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);
}