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faux   Latest Version rustc 1.45+ docs

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

See the API docs for more information.

faux is in its early alpha stages, so there are no guarantees of API stability.


faux will modify existing code at compile time to transform structs and their methods into mockable versions of themselves. faux makes liberal use of unsafe Rust features, so it is only recommended for use inside of tests. Add faux as a dev-dependency in Cargo.tomlto prevent usage in production code:

faux = "0.0.7"

faux provides two attributes: create and methods. Use these attributes for tagging your struct and its impl block respectively. 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 {
    // creates a mockable version of `UserClient`
    // generates an associated function, `UserClient::faux`, to create a mocked instance
    pub struct UserClient { /* data of the client */ }

    pub struct User {
        pub name: String

    // creates mockable version of every method in the impl
    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();

    // set up what the mock should return
    faux::when!(client.fetch).safe_then(|id| {
        assert_eq!(id, 3, "expected UserClient.fetch to receive user #3");
        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)].

Interactions With Other Proc Macros

faux makes no guarantees that it will work with other macro libraries. faux in theory should "just" work although with some caveats, in particular if they modify the signature of methods.

Unfortunately, the order of proc macros is not specified. However, in practive 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].

If faux does its expansion first then faux will effectively ignore the other macro and expand based on the code that the user wrote. If you want faux to treat the code in the impl block (or the struct) as-is, before the expansion then put it on the top.

If faux does its expansion after, then faux will morph the expanded version of the code, which might have a different signature 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 modifies the signature of the function to a signature that faux cannot handle (explicit lifetimes) then having async-trait do its expansion before faux would make faux not work. Note that even if faux could handle explicit lifetimes, our signature now it's 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 */

Since no expansions came before, faux sees an async function, which it supports. faux does its magic assuming this is a normal async function, and then async-trait does its magic to convert the signature to something that can work on trait impls.

If you find a procedural macro that faux cannot handle please submit 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. It aims to create mocks out of user-defined structs, avoiding extra production code that exists solely for tests. In particular, faux does not rely on trait definitions for every mocked object, which would pollute their function signatures with either generics or trait objects.


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