trait A is not implemented for Box<dyn A>

[frank-king]

Hi folks, I met a compile error when doing this:

trait A { }

trait B: A {
    fn as_a(&self) -> &dyn A;
}


mockall::mock! {
    Foo {}
    impl A for Foo {}
    impl B for Foo {
        fn as_a(&self) -> &dyn A;
    }
}

The compile error is:

error[E0277]: the trait bound `Box<dyn A>: A` is not satisfied
  --> src/lib.rs:11:1
   |
11 | / mockall::mock! {
12 | |     Foo {}
13 | |     impl A for Foo {}
14 | |     impl B for Foo {
15 | |         fn as_a(&self) -> &dyn A;
16 | |     }
17 | | }
   | |_^ the trait `A` is not implemented for `Box<dyn A>`
   |
   = note: required for the cast to the object type `dyn A`
   = note: this error originates in the macro `mockall::mock` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.
error: could not compile `mocktest` due to previous error

I ran cargo expand to see what was happening, and find that:

impl __mock_MockFoo_B::__as_a::Expectation {
    /// Call this [`Expectation`] as if it were the real method.
    pub(in super::super) fn call(&self) -> &Box<dyn A> {
         /* ... */
    }
}
// ...
impl B for MockFoo {
    fn as_a(&self) -> &dyn A {
        let no_match_msg = /* something irrelavent ... */;
        self.B_expectations.as_a.call().expect(&no_match_msg)
    }
}

Here Expectation::call returns &Box<dyn A> but B::as_a requires &dyn A, and Box<dyn A>: A is not satisfied.

The solution is simple. I just add something like this:

impl<T: A + ?Sized> A for Box<T> {}

Then it compiles.

---

But is it possible to make this change in the generated code in MockFoo::as_a?

-        self.B_expectations.as_a.call().expect(&no_match_msg)
+        &*self.B_expectations.as_a.call().expect(&no_match_msg)

After that, there is no extra impl<T: A + ?Sized> A for Box<T> {} needed.

[asomers]

Yeah, I think that will work. Since you've already delved in so far, would you like to submit a PR? You can extend the test case that's already in mockall/tests/mock_return_dyn_trait.rs .

[frank-king]

No problem. I'll try opening a PR later.