Attribute Macro double

#[double]

Import a mock type in test mode, or a real type otherwise.

In a regular build, this macro is a no-op. But when #[cfg(test)], it substitutes “MockXXX” for any imported “XXX”. This makes it easy to to use mock objects, especially structs, in your unit tests.

This macro uses the same naming convention as mockall, but doesn’t strictly require it. It can work with hand-built Mock objects, or any other crate using the same naming convention.

This is the most common way to use #[double]. In order to replace a type, it must come from a separate module. So to mock type Foo, place it into a submodule, along with its mock counterpart. Then simply import Foo with #[double] like this:

mod foo {
    pub(super) struct Foo {
        // ...
    }
    #[cfg(test)]
    pub(super) struct MockFoo {
        // ...
    }
}
#[double]
use foo::Foo;

That will expand to the following:

#[cfg(not(test))]
use foo::Foo;
#[cfg(test)]
use foo::MockFoo as Foo;

#[double] can handle deeply nested paths,

#[double]
use foo::bar::Baz;

grouped imports,

#[double]
use foo::bar::{
    Baz,
    Bean
};

type aliases,

#[double]
type Foo = bar::Baz;

and renamed imports, too. With renamed imports, it isn’t even necessary to declare a submodule.

#[double]
use Foo as Bar;

Finally, #[double] can also import entire mocked modules, not just structures. In this case the naming convention is different. It will replace “xxx” with “mock_xxx”. For example:

mod foo {
    pub mod inner {
        // ...
    }
    pub mod mock_inner {
        // ...
    }
}
#[double]
use foo::inner;

will expand to:

#[cfg(not(test))]
use foo::inner;
#[cfg(test)]
use foo::mock_inner as inner;