Rustdoc fails to compile compilable crates with #![feature(type_alias_impl_trait)]

[WaffleLapkin]

I was trying to use #![feature(type_alias_impl_trait)] in my crates, but I've recently found out that rustdoc fails to compile their doc (though cargo build compiles them just fine)

First example (playground):

#![feature(type_alias_impl_trait)]
use std::future::Future;

struct Ty;
trait Trait {
    type Fut;

    fn fut() -> Self::Fut;
}

impl Trait for Ty {
    type Fut = impl Future<Output = Ty>;

    fn fut() -> Self::Fut {
        async { Ty }
    }
}

cargo doc error:

% cargo doc
 Documenting rustfmt_brakes_because_of_futures v0.1.0 (/home/waffle/projects/repos/rustfmt_brakes_because_of_futures)
error[E0277]: `()` is not a future
  --> src/lib.rs:12:16
   |
12 |     type Fut = impl Future<Output = Ty>;
   |                ^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
   |
   = help: the trait `std::future::Future` is not implemented for `()`
   = note: the return type of a function must have a statically known size

Second example (playground):

#![feature(type_alias_impl_trait)]
use std::future::Future;

struct Ty;
trait Trait {
    type Fut;

    // The `Option<>` is important here - without it it's essentially first example.
    // (in my original crates it was `Result` and nto option, but it doesn't change anything)
    fn fut() -> Option<Self::Fut>;
}

impl Trait for Ty {
    type Fut = impl Future<Output = Ty>;

    fn fut() -> Option<Self::Fut> {
        Some(async { Ty })
    }
}

cargo doc error:

% cargo doc
 Documenting rustfmt_brakes_because_of_futures v0.1.0 (/home/waffle/projects/repos/rustfmt_brakes_because_of_futures)
error: could not find defining uses
  --> src/lib.rs:12:16
   |
12 |     type Fut = impl Future<Output = Ty>;
   |                ^^^^^^^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

[jyn514]

Both your examples work for me on master. This is a duplicate of #65863 which was fixed in #73566. Can you try updating nightly?

[WaffleLapkin]

@jyn514 Oh. I've run rustup update yesterday, but it didn't update nightly for some reason (probably missing rustfmt?)

Yes, update of the nightly compiler fixed the issue for me, sorry for bothering