Rustdoc renders async fn incorrectly

[ebkalderon]

Summary

While experimenting with async fn and await!() on nightly Rust, I noticed that cargo doc renders them incorrectly. It displays the following:

pub async fn foo(&self, bar: Bar) -> impl Future<Output = Option<Baz>> + '_

I expected the output to be something like this:

pub async fn foo(&self, bar: Bar) -> Option<Baz>

I believe having async fn displayed as returning an impl Future like that is misleading because the async keyword at the beginning makes it seem like the return value is a nested future, when it really isn't. That is, it erroneously suggests that the desugared form of the async fn is:

pub fn foo(&self, bar: Bar) -> impl Future<Output = impl Future<Output = Option<Baz>> + '_> + '_

Toolchain information

• rustc 1.33.0-nightly (b43986184 2019-01-11)
• rustdoc 1.33.0-nightly (b43986184 2019-01-11)
• macOS Mojave 10.14.2 (x86_64-apple-darwin)

How to reproduce

The minimal verifiable example for this issue is essentially creating a new Cargo project with some trivial async fn code in it, and then running cargo doc --open.