A simple utility library for transitioning enums into and out of occupied states

occupied provides compile-time guaranteed ways to interact with inserting and removing items into Option. This simplifies more complicated access patterns, when you're interacting with a handful of options and don't want to .take() anything out of them until they've all been verified somehow.

Suppose you had an array of options, and wanted to unwrap them all, but only if they're all Some, and leave them all untouched otherwise. If you own the array, this is easy to do with a match, but if not, you have to either:

1. Manually check .is_some() on all of them, and then .unwrap() them only after they've all been checked, or
2. .take() them one-by-one, and take care to restore each option to its original state after.

occupied provides a way to preserve the is_some() check within the type system, making it checked at compile-time, without actually mutating the option.

use occupied::OptionExt as _;

fn try_unwrap_all<T>(options: &mut [Option<T>; 4]) -> Option<[T; 4]> {
    // (this code will be simpler when `array::try_map` is available)
    let [opt1, opt2, opt3, opt4] = options;

    // Create an of `Occupied` instances, guaranteeing that the underlying
    // options are `Some`
    let confirmed = [
        opt1.peek_some()?,
        opt2.peek_some()?,
        opt3.peek_some()?,
        opt4.peek_some()?,
    ];

    // `.take()` all of the values
    Some(confirmed.map(|item| item.take()))
}

let mut opts = [Some(1), Some(2), Some(3), None];

assert_eq!(try_unwrap_all(&mut opts), None);
assert_eq!(opts, [Some(1), Some(2), Some(3), None]);

opts[3] = Some(4);


assert_eq!(try_unwrap_all(&mut opts), Some([1, 2, 3, 4]));
assert_eq!(opts, [None, None, None, None]);