I have a trait like such:

#[cfg_attr(test, automock)]
trait Foo {
    #[cfg_attr(test, concretize)]
    fn for_each<F>(&self, processor: F)
    where
        Self: Sized,
        F: FnMut(&u32);
}

#[derive(Default)]
struct UnderTest;

impl UnderTest {
   fn do_something(&self, foo: &Foo) {
     foo.for_each(|x| {
         eprintln!("Invoked with {x}");
     })
   }
}

In my test I want to do something like:

#[cfg(test)]
fn do_something() {
   let mut foo = MockFoo::new();
   foo.expect_for_each().times(1).returning(|cb| {
     cb(&0);
     cb(&1);
   });

   let t = UnderTest::default();
   t.do_something(&foo);
}

The problem is that when I compile this, there’s an error because the returning closure receives a &FnMut instead of a &mut FnMut so there’s no way to invoke it. Am I missing something obvious or is it impossible to actually mock this situation?