Container ownership transfer

Meaning

An owned list takes ownership of a move-only element. Inserting the same owner again is a use after move; inserting an ordinary borrowed parameter attempts to move something the function does not own.

The container needs an owner for every move-only element it stores. A borrowed view cannot be silently promoted to owned data, and one owner cannot populate two positions.

Why it happens

list_push, list_set, and list_set_exclusive are sometimes mistaken for copy operations. They copy scalar/enum/array elements, but transfer strings, structs, lists, and owned optional values. list_set_exclusive additionally reclaims the displaced boxed struct and is therefore accepted only while the List remains an unobserved local owner.

Invalid code

struct Item { value: Int }
fn main() -> Int {
    let items: List<Item> = list_new()
    let item = Item { value: 1 }
    list_push(items, item)
    list_push(items, item)
    return 0
}

Correct code

struct Item { value: Int }
fn hold(items: List<Item>, sink item: Item) { list_push(items, item) }
fn main() -> Int {
    let items: List<Item> = list_new()
    hold(items, Item { value: 1 })
    return list_len(items)
}

Common fixes

Create a separate element for each insertion, or mark a helper parameter sink when it transfers ownership into the list.

If the helper should preserve the caller's value, redesign it to inspect rather than store, or construct a distinct element from copyable fields. Use --verify for lifecycle-sensitive replacement of owned list elements because destruction behavior remains experimental at that edge.