---
gpt: Crafted by [Genie](https://marketplace.visualstudio.com/items?itemName=genieai.chatgpt-vscode)
ID: q6aKyg2zki0Jt4oexqSc6ZK171VeDzuS
---

<https://twitter.com/tim_abell/status/1651286661509136386?s=19>

## You

how do you inject dependencies for testing in rust

## Genie

In Rust, one way to inject dependencies for testing is through the use of traits and trait objects. The idea is to define a trait for each dependency that your code depends on, and then provide mock implementations of these traits for testing purposes. Here's an example:

```rust
// Define a trait for the dependency
trait MyDependency {
    fn some_method(&self) -> i32;
}

// Your code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: T,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep }
    }

    fn do_something(&self) -> i32 {
        self.dep.some_method()
    }
}

// Mock implementation of the dependency for testing purposes
struct MockDependency;

impl MyDependency for MockDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Unit test
#[test]
fn my_type_works() {
    let my_type = MyType::new(MockDependency);
    assert_eq!(my_type.do_something(), 42);
}
```

In this example, we define a `MyDependency` trait that represents the dependency that our code depends on. We then define `MyType` struct which has a generic type parameter `T` that must implement `MyDependency` trait.

We can now write a mock implementation of `MyDependency`, called `MockDependency`, which we can use in our unit tests.

The key point here is that we can create a `MyType` instance with the `MockDependency` instance injected as a dependency, allowing us to test the behavior of `MyType` without actually using the real `MyDependency`.

This approach allows you to keep your production code decoupled from specific implementations of dependencies, so that it can be easily tested and maintained.

## You

how else could this be done?

## Genie

Another way to inject dependencies for testing in Rust is by using a dependency injection (DI) crate, such as `rust-inject`. 

With `rust-inject`, you can use a `Container` to register your dependencies and then inject those dependencies into your code at runtime. The `Container` can also hold mock implementations of those dependencies for testing purposes.

Here's an example:

```rust
use rust_inject::Container;

// Define a trait for the dependency
trait MyDependency {
    fn some_method(&self) -> i32;
}

// Implementation of the dependency
struct MyRealDependency;

impl MyDependency for MyRealDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Your code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: T,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep }
    }

    fn do_something(&self) -> i32 {
        self.dep.some_method()
    }
}

// Unit test
#[test]
fn my_type_works() {
    // Create a Container and register a mock implementation of the dependency
    let mut container = Container::new();
    container.register::<dyn MyDependency, _>(Box::new(MockDependency));

    // Resolve MyType instance from the container with the mock dependency
    let my_type = container.resolve::<MyType<dyn MyDependency>>();
    assert_eq!(my_type.do_something(), 42);
}
```

In this example, we define a `MyRealDependency` struct which implements `MyDependency` trait. We then define `MyType` struct which has a generic type parameter `T` that must implement `MyDependency` trait.

We can now write a mock implementation of `MyDependency`, called `MockDependency`, which we can register with a `Container` instance provided by the `rust-inject` crate.

The key point here is that we create a `MyType` instance from the `Container` with the registered mock implementation of `MyDependency`, allowing us to test the behavior of `MyType` without actually using the real `MyDependency`.

This approach may be more suitable for large-scale applications where multiple dependencies need to be injected and managed, as it provides a more formal way of handling dependency injection.

## You

are there other ways?

## Genie

Yes, there are other ways to inject dependencies for testing in Rust. Another option is to use a mocking framework like `mockers`. 

With `mockers`, you can create mock objects that mimic the behavior of the real objects and then use them to test your code. This approach can be particularly useful when you need to test complex interactions between objects.

Here's an example:

```rust
use mockers::Scenario;
use mockers_derive::mocked;

// Define a trait for the dependency
trait MyDependency {
    fn some_method(&self) -> i32;
}

// Implementation of the production dependency
struct MyRealDependency;

impl MyDependency for MyRealDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Your code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: T,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep }
    }

    fn do_something(&self) -> i32 {
        self.dep.some_method()
    }
}

// Mock implementation of the dependency
#[mocked]
trait MockDependency: MyDependency {}

#[test]
fn my_type_works() {
    // Create a scenario for mocking
    let scenario = Scenario::new();

    // Create a mock object for the dependency
    let mock_dep = scenario.create_mock::<MockDependency>();

    // Tell the mock object how to behave
    scenario.expect(mock_dep.some_method_call().and_return(42));

    // Create a MyType instance with the mock dependency
    let my_type = MyType::new(mock_dep);

    // Call a method on MyType and assert the result
    assert_eq!(my_type.do_something(), 42);
}
```

In this example, we define a `MyRealDependency` struct which implements `MyDependency` trait. We then define `MyType` struct which has a generic type parameter `T` that must implement `MyDependency` trait.

We can now use the `mockers` crate to create a mock implementation of `MyDependency`, called `MockDependency`. This creates an object that behaves like the real `MyDependency` but allows us to specify its behavior during testing.

We then create a `Scenario` and add an expectation that the `some_method` method of `mock_dep` should be called and return `42`. Then we create a `MyType` instance with the mock dependency and call its `do_something` method. Finally, we assert that the result is `42`. 

