Package with both a library and a binary?

I would like to make a Rust package that contains both a reusable library (where most of the program is implemented), and also an executable that uses it.

Assuming I have not confused any semantics in the Rust module system, what should my Cargo.toml file look like?

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();
}

Simple

Create a src/main.rs that will be used as the defacto executable. You do not need to modify your Cargo.toml and this file will be compiled to a binary of the same name as the library.

The project contents:

% tree
.
├── Cargo.toml
└── src
    ├── lib.rs
    └── main.rs

Cargo.toml

[package]
name = "example"
version = "0.1.0"
edition = "2018"

src/lib.rs

use std::error::Error;

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

src/main.rs

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

And execute it:

% cargo run -q
I'm using the library: Ok(3)

Flexible

If you wish to control the name of the binary or have multiple binaries, you can create multiple binary source files in src/bin and the rest of your library 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 in src/bin will be compiled to a binary of the same name.

The project contents:

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

Cargo.toml

[package]
name = "example"
version = "0.1.0"
edition = "2018"

src/lib.rs

use std::error::Error;

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

src/bin/mybin.rs

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

And execute it:

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

See also:

• https://stackoverflow.com/q/51785457/155423

An alternate solution is to not 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.

Here, I create a binary project that includes a library inside of it, but there are many possible ways of organizing the code:

 % tree the-binary
the-binary
├── Cargo.toml
├── src
│   └── main.rs
└── the-library
    ├── Cargo.toml
    └── src
        └── lib.rs

Cargo.toml

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

[package]
name = "the-binary"
version = "0.1.0"
edition = "2018"

[workspace]

[dependencies]
the-library = { path = "the-library" }

src/main.rs

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

the-library/Cargo.toml

[package]
name = "the-library"
version = "0.1.0"
edition = "2018"

the-library/src/lib.rs

use std::error::Error;

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

And execute it:

% cargo run -q
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 the-library and the-binary directory, the library will not be built both times — it's shared between both projects.

You can put lib.rs and main.rs to sources folder together. There is no conflict and cargo will build both things.

To resolve documentaion conflict add to your Cargo.toml:

[[bin]]
name = "main"
doc = false