src/test/ui/editions/async-block-2015.rs

async fn foo() {
//~^ ERROR `async fn` is not permitted in the 2015 edition
//~| NOTE to use `async fn`, switch to Rust 2018
//~| HELP set `edition = "2018"` in `Cargo.toml`
//~| NOTE for more on editions, read https://doc.rust-lang.org/edition-guide

    let x = async {};
    //~^ ERROR cannot find struct, variant or union type `async` in this scope
    //~| NOTE `async` blocks are only allowed in the 2018 edition
    let y = async { //~ NOTE `async` blocks are only allowed in the 2018 edition
        let x = 42;
        //~^ ERROR expected identifier, found keyword `let`
        //~| NOTE expected identifier, found keyword
        //~| HELP set `edition = "2018"` in `Cargo.toml`
        //~| NOTE for more on editions, read https://doc.rust-lang.org/edition-guide
        42
    };
    let z = async { //~ NOTE `async` blocks are only allowed in the 2018 edition
        42
        //~^ ERROR expected identifier, found `42`
        //~| NOTE expected identifier
        //~| HELP set `edition = "2018"` in `Cargo.toml`
        //~| NOTE for more on editions, read https://doc.rust-lang.org/edition-guide
    };
    y.await;
    z.await;
    x
}

fn main() {}