Can't mock methods with generic parameters more than once per mock object

[asomers]

Using the low-level technique, I can create mocks for structs with methods that have generic parameters. In different testcases, I can create expectations with different type arguments. However, I cannot create a single testcase that has multiple expectations for the same method with different type parameters. When I try, I get a panicked at 'called Option::unwrap() on a None value' error. Here's some example code:

    fn generic_parameters2() {
        struct AMock {
            e: Expectations
        }
        impl AMock {
            fn new() -> Self {
                Self {
                    e: Expectations::new()
                }
            }

            fn expect_foo<T: 'static>(&mut self) -> Method<T, T> {
                self.e.expect::<T, T>("foo")
            }

            fn foo<T: 'static>(&self, t:T) -> T {
                self.e.was_called_returning::<T, T>("foo", t)
            }

            fn then(&mut self) -> &mut Self {
                self.e.then();
                self
            }
        }

        let mut mock: AMock = AMock::new();
        mock.expect_foo::<i16>().called_once().with(-1).returning(|_| -1);
        mock.then().expect_foo::<u32>().called_once().with(0).returning(|_| 0);

        mock.foo::<i16>(-1);
        mock.foo::<u32>(1);
    }

[asomers]

I've found a workaround, but it's ugly. The basic idea is to dynamically generate a method name based on the generic parameters. It adds a bunch of ugly code to the user's Mock implementation, however. Worse, it adds an extra method call per generic parameter used to the user's test method (the latter could be eliminated if MethodName did not require a 'static lifetime).

    fn generic_parameters2() {
        struct AMock {
            e: Expectations,
            m: HashMap<any::TypeId, &'static str>
        }
        impl AMock {
            fn new() -> Self {
                Self {
                    e: Expectations::new(),
                    m: HashMap::new()
                }
            }

            fn expect_foo<T: 'static>(&mut self) -> Method<T, T> {
                let m = self.m.get(&any::TypeId::of::<T>()).unwrap();
                self.e.expect::<T, T>(m)
            }

            fn expect_foo_type<T: 'static>(&mut self, name: &'static str) {
                self.m.insert(any::TypeId::of::<T>(), name);
            }

            fn foo<T: 'static>(&self, t:T) -> T {
                let m = self.m.get(&any::TypeId::of::<T>()).unwrap();
                self.e.was_called_returning::<T, T>(m, t)
            }

            fn then(&mut self) -> &mut Self {
                self.e.then();
                self
            }
        }

        let mut mock: AMock = AMock::new();
        mock.expect_foo_type::<i16>("foo_i16");
        mock.expect_foo_type::<u32>("foo_u32");
        mock.expect_foo::<i16>().called_once().with(-1).returning(|_| -1);
        mock.expect_foo::<u32>().called_once().with(1).returning(|_| 0);

        mock.foo::<i16>(-1);
        mock.foo::<u32>(1);
    }

[jasongrlicky]

Hey Alan - wow, that's quite a workaround! A goal with simulacrum was to not have to modify the existing struct's method signatures, so that last point in particular is a hassle. Still, I appreciate you bringing this workaround to my attention.

[asomers]

If you change MethodName to be an owned string rather than a 'static reference, then the expect_foo_type calls would be unnecessary.

[jasongrlicky]

Wow, that is such a simple solution, can't believe I didn't see that before! Thanks for the suggestion. I've put a couple of hours on my calendar tomorrow to spend some time maintaining simulacrum, so hopefully I'll be able to integrate your pull request from June as well as see about fixing this.

[jasongrlicky]

Ok, as of 872b48d, MethodName is now an owned strings rather than 'static. Hopefully that will make the workaround less painful (let me know if it doesn't!), and I have put down some time next week to look at integrating the approach from your workaround into the code. Thanks so much again @asomers!