Can't set simultaneous expectations of different types on generic method whose generic parameters appear neither in args nor in return type

[twz123]

Related to #236, if generic args are neither referenced in method arguments nor in the return type, it's not possible to set expectations for different types.

I've created a test case that shows the problem. The regular_impl test shows a regular trait implementation and passes. The generic_only test uses a mock!ed trait implementation along with expectations and panics:

thread 'generic_only' panicked at 'called Result::unwrap() on an Err value: TypeMismatch { expected: "[ONLY ON NIGHTLY]", found: "[ONLY ON NIGHTLY]" }'

Following the code:

use mockall::*;

trait Bar {
    const BAZ: &'static str;
}

impl Bar for i32 {
    const BAZ: &'static str = "BAZ/i32";
}

impl Bar for u32 {
    const BAZ: &'static str = "BAZ/u32";
}

trait Foo {
    fn foo<T: Bar + 'static>(&self) -> &'static str;
}

mock! {
    MyStruct {}
    impl Foo for MyStruct {
        fn foo<T: Bar + 'static>(&self) -> &'static str;
    }
}

#[test]
fn generic_only() {
    let mut mock = MockMyStruct::new();
    mock.expect_foo::<i32>().return_const("foo/i32");
    mock.expect_foo::<u32>().return_const("foo/u32");
    assert_eq!("foo/i32", mock.foo::<i32>());
    assert_eq!("foo/u32", mock.foo::<u32>());
}

#[test]
fn regular_impl() {
    struct RegularFoo {}
    impl Foo for RegularFoo {
        fn foo<T: Bar>(&self) -> &'static str {
            T::BAZ
        }
    }

    let regular_foo = RegularFoo {};
    assert_eq!("BAZ/i32", regular_foo.foo::<i32>());
    assert_eq!("BAZ/u32", regular_foo.foo::<u32>());
}

[asomers]

Wow! That bug title's a mouthful. First of all, your panic message will be more informative if you build mockall with the nightly crate feature. Secondly, from Mockall's perspective that method isn't really generic at all. So the fact that mock.expect_foo::<T> even compiles indicates a bug. The expectation should be concrete. I'll fix it.

[asomers]

@twz123 could you please test your application using Mockall at 274a539 ? And expect_foo isn't concrete after all; it should be generic just the way you wrote it.

[twz123]

@asomers Works as expected! Thanks for fixing 👍