Path and module system changes

Summary

Paths in use declarations now work the same as other paths.
Paths starting with :: must now be followed with an external crate.
Paths in pub(in path) visibility modifiers must now start with crate, self, or super.

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 in 99% of circumstances.
The crate keyword refers to the current crate.
Paths may start with a crate name, even within submodules.
Paths starting with :: must reference an external crate.
A foo.rs and foo/ subdirectory may coexist; mod.rs is no longer needed when placing submodules in a subdirectory.
Paths in use declarations work the same as other paths.

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!

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.

An exception

There's one exception to this rule, and that's the "sysroot" crates. These are the crates distributed with Rust itself.

Usually these are only needed in very specialized situations. Starting in 1.41, rustc accepts the --extern=CRATE_NAME flag which automatically adds the given crate name in a way similar to extern crate. Build tools may use this to inject sysroot crates into the crate's prelude. Cargo does not have a general way to express this, though it uses it for proc_macro crates.

Some examples of needing to explicitly import sysroot crates are:

std: Usually this is not necessary, because std is automatically imported unless the crate is marked with #![no_std].
core: Usually this is not necessary, because core is automatically imported, unless the crate is marked with #![no_core]. For example, some of the internal crates used by the standard library itself need this.
proc_macro: This is automatically imported by Cargo if it is a proc-macro crate starting in 1.42. extern crate proc_macro; would be needed if you want to support older releases, or if using another build tool that does not pass the appropriate --extern flags to rustc.
alloc: Items in the alloc crate are usually accessed via re-exports in the std crate. If you are working with a no_std crate that supports allocation, then you may need to explicitly import alloc.
test: This is only available on the nightly channel, and is usually only used for the unstable benchmark support.

Macros

One other use for extern crate was to import macros; that's no longer needed. Macros may be imported with use like any other item. For example, the following use of extern crate:

#[macro_use]
extern crate bar;

fn main() {
    baz!();
}

Can be changed to something like the following:

use bar::baz;

fn main() {
    baz!();
}

Renaming crates

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 self::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.

Extern crate paths

Previously, using an external crate in a module without a use import required a leading :: on the path.

// Rust 2015

extern crate chrono;

fn foo() {
    // this works in the crate root
    let x = chrono::Utc::now();
}

mod submodule {
    fn function() {
        // but in a submodule it requires a leading :: if not imported with `use`
        let x = ::chrono::Utc::now();
    }
}

Now, extern crate names are in scope in the entire crate, including submodules.

// Rust 2018

fn foo() {
    // this works in the crate root
    let x = chrono::Utc::now();
}

mod submodule {
    fn function() {
        // crates may be referenced directly, even in submodules
        let x = chrono::Utc::now();
    }
}

If you have a local module or item with the same name as an external crate, a path beginning with that name will be taken to refer to the local module or item. To explicitly refer to the external crate, use the ::name form.

No more `mod.rs`

In Rust 2015, if you have a submodule:

// This `mod` declaration looks for the `foo` module in
// `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 the restriction that a module with submodules must be named mod.rs is lifted. 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.

Rust 2015	Rust 2018
. ├── lib.rs └── foo/ ├── mod.rs └── bar.rs	. ├── lib.rs ├── foo.rs └── foo/ └── bar.rs

`use` paths

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 scope. 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 Rust 2018, paths in use declarations and in other code work the same way, both in the top-level module and in any submodule. You can use a relative path from the current scope, 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

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) => { ... }
    }
}

The same code will also work completely unmodified in a submodule:

// Rust 2018

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.

The Rust Edition Guide