Container ownership transfer

Meaning

A Vec 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

push, insert and set are sometimes mistaken for copy operations. They copy scalar/enum/array elements, but transfer strings, structs, vectors, and owned optional values. replace, the reclaiming replacement, additionally reclaims the displaced boxed struct and is therefore accepted only while the Vec remains an unobserved local owner.

Invalid code

struct Item { value: Int }
fn main() -> Int {
    let items: Vec<Item>
    let item = Item { value: 1 }
    items.push(item)
    items.push(item)
    return 0
}

Correct code

struct Item { value: Int }
fn hold(inout items: Vec<Item>, sink item: Item) { items.push(item) }
fn main() -> Int {
    let mut items: Vec<Item>
    hold(items, Item { value: 1 })
    return items.length - 1
}

Common fixes

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

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 Vec elements because destruction behavior remains experimental at that edge.