Why

• Unit tests have to finish fast and must not be affected by their surroundings.

So

• Use the mockall crate to strip out external coupling.

How

[dev-dependencies]
mockall = "*"

main.rs

#[cfg(test)]
use mockall::{automock, mock, predicate::*};

#[cfg_attr(test, automock)]
trait MyTrait {
  fn foo(&self, x: u32) -> u32;
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn mytest() {
    let mut mock = MockMyTrait::new();
    mock.expect_foo()
    .with(eq(4))
      .times(1)
      .returning(|x| x + 1);

    assert_eq1!(5, mock.foo(4));
  }
}