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#300 in Procedural macros

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Used in entrait

MIT license

732 lines


A proc macro to ease development using Inversion of Control patterns in Rust.

entrait is used to generate a trait from the definition of a regular function. The main use case for this is that other functions may depend upon the trait instead of the concrete implementation, enabling better test isolation.

The macro looks like this:

fn my_function<D>(deps: &D) {

which generates the trait MyFunction:

trait MyFunction {
    fn my_function(&self);

my_function's first and only parameter is deps which is generic over some unknown type D. This would correspond to the self parameter in the trait. But what is this type supposed to be? The trait gets automatically implemented for ::implementation::Impl(T):

use implementation::Impl;
struct App;

fn my_function<D>(deps: &D) { // <--------------------.
}                             //                      |
                              //                      |
// Generated:                                         |
// trait MyFunction {                                 |
//     fn my_function(&self);                         |
// }                                                  |
//                                                    |
// impl<T> MyFunction for ::implementation::Impl<T> { |
//     fn my_function(&self) {                        |
//         my_function(self) // calls this! ----------´
//     }
// }

let app = Impl::new(App);

The advantage of this pattern comes into play when a function declares its dependencies, as trait bounds:

fn foo(deps: &impl Bar) {

fn bar<D>(deps: &D) {

The functions may take any number of parameters, but the first one is always considered specially as the "dependency parameter".

Functions may also be non-generic, depending directly on the App:

use implementation::Impl;

struct App { something: SomeType };
type SomeType = u32;

fn generic(deps: &impl Concrete) -> SomeType {

fn concrete(app: &App) -> SomeType {

let app = Impl::new(App { something: 42 });
assert_eq!(42, app.generic());

These kinds of functions may be considered "leaves" of a dependency tree.


The entrait crate is a building block of a design pattern - the entrait pattern. The entrait pattern is simply a convenient way to achieve unit testing of business logic.

Entrait is not intended for achieving polymorphism. If you want that, you should instead hand-write a trait.

Entrait should only be used to define an abstract computation that has a single implementation in realase mode, but is mockable in test mode.

entrait does not implement Dependency Injection (DI). DI is a strictly object-oriented concept that will often look awkward in Rust. The author thinks of DI as the "reification of code modules": In a DI-enabled programming environment, code modules are grouped together as objects and other modules may depend upon the interface of such an object by receiving some instance that implements it. When this pattern is applied successively, one ends up with an in-memory dependency graph of high-level modules.

entrait tries to turn this around by saying that the primary abstraction that is depended upon is a set of functions, not a set of code modules.

An architectural consequence is that one ends up with one ubiquitous type that represents a running application that implements all these function abstraction traits. But the point is that this is all loosely coupled: Most function definitions themselves do not refer to this god-like type, they only depend upon traits.

async support

Since Rust at the time of writing does not natively support async methods in traits, you may opt in to having #[async_trait] generated for your trait:

#[entrait(Foo, async_trait=true)]
async fn foo<D>(deps: &D) {

This is designed to be forwards compatible with real async fn in traits. When that day comes, you should be able to just remove the async_trait=true to get a proper zero-cost future.

Trait visibility

by default, entrait generates a trait that is module-private (no visibility keyword). To change this, just put a visibility specifier before the trait name:

use entrait::*;
#[entrait(pub Foo)]   // <-- public trait
fn foo<D>(deps: &D) { // <-- private function

Mock support


Entrait works best together with unimock, as these two crates have been designed from the start with each other in mind.

Unimock exports a single mock struct which can be passed in as parameter to every function that accept a deps parameter (given that entrait is used with unimock support everywhere). To enable mocking of entraited functions, they get reified and defined as a type called Fn inside a module with the same identifier as the function: entraited_function::Fn.

Unimock support is enabled by importing entrait from the path entrait::unimock::*.

use entrait::unimock::*;
use unimock::*;

fn foo<D>(_: &D) -> i32 {
fn bar<D>(_: &D) -> i32 {

fn my_func(deps: &(impl Foo + Bar)) -> i32 {
    deps.foo() + deps.bar()

let mocked_deps = mock([

assert_eq!(42, my_func(&mocked_deps));

Entrait with unimock supports unmocking. This means that the test environment can be partially mocked!

use entrait::unimock::*;
use unimock::*;
use std::any::Any;

fn say_hello(deps: &impl FetchPlanetName, planet_id: u32) -> Result<String, ()> {
    Ok(format!("Hello {}!", deps.fetch_planet_name(planet_id)?))

fn fetch_planet_name(deps: &impl FetchPlanet, planet_id: u32) -> Result<String, ()> {
    let planet = deps.fetch_planet(planet_id)?;

pub struct Planet {
    name: String

fn fetch_planet(deps: &impl Any, planet_id: u32) -> Result<Planet, ()> {
    unimplemented!("This doc test has no access to a database :(")

let hello_string = say_hello(
            .answers(|_| Ok(Planet {
                name: "World".to_string(),

assert_eq!("Hello World!", hello_string);


If you instead wish to use a more established mocking crate, there is also support for mockall.

Just import entrait from entrait::mockall:* to have those mock structs autogenerated:

use entrait::mockall::*;

fn foo<D>(_: &D) -> u32 {

fn my_func(deps: &impl Foo) -> u32 {

fn main() {
    let mut deps = MockFoo::new();
    deps.expect_foo().returning(|| 42);
    assert_eq!(42, my_func(&deps));

conditional mock implementations

Most often, you will only need to generate mock implementations in test code, and skip this for production code. For this configuration there are more alternative import paths:

  • use entrait::unimock_test::* puts unimock support inside #[cfg_attr(test, ...)].
  • use entrait::mockall_test::* puts mockall support inside #[cfg_attr(test, ...)].


This section lists known limitations of entrait:

Cyclic dependency graphs

Cyclic dependency graphs are impossible with entrait. In fact, this is not a limit of entrait itself, but with Rust's trait solver. It is not able to prove that a type implements a trait if it needs to prove that it does in order to prove it.

While this is a limitation, it is not necessarily a bad one. One might say that a layered application architecture should never contain cycles. If you do need recursive algorithms, you could model this as utility functions outside of the entraited APIs of the application.

Crate compatibility

As entrait is just a macro, it does not pull in any dependencies besides the code needed to execute the macro. But in order to compile the generated code, some additional dependencies will be needed alongside entrait. The following table shows compatible major versions:

entrait implementation unimock (optional) mockall (optional)
0.3 0.1 0.2 0.11
0.2 - 0.1 0.11
0.1 - - 0.11

License: MIT


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