This approach may be more suitable for applications where most of the dependencies are already pre-built and only require slight modification to handle edge cases or testing purposes.

## You

any other ways?

## Genie

Yes, another way to inject dependencies for testing in Rust is to use the `Arc<RwLock<dyn Trait>>` pattern. This approach allows you to create a shared reference-counted pointer to the dependency that can be safely shared across multiple threads.

Here's an example:

```rust
use std::sync::{Arc, RwLock};

// Define a trait for the dependency
trait MyDependency {
    fn some_method(&self) -> i32;
}

// Implementation of the production dependency
struct MyRealDependency;

impl MyDependency for MyRealDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Your code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: Arc<RwLock<T>>,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep: Arc::new(RwLock::new(dep)) }
    }

    fn do_something(&self) -> i32 {
        let guard = self.dep.read().unwrap();
        guard.some_method()
    }
}

#[test]
fn my_type_works() {
    // Create a MyRealDependency instance
    let real_dep = MyRealDependency;

    // Wrap the real dependency in an Arc and create a MyType instance with it
    let arc_dep = Arc::new(RwLock::new(real_dep));
    let my_type = MyType::new(arc_dep.clone());

    // Call a method on MyType and assert the result
    assert_eq!(my_type.do_something(), 42);
}
```

In this example, we define a `MyRealDependency` struct which implements `MyDependency` trait. We then define `MyType` struct which has a generic type parameter `T` that must implement `MyDependency` trait.

We can now create a shared reference-counted pointer to `MyRealDependency` using `Arc<RwLock<MyRealDependency>>`. We then wrap the shared reference-counted pointer in another `Arc<RwLock<dyn MyDependency>>` and pass it to `MyType` constructor.

We call `some_method()` on the guard obtained from read-locking the `RwLock` in `do_something()` and assert that its value is equal to `42`.

This approach is helpful when you need to share resources such as DB connections, network connections, or files across multiple threads. It provides an efficient way to handle concurrency while testing your code.

## You

what about with conditional compilation

## Genie

Conditional compilation in Rust allows you to compile different parts of your code depending on certain conditions. You can use conditionals to control whether or not certain blocks of code are included in the final binary.

When it comes to dependency injection, conditional compilation can be used to inject different dependencies into your code depending on whether you're building for production or testing.

Here's an example:

```rust
trait MyDependency {
    fn some_method(&self) -> i32;
}

// Conditional compilation for production
#[cfg(not(test))]
struct MyRealDependency;

#[cfg(not(test))]
impl MyDependency for MyRealDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Conditional compilation for testing
#[cfg(test)]
struct MyMockDependency;

#[cfg(test)]
impl MyDependency for MyMockDependency {
    fn some_method(&self) -> i32 {
        24
    }
}

// Your code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: T,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep }
    }

    fn do_something(&self) -> i32 {
        self.dep.some_method()
    }
}

#[test]
fn my_type_works() {
    // Create a MyMockDependency instance and pass it to MyType
    let mock_dep = MyMockDependency;
    let my_type = MyType::new(mock_dep);

    // Call a method on MyType and assert the result
    assert_eq!(my_type.do_something(), 24);
}

fn main() {
    // Create a MyRealDependency instance and pass it to MyType
    let real_dep = MyRealDependency;
    let my_type = MyType::new(real_dep);

    // Call a method on MyType and print the result
    let result = my_type.do_something();
    println!("{}", result);
}
```

In this example, we define a `MyDependency` trait for the dependency. We then use conditional compilation to define different implementations of `MyDependency` depending on whether or not we're building for testing.

If we're building for production (`#[cfg(not(test))]`), we define `MyRealDependency` which implements `MyDependency` and returns `42` from `some_method()`. If we're building for testing (`#[cfg(test)]`), we define `MyMockDependency` which also implements `MyDependency` but returns `24` from `some_method()`.

We then define `MyType` struct which depends on a type that implements `MyDependency` trait. We use generics to ensure that the dependency can be either `MyRealDependency` or `MyMockDependency` depending on the configuration.

We create instances of `MyType` in the `main()` function and in the `my_type_works()` test function and call the same method on both instances. Depending on the configuration, the calls will return different values.

Conditional compilation is a powerful feature in Rust that allows you to control which parts of your code are included in the final binary based on certain conditions such as the target platform or the build configuration. This makes it easy to inject different dependencies into your code for testing or production purposes.


## You

show dependeny injection for tests with rust macros

## Genie

Rust macros can be used to write dependency injection code for tests. In Rust, we can use `macro_rules!` macro to define simple declarative macros or we can use the `proc_macro` crate to implement more powerful procedural macros.

