printf 'abcdefghij' > c1; printf 'abcdeXghiY' > c2

$ ./target/release/diffutils cmp
Usage: cmp <from> <to>
# Exit code: 2
$ ./target/release/diffutils cmp --bogus c1 c2
unrecognized option '--bogus'
# Exit code: 2
$ ./target/release/diffutils cmp c1 c2 3 4 extra
Usage: cmp <from> <to>
# Exit code: 2

$ LC_ALL=C cmp
cmp: missing operand after 'cmp'
cmp: Try 'cmp --help' for more information.
# Exit code: 2
$ LC_ALL=C cmp --bogus c1 c2
cmp: unrecognized option '--bogus'
cmp: Try 'cmp --help' for more information.
# Exit code: 2
$ LC_ALL=C cmp c1 c2 3 4 extra
cmp: extra operand 'extra'
cmp: Try 'cmp --help' for more information.
# Exit code: 2

fn usage_string(executable: &str) -> String {
    format!("Usage: {executable} <from> <to>")
}
...
return Err(format!("unrecognized option '{}'", param.to_string_lossy()));

#[test]
fn cmp_usage_errors_match_gnu() -> Result<(), Box<dyn std::error::Error>> {
    let mut cmd = cargo_bin_cmd!("diffutils");
    cmd.arg("cmp");
    cmd.assert().code(predicate::eq(2)).stderr(predicate::eq(
        "cmp: missing operand after 'cmp'\ncmp: Try 'cmp --help' for more information.\n",
    ));

    let mut cmd = cargo_bin_cmd!("diffutils");
    cmd.arg("cmp").arg("--bogus").arg("a").arg("b");
    cmd.assert().code(predicate::eq(2)).stderr(predicate::eq(
        "cmp: unrecognized option '--bogus'\ncmp: Try 'cmp --help' for more information.\n",
    ));
    Ok(())
}