Mock trait method with Fn(T) + Clone + Send + 'static

[grongor]

Hello 👋

I have a trait that takes a closure which it later uses in multiple places on multiple threads. mockall generates argument types like Fn(Box<dyn Fn(T, bool)>) so I'm unable to use non-*_st methods like returning.

Simplified example:

#[cfg_attr(test, automock)]
pub trait Pinger<T: Send + 'static> {
    fn start<F: Fn(T, bool) + Clone + Send + 'static>(&mut self, done_fn: F);
    // ...other non-relevant methods
}

#[cfg(test)]
mod test {
    use std::sync::{Arc, OnceLock};

    use crate::pinger::MockPinger;

    #[tokio::test]
    async fn test_some_other_thing() {
        let done_fn = Arc::new(OnceLock::new());
        let mut pinger = MockPinger::<bool>::new();
        {
            let done_fn = Arc::clone(&done_fn);
            pinger.expect_start().returning(move |f| assert!(done_fn.set(f).is_ok()));
        }

        // setup other mock methods to call done_fn (this trait is used internally by the tested object)
        
        // let thing = OtherThing::new(pinger);
        // assert!(thing.do_stuff());
    }
}

The error I'm encoutering:

error[E0277]: `dyn Fn(bool, bool)` cannot be sent between threads safely
   --> crates/lib/src/pinger.rs:218:45
    |
218 |             pinger.expect_start().returning(move |f| assert!(done_fn.set(f).is_ok()));
    |                                   --------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Fn(bool, bool)` cannot be sent between threads safely
    |                                   |
    |                                   required by a bound introduced by this call
    |
    = help: the trait `std::marker::Send` is not implemented for `dyn Fn(bool, bool)`, which is required by `{closure@crates/lib/src/pinger.rs:218:45: 218:53}: std::marker::Send`
    = note: required for `std::ptr::Unique<dyn Fn(bool, bool)>` to implement `std::marker::Send`
note: required because it appears within the type `Box<dyn Fn(bool, bool)>`
   --> /home/grongor/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:237:12
    |
237 | pub struct Box<
    |            ^^^
    = note: required for `OnceLock<Box<dyn Fn(bool, bool)>>` to implement `std::marker::Sync`
    = note: 1 redundant requirement hidden
    = note: required for `Arc<OnceLock<Box<dyn Fn(bool, bool)>>>` to implement `std::marker::Send`
note: required because it's used within this closure
   --> crates/lib/src/pinger.rs:218:45
    |
218 |             pinger.expect_start().returning(move |f| assert!(done_fn.set(f).is_ok()));
    |                                             ^^^^^^^^
note: required by a bound in `__mock_MockPinger_Pinger_13065831140148901594::__start::Expectation::<T>::returning`
   --> crates/lib/src/pinger.rs:201:18
    |
201 | #[cfg_attr(test, automock)]
    |                  ^^^^^^^^ required by this bound in `Expectation::<T>::returning`

For now I'm ok with Rc<OnceCell<_>> and the _*st methods, but I wonder if I'm doing something wrong or if this is some limitation of mockall or the automock derive attr.

[asomers]

So the closure that you're passing to returning isn't Send. Why it isn't is a little harder to determine. What happens if you add the Sync bound to T and/or F ?

[grongor]

I don't think the closure passed to the returning is the issue (and because it is move and only uses let done_fn = Arc::clone(&done_fn);, which points to OnceLock, I'd say it is Send).

Adding Sync bound to either has no effect.

[asomers]

There are two three problems:

1. You've found a bug. Mockall isn't handling the additional bounds on F correctly. I'll fix that shortly.
2. The Clone trait isn't an automatic trait, so you can't turn F into a trait object. Mockall relies on being able to do that.
3. F does also need Sync

For a solution, you can either wait for my patch and remove the Clone, or you can use returning_st instead of returning. It has the constraint that the function must be called from the same thread that set the expectation, but it doesn't require the closure to be Send.

[asomers]

@grongor try the branch in #606 . You'll still probably have to remove the + Clone.

[grongor]

Thanks for the quick fix! I tried it and it seems to add Send properly now. The thing is that I need the Clone there 😄 and with the patch it looks like I'm not able to generate the mock at all (error[E0225]: only auto traits can be used as additional traits in a trait object), so...what are my options here? Am I able to generate the mock some other way (instead of the automock derive) or do I have to skip mockall entirely for this specific case?

[asomers]

That error is a limitation of the Rust language, not of Mockall. So I can't fix it for you. However, I think you can solve this by creating a new trait and then define Pinger in terms of that. Like this:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1f412bdb5680b0bb26b0f38a8cd0f9a6