Here's an example of a simple declarative macro that uses dependency injection for testing:

```rust
trait MyDependency {
    fn some_method(&self) -> i32;
}

struct MyRealDependency;

impl MyDependency for MyRealDependency {
    fn some_method(&self) -> i32 {
        42
    }
}

// Define a macro that injects a test dependency
macro_rules! inject_test_dependency {
    () => {
        struct MyMockDependency;

        impl MyDependency for MyMockDependency {
            fn some_method(&self) -> i32 {
                24
            }
        }

        let _ = MyMockDependency;
    };
}

// Our code depends on a type that implements the MyDependency trait
struct MyType<T: MyDependency> {
    dep: T,
}

impl<T: MyDependency> MyType<T> {
    fn new(dep: T) -> Self {
        Self { dep }
    }

    fn do_something(&self) -> i32 {
        self.dep.some_method()
    }
}

#[test]
fn my_type_works() {
    // Inject the test dependency using the inject_test_dependency! macro
    inject_test_dependency!();

    // Create a MyMockDependency instance and pass it to MyType
    let mock_dep = MyMockDependency;
    let my_type = MyType::new(mock_dep);

    // Call a method on MyType and assert the result
    assert_eq!(my_type.do_something(), 24);
}

fn main() {
    // Create a MyRealDependency instance and pass it to MyType
    let real_dep = MyRealDependency;
    let my_type = MyType::new(real_dep);

    // Call a method on MyType and print the result
    let result = my_type.do_something();
    println!("{}", result);
}
```

In this example, we define a `MyDependency` trait for the dependency. We then define `MyRealDependency` which implements `MyDependency` and returns `42` from `some_method()`.

We then define a macro called `inject_test_dependency!` which defines a test implementation of `MyDependency` called `MyMockDependency`. The macro creates an instance of `MyMockDependency` to ensure that it is included in the compiled binary.

We then define `MyType` struct which depends on a type that implements `MyDependency` trait. We use generics to make sure that the dependency can be either `MyRealDependency` or `MyMockDependency`.

In the `my_type_works()` test function, we use the `inject_test_dependency!` macro to inject the test dependency and create an instance of `MyMockDependency` which we pass to `MyType`. We call a method on `MyType` and assert that the result is `24`, which is the value returned by the test implementation of `MyDependency`.

Note that macros can be powerful tools in Rust because they allow you to generate code at compile-time. However, they can also be tricky to use properly so be sure to consult the Rust documentation before attempting to write your own macros.


## You

list all the ways you know of to do dependency injection for tests in rust

## Genie

There are a few ways to do dependency injection for tests in Rust. Here are some of the most common ways:

1. **Constructor Injection**: Pass in the dependencies as parameters to the constructor of the object being tested. This technique uses the power of Rust's generics and traits to make it possible to inject different implementations of the same trait.

    Example:

   ```rust
   trait MyDependency {
       fn some_method(&self) -> i32;
   }

   struct MyRealDependency;

   impl MyDependency for MyRealDependency {
       fn some_method(&self) -> i32 {
           42
       }
   }

   // Our code depends on a type that implements the MyDependency trait
   struct MyType<T: MyDependency> {
       dep: T,
   }

   impl<T: MyDependency> MyType<T> {
       fn new(dep: T) -> Self {
           Self { dep }
       }

       fn do_something(&self) -> i32 {
           self.dep.some_method()
       }
   }

   #[test]
   fn my_type_works() {
       // Create a MyRealDependency instance and pass it to MyType
       let real_dep = MyRealDependency;
       let my_type = MyType::new(real_dep);

       // Call a method on MyType and assert the result
       assert_eq!(my_type.do_something(), 42);
   }
   ```

2. **Trait Object Injection**: Pass in the dependencies as trait objects (i.e., a reference or a pointer to a trait). This technique allows you to change the implementation of the trait at runtime, which can be useful for testing.

    Example:

   ```rust
   trait MyDependency {
       fn some_method(&self) -> i32;
   }

   struct MyRealDependency;

   impl MyDependency for MyRealDependency {
       fn some_method(&self) -> i32 {
           42
       }
   }

   // Our code depends on a trait object that implements the MyDependency trait
   struct MyType {
       dep: Box<dyn MyDependency>,
   }

   impl MyType {
       fn new(dep: Box<dyn MyDependency>) -> Self {
           Self { dep }
       }

       fn do_something(&self) -> i32 {
           self.dep.some_method()
       }
   }

   #[test]
   fn my_type_works() {
       // Create a MyRealDependency instance and pass it to MyType as a trait object
       let real_dep: Box<dyn MyDependency> = Box::new(MyRealDependency);
       let my_type = MyType::new(real_dep);

       // Call a method on MyType and assert the result
       assert_eq!(my_type.do_something(), 42);
   }
   ```

3. **Global Variable Injection**: Define a global variable that holds the dependency, and reference it from the test. This technique is simple but can be dangerous because the global state can affect the behavior of other tests.

    Example:

   ```rust
   trait MyDependency {
       fn some_method(&self) -> i32;
   }

   struct MyRealDependency;

   impl MyDependency for MyRealDependency {
       fn some_method(&self) -> i32 {
           42
       }
   }

   // Define a global variable holding the dependency
   static DEP: MyRealDependency = MyRealDependency;

   // Our code depends on a type that uses the global variable
   struct MyType;

   impl MyType {
       fn do_something(&self) -> i32 {
           DEP.some_method()
       }
   }

   #[test]
   fn my_type_works() {
       // Call a method on MyType and assert the result
       assert_eq!(MyType.do_something(), 42);
   }
   ```