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`.
```

src/test/ui/existential_types/nested_existential_types.rs

@@ -0,0 +1,31 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(existential_type)]
+// compile-pass
+mod my_mod {
+  use std::fmt::Debug;
+
+  pub existential type Foo: Debug;
+  pub existential type Foot: Debug;
+
+  pub fn get_foo() -> Foo {
+      5i32
+  }
+
+  pub fn get_foot() -> Foot {
+      get_foo()
+  }
+}
+
+fn main() {
+    let _: my_mod::Foot = my_mod::get_foot();
+}
+

src/test/ui/existential_types/no_inferrable_concrete_type.stderr

@@ -1,10 +1,15 @@
-error[E0391]: cycle detected when normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`
+error[E0391]: cycle detected when processing `Foo`
   --> $DIR/no_inferrable_concrete_type.rs:16:1
    |
 LL | existential type Foo: Copy; //~ cycle detected
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: ...which again requires normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: Foo }`, completing the cycle
+note: ...which requires processing `bar`...
+  --> $DIR/no_inferrable_concrete_type.rs:19:23
+   |
+LL | fn bar(x: Foo) -> Foo { x }
+   |                       ^^^^^
+   = note: ...which again requires processing `Foo`, completing the cycle
 
 error: aborting due to previous error