Rust package with both a library and a binary?

Tok:tmp doug$ du -a

8   ./Cargo.toml
8   ./src/bin.rs
8   ./src/lib.rs
16  ./src

Cargo.toml:

[package]
name = "mything"
version = "0.0.1"
authors = ["me <[email protected]>"]

[lib]
name = "mylib"
path = "src/lib.rs"

[[bin]]
name = "mybin"
path = "src/bin.rs"

src/lib.rs:

pub fn test() {
    println!("Test");
}

src/bin.rs:

extern crate mylib; // not needed since Rust edition 2018

use mylib::test;

pub fn main() {
    test();
}

You can also just put binary sources in src/bin and the rest of your sources in src. You can see an example in my project. You do not need to modify your Cargo.toml at all, and each source file will be compiled to a binary of the same name.

The other answer’s configuration is then replaced by:

$ tree
.
├── Cargo.toml
└── src
    ├── bin
    │   └── mybin.rs
    └── lib.rs

Cargo.toml

[package]
name = "example"
version = "0.0.1"
authors = ["An Devloper <[email protected]>"]

src/lib.rs

use std::error::Error;

pub fn really_complicated_code(a: u8, b: u8) -> Result<u8, Box<Error>> {
    Ok(a + b)
}

src/bin/mybin.rs

extern crate example; // Optional in Rust 2018

fn main() {
    println!("I'm using the library: {:?}", example::really_complicated_code(1, 2));
}

And execute it:

$ cargo run --bin mybin
I'm using the library: Ok(3)

---

Additionally, you can just create a src/main.rs that will be used as the defacto executable. Unfortunately, this conflicts with the cargo doc command:

Cannot document a package where a library and a binary have the same name. Consider renaming one or marking the target as doc = false

An alternate solution is to not actually try to cram both things into one package. For slightly larger projects with a friendly executable, I've found it very nice to use a workspace

We create a binary project that includes a library inside of it:

the-binary
├── Cargo.lock
├── Cargo.toml
├── mylibrary
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
└── src
    └── main.rs

Cargo.toml

This uses the [workspace] key and depends on the library:

[package]
name = "the-binary"
version = "0.1.0"
authors = ["An Devloper <[email protected]>"]

[workspace]

[dependencies]
mylibrary = { path = "mylibrary" }

src/main.rs

extern crate mylibrary;

fn main() {
    println!("I'm using the library: {:?}", mylibrary::really_complicated_code(1, 2));
}

mylibrary/src/lib.rs

use std::error::Error;

pub fn really_complicated_code(a: u8, b: u8) -> Result<u8, Box<Error>> {
    Ok(a + b)
}

And execute it:

$ cargo run
   Compiling mylibrary v0.1.0 (file:///private/tmp/the-binary/mylibrary)
   Compiling the-binary v0.1.0 (file:///private/tmp/the-binary)
    Finished dev [unoptimized + debuginfo] target(s) in 0.73 secs
     Running `target/debug/the-binary`
I'm using the library: Ok(3)

---

There are two big benefits to this scheme:

1. The binary can now use dependencies that only apply to it. For example, you can include lots of crates to improve the user experience, such as command line parsers or terminal formatting. None of these will "infect" the library.

2. The workspace prevents redundant builds of each component. If we run cargo build in both the mylibrary and the-binary directory, the library will not be built both times — it's shared between both projects.