Crate mocktopus [] [src]

Mocking framework for Rust (currently only nightly)

mod hello_world {
    pub fn world() -> &'static str {

    pub fn hello_world() -> String {
        format!("Hello {}!", world())

fn mock_test() {
    hello_world::world.mock_safe(|| MockResult::Return("mocking"));

    assert_eq!("Hello mocking!", hello_world::hello_world());


This is a user guide showing Rust project set up for testing with mocks.

It is highly recommended to use mocks ONLY for test runs and NEVER in release builds! Mocktopus is not designed for high performance and will slow down code execution.

Note: this guide shows set up of mocking for test builds only.


Add Mocktopus dev-dependency to project's Cargo.toml:

mocktopus = "0.1.0"

Enable procedural macros in crate root:

#![cfg_attr(test, feature(proc_macro))]

Import Mocktopus:

extern crate mocktopus;

This import MUST NOT be aliased, mocking framework depends on Mocktopus root being visible as mocktopus.

Making functions mockable

To make functions mockable they must be annotated with provided procedural macros. See documentation for all their possibilities and rules.

To use these macros import them into namespace:

use mocktopus::macros::*;

It is possible to annotate modules, which makes all their potentially mockable content mockable. To make every function in project mockable annotate each module in its root:

#[cfg_attr(test, mockable)]
mod my_module;


Import tools for mocking in test module:

mod tests {
    use mocktopus::mocking::*;

Among others this imports trait Mockable. It is implemented for all functions and provides an interface for setting up mocks:

fn my_test() {
    my_function.mock_safe(|| MockResult::Return(1));

    assert_eq!(1, my_function());

It is also possible to mock struct methods, either from own impls, traits or trait defaults:

// Mocking method
MyStruct::my_method.mock_safe(|| MockResult::Return(1));
// Mocking trait method
MyStruct::my_trait_method.mock_safe(|| MockResult::Return(2));
// Mocking default trait method
MyStruct::my_trait_default_method.mock_safe(|| MockResult::Return(3));

Mocking with mock_safe is simplest, but the Mockable trait has more, see documantation.

Mocking range

Every mock works only in thread, in which it was set. All Rust test runs are executed in independent threads, so mocks do not leak between them:

#[cfg_attr(test, mockable)]
fn common_fn() -> u32 {

fn common_fn_test_1() {
    assert_eq!(0, common_fn());

    common_fn.mock_safe(|| MockResult::Return(1));

    assert_eq!(1, common_fn());

fn common_fn_test_2() {
    assert_eq!(0, common_fn());

    common_fn.mock_safe(|| MockResult::Return(2));

    assert_eq!(2, common_fn());

Mock closure

mock_safe has single argument: a closure, which takes same input as mocked function and returns a MockResult. Whenever the mocked function is called, its inputs are passed to the closure:

#[cfg_attr(test, mockable)]
fn my_function_1(_: u32) {

fn my_function_1_test() {
    my_function_1.mock_safe(|x| {
        assert_eq!(2, x);

    my_function_1(2); // Passes
    my_function_1(3); // Panics

If the closure returns MockResult::Return, the mocked function does not run. It immediately returns with a value, which is passed inside MockResult::Return:

#[cfg_attr(test, mockable)]
fn my_function_2() -> u32 {

fn my_function_2_test() {
    my_function_2.mock_safe(|| MockResult::Return(3));

    assert_eq!(3, my_function_2());

If the closure returns MockResult::Continue, the mocked function runs normally, but with changed arguments. The new arguments are returned from closure in tuple inside MockResult::Continue:

#[cfg_attr(test, mockable)]
fn my_function_3(x: u32, y: u32) -> u32 {
    x + y

fn my_function_3_test() {
    my_function_3.mock_safe(|x, y| MockResult::Continue((x, y + 1)));

    assert_eq!(3, my_function_3(1, 1));

Mocking generics

When mocking generic functions, all its generics must be defined and only this variant will be affected:

#[cfg_attr(test, mockable)]
fn generic_fn<T: Display>(t: T) -> String {

fn generic_fn_test() {
    generic_fn::<u32>.mock_safe(|_| MockResult::Return("mocked".to_string()));

    assert_eq!("1", generic_fn(1i32));
    assert_eq!("mocked", generic_fn(1u32));

The only exception are lifetimes, they are ignored:

#[cfg_attr(test, mockable)]
fn lifetime_generic_fn<'a>(string: &'a String) -> &'a str {

fn lifetime_generic_fn_test() {
    lifetime_generic_fn.mock_safe(|_| MockResult::Return("mocked"));

    assert_eq!("mocked", lifetime_generic_fn(&"not mocked".to_string()));

Same rules apply to methods and structures:

struct GenericStruct<'a, T: Display + 'a>(&'a T);

#[cfg_attr(test, mockable)]
impl<'a, T: Display + 'a> GenericStruct<'a, T> {
    fn to_string(&self) -> String {

static VALUE: u32 = 1;

fn lifetime_generic_fn_test() {
    GenericStruct::<u32>::to_string.mock_safe(|_| MockResult::Return("mocked".to_string()));

    assert_eq!("mocked", GenericStruct(&VALUE).to_string());
    assert_eq!("mocked", GenericStruct(&2u32).to_string());
    assert_eq!("2", GenericStruct(&2i32).to_string());



For use in tested code: tools making items mockable


For use in testing code: mocking tools


For use in testing code: helper tools for writing tests using mocking