Accept units (k, m, g) for --chunk-size.
https://man.freebsd.org/cgi/man.cgi?sektion=0&arch=default&format=html&query=expand_number
Add option -k/--keep-going to continue in case of IO errors such as permission denied.
Add option -e/--errexit to immediately exit >0 on the first denied difference.
Add option to scan ahead and show progress on standard error.
Hash and check symlinks targets?
https://doc.rust-lang.org/std/fs/fn.read_link.html
https://unix.stackexchange.com/questions/63876/changing-the-timestamp-of-a-symlink
Estimate default number of worker threads based on disk read speed and CPU speed.
Test effect of stdout buffering and/or locking on performance with high verbosity.
https://rust-cli.github.io/book/tutorial/output.html#a-note-on-printing-performance
Generate a man page.
https://rust-cli.github.io/book/in-depth/docs.html
https://lib.rs/crates/clap_mangen
Use common exit codes.
https://man.openbsd.org/sysexits
https://github.com/dtolnay/anyhow/issues/247