Change:
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`

Discussion:
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.
Error: Label regression-from-stable-to-nightly can only be set by Rust team members Please let know if you're having trouble with this bot.
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.