Macro rerun_if_changed

macro_rules! rerun_if_changed {
    ($($tokens:tt)*) => { ... };
}

The rerun_if_changed instruction tells Cargo to re-run the build script if the file at the given path has changed. Currently, Cargo only uses the filesystem last-modified “mtime” timestamp to determine if the file has changed. It compares against an internal cached timestamp of when the build script last ran.

If the path points to a directory, it will scan the entire directory for any modifications.

If the build script inherently does not need to re-run under any circumstance, then emitting cargo:rerun-if-changed=build.rs is a simple way to prevent it from being re-run (otherwise, the default if no rerun-if instructions are emitted is to scan the entire package directory for changes). Cargo automatically handles whether or not the script itself needs to be recompiled, and of course the script will be re-run after it has been recompiled. Otherwise, specifying build.rs is redundant and unnecessary.

For more information, see this cargo reference.