In any sensible API, saveToDisk would return an error status (a Result type in R...

veber-alex · on July 5, 2023

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 · on July 5, 2023

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 · on July 5, 2023

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 · on July 6, 2023

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