Auto merge of #53588 - tristanburgess:52985_diagnostics_no_concrete_type_behind_exist...

Auto merge of #53588 - tristanburgess:52985_diagnostics_no_concrete_type_behind_existential_type, r=oli-obk

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`.
```