Auto-Reloading Development Server

During development it can be very handy to have cargo automatically recompile the code on changes. This can be accomplished very easily by using watchexec.

The following command runs/restarts cargo run every time a file with the rs extension changes inside the current directory (including subdirectories):

watchexec -e rs -r cargo run

If you want to watch all files in a specific directory, you can use the the -w argument:

watchexec -w src -r cargo run

Historical Note

An old version of this page recommended using a combination of systemfd and listenfd, but this has many gotchas and was difficult to integrate properly, especially when part of a broader development workflow. We consider watchexec to be sufficient for auto-reloading purposes.

If you still want to use systemfd/listenfd, keep these common gotchas in mind:

• It requires code changes to accept an externally provided listener, so you need extra setup (and often an extra dependency) in your server code instead of a simple bind.
• When a watcher process sits between systemfd and your server (for example, cargo watch or watchexec), you must disable the LISTEN_PID check (for example, systemfd --no-pid). Otherwise listenfd ignores the passed socket and your app falls back to binding a port that is already in use.
• systemd socket activation itself is Linux-only. systemfd emulates socket passing on macOS/Linux and uses a custom protocol on Windows, so behavior differs across platforms.
• Because systemfd keeps the listening socket open while a rebuild happens, the port can look "open" even when your app is not running yet; requests may hang during slow or failed rebuilds.