supported right now. You can try moving the opaque type and the item that actually registers a hidden type into a new submodule
+  --> $DIR/issue-103181-2.rs:20:10
+   |
+LL | async fn iceice<A, B>()
+   |          ^^^^^^
+note: required for `impl Future<Output = ()>` to implement `SendFuture`
+  --> $DIR/issue-103181-2.rs:7:17
+   |
+LL | impl<Fut: Send> SendFuture for Fut {
+   |           ----  ^^^^^^^^^^     ^^^
+   |           |
+   |           unsatisfied trait bound introduced here
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0425`.

tests/ui/type-alias-impl-trait/auto-trait-leakage3.rs

@@ -6,13 +6,15 @@
 mod m {
     pub type Foo = impl std::fmt::Debug;
     //~^ ERROR: cycle detected when computing type of `m::Foo::{opaque#0}` [E0391]
+    //~| ERROR: cycle detected when computing type of `m::Foo::{opaque#0}` [E0391]
 
     pub fn foo() -> Foo {
         22_u32
     }
 
     pub fn bar() {
         is_send(foo());
+        //~^ ERROR: cannot check whether the hidden type of `auto_trait_leakage3[211d]::m::Foo::{opaque#0}
     }
 
     fn is_send<T: Send>(_: T) {}