rustdoc can't handle existential types

[QuietMisdreavus]

#![feature(existential_type)]

pub trait SomeTrait {
    fn some_fn(&self) {}
    
    type SomeType;
}

struct SomeStruct;

impl SomeTrait for SomeStruct {
    type SomeType = String;
}

pub existential type Asdf: SomeTrait;
pub fn asdf() -> Asdf { SomeStruct }

This sample compiles just fine, but when you run it in rustdoc...

$ rustdoc +nightly c.rs
error[E0277]: the trait bound `(): SomeTrait` is not satisfied
  --> c.rs:15:1
   |
15 | pub existential type Asdf: SomeTrait;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `SomeTrait` is not implemented for `()`
   |
   = note: the return type of a function must have a statically known size

error: Compilation failed, aborting rustdoc

$ rustdoc +nightly --version
rustdoc 1.34.0-nightly (f66e4697a 2019-02-20)

My personal guess is that the fact that rustdoc runs everybody_loops on the function that defines this type means that it no longer returns the actual type - instead it returns !, though the compiler picks up () for the error message.

[QuietMisdreavus]

cc #34511

[oli-obk]

The same thing happens for plain impl trait:

trait Foo {}

fn foo() -> impl Foo {
    loop {}
}

gives

error[E0277]: the trait bound `(): Foo` is not satisfied
 --> src/lib.rs:3:13
  |
3 | fn foo() -> impl Foo {
  |             ^^^^^^^^ the trait `Foo` is not implemented for `()`
  |
  = note: the return type of a function must have a statically known size

I'm not sure how to handle this, even if I turn on #![feature(never_type)], I get

error[E0277]: the trait bound `!: Foo` is not satisfied
 --> src/lib.rs:5:13
  |
5 | fn foo() -> impl Foo {
  |             ^^^^^^^^ the trait `Foo` is not implemented for `!`
  |
  = note: the return type of a function must have a statically known size

[alecmocatta]

see also #58011

[varkor]

Closing as a duplicate of #65863 (which has more information).