Run multiple targets using Rust cargo

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If you have used package.json from a NodeJS app, you know you can add as many command as you want to support multiple targets (or builds). When I first come to Rust, I have been stuck with rustc for a while, because I find easy to run any program I want to:

> rustc hello.rs
> ./hello
Hello world!

If I have another file, such as main.rs in the folder:

> tree
├── hello.rs
├── main.rs

I can simply switch to run that program:

> rustc main.rs 
> ./main

My style of coding is mostly working with lots of un-related pieces of snippets these days, when I experiment, learn, or blog stuff. Adding a repo or project for them can be deferred to very very late, if needed.

Single target

This process is easy with less overhead. However, it can become difficult in Rust when the program involves dependencies, especially external dependencies. This is where cargo comes to play, which can be driven by a Cargo.toml configuration file:

[package]
name = "rs"
version = "0.1.0"
edition = "2021"
[dependencies]
assert_cmd = "1"

Given that, we can run various cargo commands, such as run to execute the default main app right away:

> cargo run

This is quite convenient. But what happen to my hello app with another main function in it?

fn main() {
  println!("Hello world!")
}

More targets

I couldn’t figure out how to add another build line in the Cargo.toml, at least not yet. However, it seems there’s a secret folder bin under src we can specify different target.

> tree src
src
├── bin
│   ├── hello.rs
├── main.rs

If we move the hello.rs under the /src/bin folder, we can use the same cargo command to run it:

> cargo run --bin hello
Hello world!

The bin flag is for us to switch to another target instead of the default main.rs. Moreover this flag can be used for other cargo command, such as building for production run:

> cargo build --release --bin hello

You can even run integration test against it if you put a file under tests folder:

use assert_cmd::Command;
#[test]
fn runs() {
  let mut cmd = Command::cargo_bin("hello").unwrap();
  cmd.assert().success();
}

Now cargo test should run this new test against the new target hello .

Conclusion

Rust cargo supports a flag bin for us to specify a new target instead of the default main .