async-await: mutually recursive functions compile sometimes

[mb64]

In the following code:

async fn rec_1() {
    rec_2().await;
}

async fn rec_2() {
    rec_1().await;
}

This gives an error when it is a part of a lib crate. However, as a part of a bin crate, it compiles.

Here's an erroneously compiling playground link and a correctly erroring playground link

[matprec]

I guess that's because in the binary, it knows that it will be unused. If you were to actually use it, it errors

[cramertj]

Tagging as blocking because fixing this is a potential future-compat hazard.