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 withcrate
,self
, orsuper
.
Motivation
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
andfoo/
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 necessary, becausestd
is automatically imported unless the crate is marked with#![no_std]
.core
: Usually this is not necessary, becausecore
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 torustc
.alloc
: Items in thealloc
crate are usually accessed via re-exports in thestd
crate. If you are working with ano_std
crate that supports allocation, then you may need to explicitly importalloc
.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 begining 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.