1 unstable release

Uses new Rust 2021

0.1.0 Jul 4, 2022

#145 in Testing


581 lines

Lightweight Mocking / Spying Library for Rust

Build Status License: MIT OR Apache-2.0 rust 1.57+ required

Documentation: crate docs (main)

Mocking in Rust is somewhat hard compared to object-oriented languages. Since there is no implicit / all-encompassing class hierarchy, Liskov substitution principle does not apply, thus making it generally impossible to replace an object with its mock. A switch is only possible if the object consumer explicitly opts in via parametric polymorphism or dynamic dispatch.

What do? Instead of trying to emulate mocking approaches from the object-oriented world, this crate opts in for another approach, somewhat similar to remote derive from serde. Mocking is performed on function / method level, with each function conditionally proxied to a mock that has access to function args and can do whatever: call the "real" function (e.g., to spy on responses), maybe with different args and/or after mutating args; substitute with a mock response, etc. Naturally, mock logic can be stateful (e.g., determine a response from the predefined list; record responses for spied functions etc.)


Add this to your Crate.toml:

mimicry = "0.1.0"

Example of usage:

use mimicry::{mock, CallReal, Mock, Mut};

// Tested function
#[mock(using = "SearchMock")]
fn search(haystack: &str, needle: char) -> Option<usize> {
    haystack.chars().position(|ch| ch == needle)

// Mock logic
#[derive(Default, Mock)]
// ^ Indicates that the mock state is wrapped in a wrapper with 
// internal mutability.
struct SearchMock {
    called_times: usize,

impl SearchMock {
    // Implementation of mocked function, which the mocked function
    // will delegate to if the mock is set.
    fn search(
        this: &Mut<Self>,
        haystack: &str,
        needle: char,
    ) -> Option<usize> {
        this.borrow().called_times += 1;
        if haystack == "test" {
        } else {
            let new_needle = if needle == '?' { 'e' } else { needle };
            this.call_real(|| search(haystack, new_needle))

// Test code.
let guard = SearchMock::default().set_as_mock();
assert_eq!(search("test", '?'), Some(42));
assert_eq!(search("needle?", '?'), Some(1));
assert_eq!(search("needle?", 'd'), Some(3));
let recovered = guard.into_inner();
assert_eq!(recovered.called_times, 3);

See crate docs for more details and examples of usage.


  • Can mock functions / methods with a wide variety of signatures, including generic functions (with not necessarily 'static type params), functions returning non-'static responses and responses with dependent lifetimes, such as in fn(&str) -> &str, functions with impl Trait args etc.
  • Can mock methods in impl blocks, including trait implementations.
  • Single mocking function can mock multiple functions, provided that they have compatible signatures.
  • Whether mock state is shared across functions / methods, is completely up to the test writer. Functions for the same receiver type / in the same impl block may have different mock states.
  • Mocking functions can have wider argument types than required from the signature of function(s) being mocked. For example, if the mocking function doesn't use some args, they can be just replaced with unconstrained type params.


  • You still cannot mock types from other crates.
  • Even if mocking logic does not use certain args, they need to be properly constructed, which, depending on the case, may defy the reasons behind using mocks.
  • Very limited built-in matching / verifying. With the chosen approach, it is frequently easier and more transparent to just use match statements. As a downside, if matching logic needs to be customized across tests, it's (mostly) up to the test writer.


mockall, simulacrum, mocktopus, mockiato etc. provide more traditional approach to mocking based on configuring expectations for called functions / methods.


Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in mimicry by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


~100K SLoC