52985 diagnostics no concrete type behind existential type
@oli-obk FYI. See below for new cycle error generated.
```rust
error[E0391]: cycle detected when processing `Foo`
--> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:3:1
|
3 | existential type Foo: Copy;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...which requires processing `bar`...
--> /dev/staging/existential_type_no_concrete_type_nouse_potential.rs:6:23
|
6 | fn bar(x: Foo) -> Foo {
| _______________________^
7 | | x
8 | | }
| |_^
= note: ...which again requires processing `Foo`, completing the cycle
error: aborting due to previous error
For more information about this error, try `rustc --explain E0391`.
```