Path clarity - The Edition Guide

# Path clarity

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.

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

#### 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 neccesary, 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. Check the macro section for more.

#### 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();
        }
    }

### 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.

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.