unit testing - How to mock specific methods but not all of them in Rust?

Question

I have troubles figuring out unit tests for the methods of the target struct.

I have a method random_number that returns a random value based on the attribute of the struct and there is another method plus_one that takes the result of the first method and does something with it:

pub struct RngTest {
    pub attr: u64,
}

impl RngTest {
    pub fn random_number(&self) -> u64 {
        let random = 42; // lets pretend it is random
        return random * self.attr;
    }

    pub fn plus_one(&self) -> u64 {
        return self.random_number() + 1;
    }
}

Having a unit test for the first method, what is the strategy to test the other? I want to mock self.random_number() output for the unit test of plus_one() to have sane code in unit tests. There is a nice post that compares different mocking libraries and concludes (sadly enough) that none of them is really good to stand out from the others.

The only thing I learned while reading instructions for these libraries is that the only way I can mock methods is by moving them to a trait. I didn't see any example in these libraries (I looked at 4 or 5 of them) where they test a case similar to this.

After moving these methods to a trait (even as they are), how do I mock random_number to unit test the output of RngTest::plus_one?

pub trait SomeRng {
    fn random_number(&self) -> u64 {
        let random = 42; // lets pretend it is random
        return random * self.attr;
    }

    fn plus_one(&self) -> u64 {
        return self.random_number() + 1;
    }
}

impl SomeRng for RngTest {}

Answer 1

How to mock specific methods but not all of them in Rust?

As you have already learned, you cannot replace methods on a type. The only thing you can do is move the methods to a trait and then provide production and test-specific implementations of that trait. How you structure the trait determines the granularity of what you are able to test.

Trait with a default implementation

Depending on your use case, you might be able to use a default implementation:

trait SomeRng {
    fn random_number(&self) -> u64;

    fn plus_one(&self) -> u64 {
        self.random_number() + 1
    }
}

struct RngTest(u64);
impl SomeRng for RngTest {
    fn random_number(&self) -> u64 {
        self.0
    }
}

#[test]
fn plus_one_works() {
    let rng = RngTest(41);
    assert_eq!(rng.plus_one(), 42);
}

Here, random_number is a required method, but plus_one has a default implementation. Implementing random_number gives you plus_one by default. You could also choose to implement plus_one if you could do it more efficiently.

What does the real rand crate do?

The real rand crate uses two traits:

• Rng

  pub trait Rng: RngCore { /* ... */ }
  

  An automatically-implemented extension trait on RngCore providing high-level generic methods for sampling values and other convenience methods.

• RngCore

  pub trait RngCore { /* ... */ }
  

  The core of a random number generator.

This splits the core interesting parts of the implementation from the helper methods. You can then control the core and test the helpers:

trait SomeRngCore {
    fn random_number(&self) -> u64;
}

trait SomeRng: SomeRngCore {
    fn plus_one(&self) -> u64 {
        self.random_number() + 1
    }
}

impl<R: SomeRngCore> SomeRng for R {}

struct RngTest(u64);
impl SomeRngCore for RngTest {
    fn random_number(&self) -> u64 {
        self.0
    }
}

#[test]
fn plus_one_works() {
    let rng = RngTest(41);
    assert_eq!(rng.plus_one(), 42);
}