Type annotations

[dbrgn]

Often I get compile errors when the compiler cannot deduce the type when using try_stream!:

error[E0698]: type inside `async fn` body must be known in this context
   --> src/server.rs:213:18
    |
213 |       let stream = try_stream! {
    |  __________________^
214 | |         while let Some(chunk) = body.data().await {
215 | |             let chunk = chunk?;
216 | |             trace!("Body: {:?}", &chunk);
...   |
219 | |         }
220 | |     };
    | |_____^ cannot infer type for type parameter `E` declared on the enum `Result`

This can be resolved by moving the code into a function that returns the appropriate generic type. However, is it also possible to put a type annotation somewhere? (impl T doesn't seem to work for annotating the type of a local variable.)

[carllerche]

I don't see you yielding anywhere in this example.

[carllerche]

Unfortunately, "terrible compilation errors" is going to be a limitation of this crate & the macro strategy. Doing better on the compilation error front will require rustc to provide stream generators.

Sorry :(

[dbrgn]

Sorry, the yield was in the part that was omitted in the error message 🙂

Sure, that's fine, I just wondered whether there's an easy way to annotate the return type, so that I won't have to create a helper function. But helper functions work just fine.

[rodrigorc]

I managed to annotate the return type by writing this:

let stream = try_stream! {
    //...
};
let _: &dyn Stream<Item=Result<_, MyError>> = &stream;

Not the prettiest code, but it gets the job done and looks better than the auxiliary function.
I've checked the generated assembly and it has zero cost.