A common problem when using async-await is to forget to invoke .await. This often shows up as a type error, e.g. in a case like this (playground):

#![feature(async_await)] async fn make_u32() -> u32 { 22 } fn take_u32(x: u32) { } async fn foo() { let x = make_u32(); take_u32(x); } fn main() { }

currently we give a rather generic error:

error[E0308]: mismatched types --> src/main.rs:13:14 | 13 | take_u32(x); | ^ expected u32, found opaque type | = note: expected type `u32` found type `impl std::future::Future` 

it'd be nice if we could suggest adding .await somewhere.