How to Mock Dependencies in Rust (mockall, mockito)

Testing
2026-04-17T12:05:42+00:00
Use the mockall crate to generate mock implementations of traits for isolated unit testing in Rust.

Use the mockall crate to generate mock implementations of traits or structs for unit testing. Add mockall to your Cargo.toml under [dev-dependencies], then use the mock! macro to define your mock struct and configure expectations in your tests.

[dev-dependencies]
mockall = "0.13.0"

use mockall::mock;

mock! {
    pub File {
        pub fn inner_read(&self, dst: &mut [u8]) -> std::io::Result<usize>;
    }
}

#[test]
fn test_read() {
    let mut mock = MockFile::default();
    mock.expect_inner_read().returning(|_| Ok(5));
    let result = mock.inner_read(&mut [0; 10]);
    assert_eq!(result.unwrap(), 5);
}