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 att

Answer 1

Thanks to @Shepmaster I came to this workaround. I have added the actual Rng to have more context.

use rand::{thread_rng, Rng}; // 0.6.5

struct RngTest(Vec);

impl RngTest {
    fn random_number(&self) -> u64 {
        let random_value = thread_rng().choose(&self.0);
        *random_value.unwrap()
    }

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

#[test]
fn plus_one_works() {
    let rng = RngTest(vec![1]);
    assert_eq!(rng.plus_one(), 2);
}

I can set an appropriate value in the object and don't need to use traits. There is a downside though - this forces me to have a special instance of my type for this specific test which I would like to avoid because my actual type has a lot of fields and I wanted to define its creation once for all the tests using speculate.