double-derive 0.1.3

A procedural macro to derive a mirror of a trait designed to make it easier to implement test doubles.
Documentation 
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// We are more interested that the code compiles and  not so much in the actual functionality.
#![allow(dead_code)]

use double_derive::double;

#[test]
fn implement_double_instead_of_original_trait() {
    // Given an original trait with a derived `DummyTrait` test double
    #[double(MyEmptyTraitDummy)]
    trait MyEmptyTrait {}

    // When implementing `DummyTrait` for a struct `MyStruct`
    struct MyStruct;
    impl MyEmptyTraitDummy for MyStruct {}

    // Then `MyStruct` also implements `OrgTrait`.
    fn use_trait(_: impl MyEmptyTrait) {
        // This function is just a placeholder to ensure the trait is used
    }
    use_trait(MyStruct);
}

#[test]
fn invoke_implemented_method_through_original_trait() {
    // Given an original trait with a method `answer`
    #[double(DummyTrait)]
    trait OrgTrait {
        fn answer(&self) -> i32;

        fn some_other_method(&self);
    }

    // When overriding default implementation of `answer` in `DummyTrait`
    struct MyStruct;
    impl DummyTrait for MyStruct {
        fn answer(&self) -> i32 {
            42
        }
    }

    // The new implementation is used than invoking `OrgTrait::answer` via `MyStruct`
    assert_eq!(42, OrgTrait::answer(&MyStruct));
}

#[tokio::test]
async fn async_method_invocation() {
    // Given an original trait with a method `answer`
    #[double(DummyTrait)]
    trait OrgTrait {
        async fn answer(&self) -> i32;

        async fn foobar(&self);
    }

    // When ovverriding default implementation of `answer` in `DummyTrait`
    struct MyStruct;
    impl DummyTrait for MyStruct {
        async fn answer(&self) -> i32 {
            42
        }
    }

    // The new implementation is used than invoking `OrgTrait::answer` via `MyStruct`
    assert_eq!(42, OrgTrait::answer(&MyStruct).await);
}

#[tokio::test]
async fn associated_method_invocation() {
    // Given an original trait with a method `answer`
    #[double(DummyTrait)]
    trait OrgTrait {
        async fn answer() -> i32;
    }

    // When ovverriding default implementation of `answer` in `DummyTrait`
    struct MyStruct;
    impl DummyTrait for MyStruct {
        async fn answer() -> i32 {
            42
        }
    }

    // The new implementation is used than invoking `OrgTrait::answer` via `MyStruct`
    assert_eq!(42, <MyStruct as OrgTrait>::answer().await);
}

#[tokio::test]
async fn impl_future_method_invocation() {
    use std::future::Future;
    // Given an original trait with a method `answer`
    #[double(DummyTrait)]
    trait OrgTrait {
        fn answer(&self) -> impl Future<Output = i32>;
    }

    // When ovverriding default implementation of `answer` in `DummyTrait`
    struct MyStruct;
    impl DummyTrait for MyStruct {
        fn answer(&self) -> impl Future<Output = i32> {
            async { 42 }
        }
    }

    // The new implementation is used than invoking `OrgTrait::answer` via `MyStruct`
    assert_eq!(42, OrgTrait::answer(&MyStruct).await);
}

// fn compile_time_error_for_default_implementation_for_impl() {
//     use std::future::Future;
//     // Given an original trait with an impl return type
//     #[double(DummyTrait)]
//     trait OrgTrait {
//         fn answer(&self) -> impl std::fmt::Display;
//     }
// }