Looking for a workaround for an FnOnce parameter with a non-static lifetime

[pentastich]

I have an trait with a function like:

    fn eval<'a, T, F>(&self, formula: F) -> Result<T, EvalError>
    where
        F: 'a + FnOnce() -> Result<T, Box<dyn std::error::Error>>;

I understand that automock can't handle the 'a lifetime, and my attempts to use the mock macro suggest that approach won't work either.

A few points about my use:

• I don't want to switch to a static lifetime because that will force most closures to use move, which will complicate use.
• For testing purposes, the only thing that I need to know is that the closure was called, and I could work around that easily enough.

So, my question isn't can I mock this, because I'm pretty sure the answer is no. My question is whether there's any way I can work around the issue without going to a static lifetime.

[asomers]

Is this function part of a public trait, part of a trait that you control, or not part of a trait at all? In the latter two cases, you can conditionally redefine the fuction to use 'static during #[cfg(test)] only. That's what I do in similar cases.
Another option is to use mock! to define a helper function that doesn't have the lifetime, like and then manually define a compliant mock method that wraps the helper. It's more work, but it will at least allow you to write expectations that your closure was called. For example:

mock!{
    pub MyStuct {
        fn __eval<T> -> Result<T, EvalError>;
    }
}
impl MockMyStruct {
    fn eval<'a, T, F>(&self, formula: F) -> Result<T, EvalError>
    where
        F: 'a + FnOnce() -> Result<T, Box<dyn std::error::Error>>
    {
         self.__eval()
    }
}

[pentastich]

It's in a trait that I control, so I'll give these options a try. The code is from what is basically a spreadsheet evaluation algorithm. I'm implemented it in several languages, and am doing a toy implementation to help me learn Rust. There's a lot of Rc<RefCell> stuff floating around, including weak pointers, so it's a good way to peek into some language corners!

Thank you!