Path clarity

Minimum Rust version: beta Minimum Rust version: nightly for "uniform paths"

The module system is often one of the hardest things for people new to Rust. Everyone has their own things that take time to master, of course, but there's a root cause for why it's so confusing to many: while there are simple and consistent rules defining the module system, their consequences can feel inconsistent, counterintuitive and mysterious.

As such, the 2018 edition of Rust introduces a few new module system features, but they end up simplifying the module system, to make it more clear as to what is going on.

Here's a brief summary:

  • extern crate is no longer needed
  • The crate keyword refers to the current crate.
  • Absolute paths begin with a crate name, where the keyword crate refers to the current crate.
  • A foo.rs and foo/ subdirectory may coexist; mod.rs is no longer needed when placing submodules in a subdirectory.

These may seem like arbitrary new rules when put this way, but the mental model is now significantly simplified overall. Read on for more details!

Additionally, in nightly, there's an additional possible tweak to paths called "Uniform paths". This is backwards compatible with the new path changes. Uniform paths have a dedicated section at the end of this guide.

More details

Let's talk about each new feature in turn.

No more extern crate

This one is quite straightforward: you no longer need to write extern crate to import a crate into your project. Before:

// Rust 2015

extern crate futures;

mod submodule {
    use futures::Future;
}

After:

// Rust 2018

mod submodule {
    use futures::Future;
}

Now, to add a new crate to your project, you can add it to your Cargo.toml, and then there is no step two. If you're not using Cargo, you already had to pass --extern flags to give rustc the location of external crates, so you'd just keep doing what you were doing there as well.

One small note here: cargo fix will not currently automate this change. We may have it do this for you in the future.

One other use for extern crate was to import macros; that's no longer needed. Check the macro section for more.

If you've been using as to rename your crate like this:

extern crate futures as f;

use f::Future;

then removing the extern crate line on its own won't work. You'll need to do this:

use futures as f;

use f::Future;

This change will need to happen in any module that uses f.

The crate keyword refers to the current crate.

In use declarations and in other code, you can refer to the root of the current crate with the crate:: prefix. For instance, crate::foo::bar will always refer to the name bar inside the module foo, from anywhere else in the same crate.

The prefix :: previously referred to either the crate root or an external crate; it now unambiguously refers to an external crate. For instance, ::foo::bar always refers to the name bar inside the external crate foo.

Changes to paths

In Rust 2018, paths in use declarations must begin with a crate name, crate, self, or super.

Code that looked like this:

// Rust 2015

extern crate futures;

use futures::Future;

mod foo {
    pub struct Bar;
}

use foo::Bar;

Now looks like this:

// Rust 2018

// 'futures' is the name of a crate
use futures::Future;

mod foo {
    pub struct Bar;
}

// 'crate' means the current crate
use crate::foo::Bar;

In addition, all of these path forms are available outside of use declarations as well, which eliminates many sources of confusion. Consider this code in Rust 2015:

// Rust 2015

extern crate futures;

mod submodule {
    // this works!
    use futures::Future;

    // so why doesn't this work?
    fn my_poll() -> futures::Poll { ... }
}

fn main() {
    // this works
    let five = std::sync::Arc::new(5);
}

mod submodule {
    fn function() {
        // ... so why doesn't this work
        let five = std::sync::Arc::new(5);
    }
}

In real code, you couldn't repeat mod submodule, and function would be defined in the first mod block.

In the futures example, the my_poll function signature is incorrect, because submodule contains no items named futures; that is, this path is considered relative. use futures:: works even though a lone futures:: doesn't! With std it can be even more confusing, as you never wrote the extern crate std; line at all. So why does it work in main but not in a submodule? Same thing: it's a relative path because it's not in a use declaration. extern crate std; is inserted at the crate root, so it's fine in main, but it doesn't exist in the submodule at all.

Let's look at how this change affects things:

// Rust 2018

// no more `extern crate futures;`

mod submodule {
    // 'futures' is the name of a crate, so this works
    use futures::Future;

    // 'futures' is the name of a crate, so this works
    fn my_poll<T, E>() -> futures::Poll {
        unimplemented!()
    }

    fn function() {
        // 'std' is the name of a crate, so this works
        let five = std::sync::Arc::new(5);
    }
}

fn main() {
    // 'std' is the name of a crate, so this works
    let five = std::sync::Arc::new(5);
}

Much more straightforward.

No more mod.rs

In Rust 2015, if you have a submodule:

///  foo.rs 
///  or 
///  foo/mod.rs

mod foo;

It can live in foo.rs or foo/mod.rs. If it has submodules of its own, it must be foo/mod.rs. So a bar submodule of foo would live at foo/bar.rs.

In Rust 2018, mod.rs is no longer needed.

///  foo.rs 
///  foo/bar.rs

mod foo;

/// in foo.rs
mod bar;

foo.rs can just be foo.rs, and the submodule is still foo/bar.rs. This eliminates the special name, and if you have a bunch of files open in your editor, you can clearly see their names, instead of having a bunch of tabs named mod.rs.

Uniform paths

Uniform paths are a nightly-only feature.

The uniform paths variant of Rust 2018 simplifies and unifies path handling compared to Rust 2015. In Rust 2015, paths work differently in use declarations than they do elsewhere. In particular, paths in use declarations would always start from the crate root, while paths in other code implicitly started from the current module. Those differences didn't have any effect in the top-level module, which meant that everything would seem straightforward until working on a project large enough to have submodules.

In the uniform paths variant of Rust 2018, paths in use declarations and in other code always work the same way, both in the top-level module and in any submodule. You can always use a relative path from the current module, a path starting from an external crate name, or a path starting with crate, super, or self.

Code that looked like this:

// Rust 2015

extern crate futures;

use futures::Future;

mod foo {
    pub struct Bar;
}

use foo::Bar;

fn my_poll() -> futures::Poll { ... }

enum SomeEnum {
    V1(usize),
    V2(String),
}

fn func() {
    let five = std::sync::Arc::new(5);
    use SomeEnum::*;
    match ... {
        V1(i) => { ... }
        V2(s) => { ... }
    }
}

will look exactly the same in Rust 2018, except that you can delete the extern crate line:

// Rust 2018 (uniform paths variant)

use futures::Future;

mod foo {
    pub struct Bar;
}

use foo::Bar;

fn my_poll() -> futures::Poll { ... }

enum SomeEnum {
    V1(usize),
    V2(String),
}

fn func() {
    let five = std::sync::Arc::new(5);
    use SomeEnum::*;
    match ... {
        V1(i) => { ... }
        V2(s) => { ... }
    }
}

With Rust 2018, however, the same code will also work completely unmodified in a submodule:

// Rust 2018 (uniform paths variant)

mod submodule {
    use futures::Future;

    mod foo {
        pub struct Bar;
    }

    use foo::Bar;

    fn my_poll() -> futures::Poll { ... }

    enum SomeEnum {
        V1(usize),
        V2(String),
    }

    fn func() {
        let five = std::sync::Arc::new(5);
        use SomeEnum::*;
        match ... {
            V1(i) => { ... }
            V2(s) => { ... }
        }
    }
}

This makes it easy to move code around in a project, and avoids introducing additional complexity to multi-module projects.

If a path is ambiguous, such as if you have an external crate and a local module or item with the same name, you'll get an error, and you'll need to either rename one of the conflicting names or explicitly disambiguate the path. To explicitly disambiguate a path, use ::name for an external crate name, or self::name for a local module or item.