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faux   Latest Version rustc 1.56+ docs

A library to create mocks out of structs.

faux allows you to mock the methods of structs for testing without complicating or polluting your code.

See the API docs for more information.

Getting Started

faux makes liberal use of unsafe Rust features, so it is only recommended for use inside tests. To prevent faux from leaking into your production code, set it as a dev-dependency in your Cargo.toml:

faux = "^0.1"

faux provides two attributes:

  • #[create]: transforms a struct into a mockable equivalent
  • #[methods]: transforms the methods in an impl block into their mockable equivalent

Use Rust's #[cfg_attr(...)] to gate these attributes to the test config only.

#[cfg_attr(test, faux::create)]
pub struct MyStructToMock { /* fields */ }

#[cfg_attr(test, faux::methods)]
impl MyStructToMock { /* methods to mock */ }


mod client {
    // #[faux::create] makes a struct mockable and
    // generates an associated `faux` function
    // e.g., `UserClient::faux()` will create a a mock `UserClient` instance
    pub struct UserClient { /* data of the client */ }

    pub struct User {
        pub name: String

    // #[faux::methods] makes every public method in the `impl` block mockable
    impl UserClient {
        pub fn fetch(&self, id: usize) -> User {
            // does some network calls that we rather not do in tests
            User { name: "".into() }

use crate::client::UserClient;

pub struct Service {
    client: UserClient,

#[derive(Debug, PartialEq)]
pub struct UserData {
    pub id: usize,
    pub name: String,

impl Service {
    fn user_data(&self) -> UserData {
        let id = 3;
        let user = self.client.fetch(id);
        UserData { id, name: user.name }

// A sample #[test] for Service that mocks the client::UserClient
fn main() {
    // create a mock of client::UserClient using `faux`
    let mut client = client::UserClient::faux();

    // mock fetch but only if the argument is 3
    // argument matchers are optional
        // stub the return value for this mock
        .then_return(client::User { name: "my user name".into() });

    // prepare the subject for your test using the mocked client
    let subject = Service { client };

    // assert that your subject returns the expected data
    let expected = UserData { id: 3, name: String::from("my user name") };
    assert_eq!(subject.user_data(), expected);

Due to constraints with rustdocs, the above example tests in main() rather than a #[test] function. In real life, the faux attributes should be gated to #[cfg(test)].


faux lets you mock the return value or implementation of:

  • Async methods
  • Trait methods
  • Generic struct methods
  • Methods with pointer self types (e.g., self: Rc<Self>)
  • Methods in external modules (but not external crates).

faux also provides easy-to-use argument matchers.

Interaction with #[derive(...)] and auto-traits.

faux mocks will auto implement Send and Sync if the real instance also implements it. Using #[derive(...)] for Clone, Debug, and Default will also work as expected. Other derivable traits are not supported as they are about data (e.g., Eq, or Hash) but faux is about mocking behavior not data. Deriving traits that are not part of the standard library is also not currently supported. An escape hatch for this is to manually write the impl for that trait. If you believe there is a derivable trait that faux should support please file an issue explaining your use case.

Clone is a bit of a special case in that it does not duplicate the stubs but instead shares them with the cloned instance. If this is not the desired behavior for cloning mocks you may instead implement Clone manually and do normal method stubbing (faux::when!(my_struct.clone()).then_return(/* something */)). Note that for the cases of exhaustable stubs (e.g., faux::when!(my_struct.foo()).once()) if either instance calls for the stub that will count as exhausting the stub as they are shared.

Interactions with other proc macros

While faux makes no guarantees that it will work with other macro libraries, it should "just" work. There are some caveats, however. For a quick solution, try making the faux attributes (e.g. #[faux::methods]) the first attribute.


If another proc-macro modifies the signature of a method before faux does its macro expansion, then it could modify the signature into something not supported by faux. Unfortunately, the order of proc macros is not specified. However, in practice it seems to expand top-down (tested in Rust 1.42).

struct Foo { /*some items here */ }

impl Foo {
    /* some methods here */

In the snippet above, #[faux::methods] will expand first followed by #[another_attribute].faux is effectively ignoring the other macro and expanding based on the code you wrote.

If #[faux::methods] performs its expansion after another macro has modified the impl block, #[faux::methods] receives the expanded code. This code might contain different method signatures than what you originally wrote. Note that the other proc macro's expansion may create code that faux cannot handle (e.g. explicit lifetimes).

For a concrete example, let's look at async-trait. async-trait effectively converts:

async fn run(&self, arg: Arg) -> Out {
    /* stuff inside */
fn run<'async>(&'async self, arg: Arg) -> Pin<Box<dyn std::future::Future<Output = Out> + Send + 'async>> {
    /* crazier stuff inside */

Because async-trait adds explicit lifetimes to the method signature, which faux cannot handle, having async-trait do its expansion first breaks faux. Note that even if faux could handle explicit lifetimes, our signature is now so unwieldy that it would make mocks hard to work with. Because async-trait just wants an async function signature, and faux does not modify function signatures, it is okay for faux to expand first.

impl MyStruct for MyTrait {
    async fn run(&self, arg: Arg) -> Out {
        /* stuff inside */

If you find a proc macro that faux cannot handle, please open an issue to see if faux is doing something unexpected that conflicts with that macro.


faux was founded on the belief that traits with single implementations are an undue burden and an unnecessary layer of abstraction. Thus, faux does not rely on trait definitions for every mocked object, which would pollute their function signatures with either generics or trait objects. faux aims to create mocks out of user-defined structs, avoiding extra production code that exists solely for tests.


This library was inspired by mocktopus, a mocking library for nightly Rust that lets you mock any function. Unlike mocktopus, faux works on stable Rust and deliberately only allows for mocking public methods in structs.


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