Suggestion for E0627's message

[Phrohdoh]

The error message for E0627 is currently:

rust/src/librustc_typeck/check/mod.rs

Line 4129 in 0a223d1

"yield statement outside of generator literal").emit();

But could be something, IMHO more helpful, like:

The containing function is not marked as async

[Arnavion]

But the error message is correct. It's about yield statements outside of generator literals.

You're getting this error because you used future-await's await! macro (which expands to code that uses yield) on a function that you didn't annotate with #[async] (which expands to code that creates a generator). The compiler can't special-case its error message just for a specific user crate macro. Native async-await syntax would have its own separate error message for await outside of async functions.

[estebank]

@Arnavion I wouldn't be against the compiler having special case code for climbing the scope tree and suggesting marking the enclosing method as #[async] if the crate futures-await is available (and even suggest adding the crate dep if it isn't).

We shouldn't be encoding information in the compiler that depends on the community's crates, as things change over time, but I feel that the usability improvement in this case is worth it. That being said, we could try to implement something along the lines of rustc_on_unimplemented for E0627 that could be generic and use messages provided by the available crates instead.

[jesskfullwood]

FWIW this error confused me a lot while trying out async/await for the first time.