async fn inside fn fails to parse

[cramertj]

This fails to parse: fn foo() { async fn bar() { } }.

[memoryruins]

Other variants:

async fn foo() { async fn bar() {} }

let inside_closure = || { async fn bar() {} };

let async_closure = async || {};

[memoryruins]

All of these examples, including your original example, parse without an issue with the new 2018 edition option enabled on the playground.

[nikomatsakis]

Closing as duplicate of #58652