Function watch_file 

pub fn watch_file(
    path: &Path,
    on_change: impl Fn(PathBuf) + Send + 'static,
) -> Result<FileWatch>

Report changes to path by calling on_change from the watcher’s thread.

Watches the parent directory, not the file. Editors and formatters write by rename-over, which destroys the inode a file watch is pinned to and leaves that watch pointed at nothing — the file keeps changing and the watcher keeps saying nothing. Watching the directory and filtering by name survives it, and also sees the file being deleted and recreated.

on_change is handed the path as it was given here, not the path the OS reported, so a caller can compare it against what it has open without worrying about how each platform spells it.