Watch Files
While you are developing, having a quick feedback loop can be invaluable. What this means is that the time between you writing some code, and getting feedback if it is syntactically incorrect or if it breaks unit tests should be as short as possible.
Often times, the development environment you use can give you fast feedback on syntaxtical issues. Some even let you define shortcuts for quickly running unit tests or other actions.
Another approach is to use a tool that watches your code for changes, and runs some command whenever you make a change. There are some situations where this is useful:
- You want to run some custom tests on the code
- You want to rebuild and relaunch your application, so that you can test it interactively.
If you build Web Frontends in Rust and use the Trunk tool to build them, you will get this for free. When you run Trunk in serve mode, it will automatically rebuild your frontend and reload your browser whenever it detects a change, to minimze your development feedback loop.
Cargo Watch
Cargo Watch is a generic tool you can use to watch your Rust projects and execute commands whenever a file changes.
You can install it using Cargo:
cargo install cargo-watch
By default, it will run cargo check when a change is detected:
# run `cargo check` whenever files change
cargo watch
You can customize it to run any command you like. Using the -x flag, you can tell
it to run any other Cargo subcommand. You can also directly give it a command to run.
cargo watch -x test
cargo watch -- just test
It also supports command chaining, where you specify multiple Cargo subcommands to run. When doing so, it will run each of them in the order you specify them, when they are successful.
cargo watch -x check -x test -x run
The repository and help text explain more commands that you can use, such as specifying which files to watch.
Reading
Chapter 1: Setup - Toolchains, IDEs, CI by Luca Palmieri
In this chapter of his book, Luca explains how to setup a real-life Rust
project. He explains that using cargo watch can reduce the perceived
compilation time, because it triggers immediately after you change and
save a file.
Cargo Issue #9339: Add Cargo watch
In this issue on the Cargo repository, there is some discussion going on to incorporate file watching functionality natively into Cargo.