Avoid leaking opaque hidden types via auto trait candidates by bit-aloo · Pull Request #159589 · rust-lang/rust

bit-aloo · 2026-07-20T07:09:03Z

rustbot · 2026-07-20T07:09:08Z

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bit-aloo · 2026-07-20T07:09:19Z

r? @lcnr

rustbot · 2026-07-25T17:42:24Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

lcnr · 2026-07-27T10:58:15Z

+    }
+}
+
+pub(in crate::solve) fn consider_auto_trait_candidate_for_opaque_ty<D, I>(


please move this function into trait_goals.rs 🤔

it is only used from there and it doesn't feel "purely structural" enough to belong in this file

View changes since the review

lcnr · 2026-07-27T10:59:32Z

+            return false;
+        }
+
+        let ty::Alias(_, ty::AliasTy { kind: ty::Opaque { def_id, .. }, .. }) =


Suggested change

let ty::Alias(_, ty::AliasTy { kind: ty::Opaque { def_id, .. }, .. }) =

let ty::Alias(ty::Rigid::Yes, ty::AliasTy { kind: ty::Opaque { def_id, .. }, .. }) =

View changes since the review

lcnr · 2026-07-27T10:59:41Z

+            return false;
+        };
+
+        !matches!(tcx.opaque_ty_origin(*def_id), hir::OpaqueTyOrigin::AsyncFn { .. })


why?

View changes since the review

If we remove this we have two failing test:

[ui] tests/ui/traits/error-reporting/leaking-vars-in-cause-code-1.rs
[ui] tests/ui/traits/next-solver/auto-with-drop_tracking_mir.rs#fail

For auto-with-drop_tracking_mir.rs, we lose the cause and .await location:

@@ -1,29 +1,12 @@ -error[E0277]: `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` cannot be shared between threads safely +error[E0277]: `impl Future<Output = ()>` cannot be sent between threads safely --> $DIR/leaking-vars-in-cause-code-1.rs:32:17 | LL | assert_send(cursed_fut()); - | ----------- ^^^^^^^^^^^^ `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` cannot be shared between threads safely + | ----------- ^^^^^^^^^^^^ `impl Future<Output = ()>` cannot be sent between threads safely | | | required by a bound introduced by this call | - = help: the trait `Sync` is not implemented for `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` - = note: required for `&dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` to implement `Send` -note: required because it's used within this `async` fn body - --> $DIR/leaking-vars-in-cause-code-1.rs:19:53 - | -LL | async fn wrap_call<P: AsyncFn + ?Sized>(filter: &P) { - | _____________________________________________________^ -LL | | filter.call().await; -LL | | } - | |_^ -note: required because it's used within this `async` fn body - --> $DIR/leaking-vars-in-cause-code-1.rs:27:23 - | -LL | async fn cursed_fut() { - | _______________________^ -LL | | wrap_call(get_boxed_fn().as_ref()).await; -LL | | } - | |_^ + = help: the trait `Send` is not implemented for `impl Future<Output = ()>` note: required by a bound in `assert_send` --> $DIR/leaking-vars-in-cause-code-1.rs:36:19 |

For leaking-vars-in-cause-code-1.rs, we also lose the nested async-function context:

@@ -1,21 +1,12 @@ -error: future cannot be sent between threads safely +error[E0277]: `impl Future<Output = ()>` cannot be sent between threads safely --> $DIR/auto-with-drop_tracking_mir.rs:25:13 | LL | is_send(foo()); - | ^^^^^ future returned by `foo` is not `Send` + | ------- ^^^^^ `impl Future<Output = ()>` cannot be sent between threads safely + | | + | required by a bound introduced by this call | -help: the trait `Sync` is not implemented for `NotSync` - --> $DIR/auto-with-drop_tracking_mir.rs:8:1 - | -LL | struct NotSync; - | ^^^^^^^^^^^^^^ -note: future is not `Send` as this value is used across an await - --> $DIR/auto-with-drop_tracking_mir.rs:16:11 - | -LL | let x = &NotSync; - | - has type `&NotSync` which is not `Send` -LL | bar().await; - | ^^^^^ await occurs here, with `x` maybe used later + = help: the trait `Send` is not implemented for `impl Future<Output = ()>` note: required by a bound in `is_send` --> $DIR/auto-with-drop_tracking_mir.rs:24:24 | @@ -24,3 +15,4 @@ error: aborting due to 1 previous error +For more information about this error, try `rustc --explain E0277`.

It seems like we should continue diagnostic traversal for AsyncFn; otherwise, the resulting diagnostic loses most of its useful context. So I’m not sure we should remove this exception. 🤔 Wanna vibe check on what you think?

I guess we should actually just leak foreign types here for diagnostics? Given that the reason we stopped the proof tree visitor here was to avoid leaking non-local closures etc, leaking them for async functions means we should leak em everywhere and just support them in the diagnostics code?

…onsider_auto_trait_candidate

…aque_ty in trait goaks

bit-aloo · 2026-08-13T12:26:33Z

@lcnr this one can get a look, almost to finish..

lcnr · 2026-08-13T15:15:33Z

@rustbot authot

rust-bors · 2026-08-24T22:31:20Z

☔ The latest upstream changes (presumably #155499) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot assigned nnethercote Jul 20, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Jul 20, 2026

rustbot assigned lcnr and unassigned nnethercote Jul 20, 2026

bit-aloo commented Jul 20, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs Outdated

lcnr reviewed Jul 20, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs Outdated

lcnr reviewed Jul 20, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

bit-aloo requested a review from lcnr July 21, 2026 07:10

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rust-cloud-vms Bot force-pushed the 2026-07-29-opaque-type branch from 2e546e0 to 7c319d5 Compare July 23, 2026 15:28

lcnr reviewed Jul 24, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs Outdated

rust-cloud-vms Bot force-pushed the 2026-07-29-opaque-type branch from 7c319d5 to c54dd33 Compare July 25, 2026 17:42

bit-aloo requested a review from lcnr July 26, 2026 00:12

lcnr reviewed Jul 27, 2026

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lcnr added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 28, 2026

rust-cloud-vms Bot force-pushed the 2026-07-29-opaque-type branch from c54dd33 to 1557821 Compare July 30, 2026 10:09

bit-aloo requested a review from lcnr July 30, 2026 10:34

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 30, 2026

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bit-aloo added 3 commits August 13, 2026 12:09

add opaque test

80e76e4

next-solver: avoid leaking opaque hidden types via auto trait probes

6c7dbaf

define consider_auto_trait_candidate_for_opaque_ty and call it from c…

764e206

…onsider_auto_trait_candidate

bit-aloo added 3 commits August 13, 2026 12:09

make the match exhaustive again

568806f

Don't let thte diagnostic leak the inner types of opaque types

f6687b0

Add description to test and move consider_auto_trait_candidate_for_op…

d974280

…aque_ty in trait goaks

rust-cloud-vms Bot force-pushed the 2026-07-29-opaque-type branch from 1557821 to d974280 Compare August 13, 2026 12:22

	let ty::Alias(_, ty::AliasTy { kind: ty::Opaque { def_id, .. }, .. }) =
	let ty::Alias(ty::Rigid::Yes, ty::AliasTy { kind: ty::Opaque { def_id, .. }, .. }) =

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bit-aloo commented Aug 13, 2026

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