why are external crates used?

tldr: You do not need to write extern crate anymore for external dependencies in Rust 2018. The code you provided without extern crate works just fine with edition = "2018" set in your Cargo.toml

No more extern crate

You no longer need to write extern crate to import a crate into your project. Before:

// Rust 2015

extern crate futures;

mod foo {
    use futures::Future;
}

After:

// Rust 2018

mod foo {
    use futures::Future;
}

Macros

One other use for extern crate was to import macros; that’s no longer needed. In Rust 2015, you would have written:

// Rust 2015

#[macro_use]
extern crate log;

fn main() {
    error!("oops");
}

Now, you write:

// Rust 2018

use log::error;

fn main() {
    error!("oops");
}

Renaming crates

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

extern crate futures as fut;

Then in Rust 2018, you simply do this:

use futures as fut;

use fut::Future;

Sysroot Crates

There’s one exception to this rule, and that’s the “sysroot” crates. These are the crates distributed with Rust itself. For now, you still need to use extern crate for these crates:

• proc_macro
• core
• std

However, extern crate std and extern crate core are already implicit, so it is very rare that you will need to declare them manually.

Finally, on nightly, you’ll need it for crates like:

• alloc
• test

Those are the only exceptions to the rule. Therefore, the code you provided without extern crate works just fine in Rust 2018:

#[global_allocator]
static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;

Setting your Rust Edition

Just because you have the latest Rust version installed does not mean that you are compiling with the latest edition. To tell Cargo to use a specific edition, set the edition key/value pair. For example:

[package]
name = "foo"
edition = "2018"

If there’s no edition key, Cargo will default to Rust 2015. But in this case, we’ve chosen 2018, and so our code is compiling with Rust 2018! Thanks to @KevinReid for pointing this out

This answer was derived from The Rust Edition Guide