Change:
Does not appear on Stable or Beta in the playground. () Errors: modify labels: +C-bug, A-diagnostics, D-verbose, E-needs-bisection, T-compiler Edit: nevermind the attempt of adding “regression-from-stable-to-nightly”, I didn’t fully read the description of that label, it doesn’t apply.
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This can be reproduced outside the playground by adding to your . I suspect that this is caused by we now continue trait selection instead of bailing out when we see an error type. I think we'll want to re-add some kind of bail-out logic, but I'm not sure where the best place would be.
I can confirm regression at modify labels to -E-needs-bisection.
(This bug leaked into beta channel while waiting for PR to land, I think.)
Removing as this have been prioritized already.
This one is already on master and beta backported so closing it as fixed in both channels.

Discussion:
) -> Result<(), SelectionError<'tcx>> { debug!("assemble_candidates_from_impls(obligation={:?})", obligation); <ins> // Essentially any user-written impl will match with an error type, // so creating `ImplCandidates` isn't useful. However, we might // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized) // This helps us avoid overflow: see issue #72839 // Since compilation is already guaranteed to fail, this is just // to try to show the 'nicest' possible errors to the user. if obligation.references_error() { return Ok(()); } </ins> self.tcx().for_each_relevant_impl( obligation.predicate.def_id(), obligation.predicate.skip_binder().trait_ref.self_ty(),

// who might care about this case, like coherence, should use // that function). if candidates.is_empty() { <ins> // If there's an error type, 'downgrade' our result from // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid // emitting additional spurious errors, since we're guaranteed // to have emitted at least one. if stack.obligation.references_error() { debug!("no results for error type, treating as ambiguous"); return Ok(None); } </ins> return Err(Unimplemented); }

// return type, which can't depend on the obligation. fn cycle1() -> impl Clone { //~^ ERROR cycle detected <del> //~| ERROR cycle detected //~| ERROR cycle detected </del> send(cycle2().clone()); //~^ ERROR cannot be sent between threads safely

| ^^^^^^^^^^^^^^^^^^^^^^^^^ = note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`... note: ...which requires computing type of `cycle2::{{opaque}}#0`... <del> --> $DIR/auto-trait-leak.rs:22:16 </del> <ins> --> $DIR/auto-trait-leak.rs:20:16 </ins> | LL | fn cycle2() -> impl Clone { |                ^^^^^^^^^^ note: ...which requires borrow-checking `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing MIR for `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires unsafety-checking `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires building MIR for `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires type-checking `cycle2`... <del> --> $DIR/auto-trait-leak.rs:22:1 </del> <ins> --> $DIR/auto-trait-leak.rs:20:1 </ins> | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^

LL | | } | |_^ <del> error[E0391]: cycle detected when computing type of `cycle1::{{opaque}}#0` --> $DIR/auto-trait-leak.rs:12:16 | LL | fn cycle1() -> impl Clone { |                ^^^^^^^^^^ | note: ...which requires borrow-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing MIR for `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires unsafety-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires building MIR for `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires type-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ = note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`... note: ...which requires computing type of `cycle2::{{opaque}}#0`... --> $DIR/auto-trait-leak.rs:22:16 | LL | fn cycle2() -> impl Clone { |                ^^^^^^^^^^ note: ...which requires borrow-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing MIR for `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires unsafety-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires building MIR for `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires type-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | 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[E0391]: cycle detected when computing type of `cycle1::{{opaque}}#0` --> $DIR/auto-trait-leak.rs:12:16 | LL | fn cycle1() -> impl Clone { |                ^^^^^^^^^^ | note: ...which requires borrow-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing MIR for `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires unsafety-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires building MIR for `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires type-checking `cycle1`... --> $DIR/auto-trait-leak.rs:12:1 | LL | fn cycle1() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ = note: ...which requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`... note: ...which requires computing type of `cycle2::{{opaque}}#0`... --> $DIR/auto-trait-leak.rs:22:16 | LL | fn cycle2() -> impl Clone { |                ^^^^^^^^^^ note: ...which requires borrow-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires processing MIR for `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires unsafety-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires building MIR for `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | LL | fn cycle2() -> impl Clone { | ^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires type-checking `cycle2`... --> $DIR/auto-trait-leak.rs:22:1 | 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 | | } | |_^ </del> error[E0277]: `std::rc::Rc<std::string::String>` cannot be sent between threads safely <del> --> $DIR/auto-trait-leak.rs:16:5 </del> <ins> --> $DIR/auto-trait-leak.rs:14:5 </ins> | LL | fn send<T: Send>(_: T) {} |            ---- required by this bound in `send`

= help: within `impl std::clone::Clone`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::string::String>` = note: required because it appears within the type `impl std::clone::Clone` <del> error: aborting due to 4 previous errors </del> <ins> error: aborting due to 2 previous errors </ins> Some errors have detailed explanations: E0277, E0391. For more information about an error, try `rustc --explain E0277`.

<ins> // Regression test for issue #72839 // Tests that we do not overflow during trait selection after // a type error occurs use std::ops::Rem; trait Foo {} struct MyStruct<T>(T); impl<T, U> Rem<MyStruct<T>> for MyStruct<U> where MyStruct<U>: Rem<MyStruct<T>> { type Output = u8; fn rem(self, _: MyStruct<T>) -> Self::Output { panic!() } } fn main() {} fn foo() { if missing_var % 8 == 0 {} //~ ERROR cannot find } </ins>

<ins> error[E0425]: cannot find value `missing_var` in this scope --> $DIR/issue-72839-error-overflow.rs:18:8 | LL |     if missing_var % 8 == 0 {} |        ^^^^^^^^^^^ not found in this scope error: aborting due to previous error For more information about this error, try `rustc --explain E0425`. </ins>