Cycle detected when checking effective visibilities when casting struct into trait object.

[Sol-Ell]

Code

struct A;

trait Trait {
    async fn f();
}

impl Trait for A {
    async fn f() {
        &A as &dyn Trait;
    }
}

Current output

error[E0391]: cycle detected when checking effective visibilities
   |
note: ...which requires computing type of `<impl at src\lib.rs:7:1: 7:17>::f::{opaque#0}`...
  --> src\lib.rs:8:5
   |
8  |     async fn f() {
   |     ^^^^^^^^^^^^
note: ...which requires computing type of opaque `<impl at src\lib.rs:7:1: 7:17>::f::{opaque#0}`...
  --> src\lib.rs:8:5
   |
8  |     async fn f() {
   |     ^^^^^^^^^^^^
note: ...which requires type-checking `<impl at src\lib.rs:7:1: 7:17>::f`...
  --> src\lib.rs:8:5
   |
8  |     async fn f() {
   |     ^^^^^^^^^^^^
   = note: ...which again requires checking effective visibilities, completing the cycle
note: cycle used when checking item types in top-level module
  --> src\lib.rs:1:1
   |
1  | / struct A;
2  | |
3  | | trait Trait {
4  | |     async fn f();
...  |
10 | |     }
11 | | }
   | |_^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

Desired output

No response

Rationale and extra context

No response

Other cases

No response

Rust Version

rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: x86_64-pc-windows-msvc
release: 1.76.0
LLVM version: 17.0.6

Anything else?

Possibly related to:

• cycle detected when checking effective visibility hides relevant diagnostics message #119502
• query cycle: cycle detected when checking effective visibilities #119346
• query cycles could be handled more like ICEs  #119321

[compiler-errors]

This is fixed on nightly:

error[E0038]: the trait `Trait` cannot be made into an object
 --> src/lib.rs:9:15
  |
9 |         &A as &dyn Trait;
  |               ^^^^^^^^^^ `Trait` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/lib.rs:4:14
  |
3 | trait Trait {
  |       ----- this trait cannot be made into an object...
4 |     async fn f();
  |              ^ ...because associated function `f` has no `self` parameter
  = help: only type `A` implements the trait, consider using it directly instead
help: consider turning `f` into a method by giving it a `&self` argument
  |
4 |     async fn f(&self);
  |                +++++
help: alternatively, consider constraining `f` so it does not apply to trait objects
  |
4 |     async fn f() where Self: Sized;
  |                  +++++++++++++++++

error[E0038]: the trait `Trait` cannot be made into an object
 --> src/lib.rs:9:9
  |
9 |         &A as &dyn Trait;
  |         ^^ `Trait` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/lib.rs:4:14
  |
3 | trait Trait {
  |       ----- this trait cannot be made into an object...
4 |     async fn f();
  |              ^ ...because associated function `f` has no `self` parameter
  = help: only type `A` implements the trait, consider using it directly instead
  = note: required for the cast from `&A` to `&dyn Trait`
help: consider turning `f` into a method by giving it a `&self` argument
  |
4 |     async fn f(&self);
  |                +++++
help: alternatively, consider constraining `f` so it does not apply to trait objects
  |
4 |     async fn f() where Self: Sized;
  |                  +++++++++++++++++

For more information about this error, try `rustc --explain E0038`.

Kind of verbose, but at least it mentions the reasons and fixes.