On: How to think about async/await in Rust

[tcfhgj]

> Haven't done too much async Rust yet but I don't think it solved this issue from what I've seen.

If you don't await your variable contains a future - how are you using that like e.g. an int, without a compiler error?

[eis → tcfhgj]

The issue arises if you don't use the returned value. Lets say there's a function "async fn saveToDisk()". You call this function before you exit the program. Now if you forget to use await on it, your program will exit without having saved the data to disk.

[phicoh → eis]

In any sensible API, saveToDisk would return an error status (a Result type in Rust). If you don't check for errors, then probably you didn't care of the data was actually saved or not.

[veber-alex → phicoh]

Futures in Rust are annotated with the #[must_use] attribute [1], same as the Result type [2].

This means the compiler will emit a warning (can be upgraded to an error) if you forget to await a future even if it doesn't return anything.

[1]: https://doc.rust-lang.org/nightly/src/core/future/future.rs.... [2]: https://doc.rust-lang.org/nightly/src/core/result.rs.html#49...

[phicoh → veber-alex]

You don't want the safety of your program to depend on whether the compiler emits a warning or not.

And turning warnings into errors just encourages people to write 'let _ = ...' to get rid of the error.

[veber-alex → phicoh]

This has nothing to do with safety, just correctness.

> And turning warnings into errors just encourages people to write 'let _ = ...' to get rid of the error.

No? writing `let _ = make_future()` will clearly not await the future, why would you do it instead of just adding `.await` ?

Using `let _ = ...` is sometimes fine for Result if you really sure you don't care about the potential error you got but it's a no go with futures.

[estebank → phicoh]

Note that the lint for un-awaited Future doesn't mention the way to silence them by assigning to _:

  warning: unused implementer of `Future` that must be used
   --> src/main.rs:9:5
    |
  9 |     foo();
    |     ^^^^^
    |
    = note: futures do nothing unless you `.await` or poll them
    = note: `#[warn(unused_must_use)]` on by default