Optional syntax for multiple arguments as well as for one optional argument

[Rémi]

Bool then function suits pretty good in terms of optional arguments, but current cmd! do not support Option with multiple arguments.

Having optional syntax available for multiple arguments as well as for one argument gives cleaner code. So instead of writing every time if condition { ... } else { ... } for several arguments we can use Option either for one argument and many arguments.
With that feature this code from documentation

let check = if true { &["--", "--check"] } else { &[][..] };
assert_eq!(
    cmd!("cargo fmt {check...}").to_string(),
    "cargo fmt -- --check"
);

let dry_run = if true { Some("--dry-run") } else { None };
assert_eq!(
    cmd!("git push {dry_run...}").to_string(),
    "git push --dry-run"
);

Can be rewritten as

let condition = true;
let check = condition.then(|| &["--", "--check"]);
assert_eq!(
    cmd!("cargo fmt {check...}").to_string(),
    "cargo fmt -- --check"
);

let dry_run = condition.then(|| "--dry-run");
assert_eq!(
    cmd!("git push {dry_run...}").to_string(),
    "git push --dry-run"
);

[Alex Kladov]

The second case, condition.then(|| "--dry-run") already works. The first case requires condition.then(|| &["--", "--check"]).into_iter().flatten().