Struct Placeholder

pub struct Placeholder;

Placeholder type to be used when constructing a PoloniusResult::Owned.

Since there is no access to the original input_borrow yet (due to the very polonius limitation that makes this crate necessary), this Placeholder is used in its stead.

use ::polonius_the_crab::*;

type StringRef = ForLt!(&str);

let map: &mut ::std::collections::HashMap<i32, String> = // ...

match polonius::<_, _, StringRef>(map, |map| match map.get(&22) {
    | Some(ret) => PoloniusResult::Borrowing(ret),
    | None => PoloniusResult::Owned {
        value: 42,
        input_borrow: /* WHAT TO PUT HERE?? */ Placeholder, // 👈👈
    },
}) {
    // `polonius` magic
    | PoloniusResult::Borrowing(dependent_entry) => {
        // ...
        stuff(dependent_entry);
    },
    | PoloniusResult::Owned {
        value,
        input_borrow: map, // we got our `map` borrow back! 🙌
    } => {
        assert_eq!(value, 42);
        stuff(map);
    },
}

Note that despite intellectually interesting w.r.t properly understanding the API, providing that input_borrow: Placeholder does not provide any valuable information to the call, and is thus rather noisy.

Hence the PoloniusResult::Owned() constructor shorthand, so as to be able to write:

PoloniusResult::Owned(42)
// is just a shorthand for:
PoloniusResult::Owned {
    value: 42,
    input_borrow: /* WHAT TO PUT HERE?? */ Placeholder, // 👈👈
}