bux-lang/examples/move_field_partial.bux

// Session 68 — partial field moves out of @[Drop] parents
// `return bag.items` / `let x = bag.items` must skip Bag_Drop (no double-free).
// Non-droppable fields (`bag.tag`) do not mark the parent moved.
import Std::Io::{PrintLine};
import Std::Array::{Array, Array_New, Array_Push, Array_Len, Array_Get};
import Std::String::{String_FromInt, String_Concat};
import Std::Test::{Test_AssertTrue, Test_Pass};

@[Drop]
struct Bag {
    items: Array<int>,
    tag: int
}

func Bag_Drop(self: *Bag) {
    Array_Drop<int>(&self.items);
}

// Move droppable field out via return
func TakeItems() -> Array<int> {
    var items: Array<int> = Array_New<int>(4);
    Array_Push<int>(&items, 42);
    let bag: Bag = Bag { items: items, tag: 7 };
    return bag.items;
}

// Move droppable field via let; read non-droppable tag after
func PeekTagAndTake() -> int {
    var items: Array<int> = Array_New<int>(2);
    Array_Push<int>(&items, 1);
    let bag: Bag = Bag { items: items, tag: 99 };
    let moved: Array<int> = bag.items;
    let t: int = bag.tag;
    discard Array_Len<int>(&moved);
    return t;
}

// Whole-struct return still moves bag (existing path)
func MakeBag() -> Bag {
    var items: Array<int> = Array_New<int>(2);
    Array_Push<int>(&items, 10);
    Array_Push<int>(&items, 20);
    let bag: Bag = Bag { items: items, tag: 3 };
    return bag;
}

func Main() -> int {
    let taken: Array<int> = TakeItems();
    Test_AssertTrue(Array_Len<int>(&taken) == 1);
    Test_AssertTrue(Array_Get<int>(&taken, 0) == 42);

    let tag: int = PeekTagAndTake();
    Test_AssertTrue(tag == 99);

    let b: Bag = MakeBag();
    Test_AssertTrue(Array_Len<int>(&b.items) == 2);
    Test_AssertTrue(Array_Get<int>(&b.items, 0) == 10);
    Test_AssertTrue(b.tag == 3);

    PrintLine(String_Concat("ok=", String_FromInt(tag as int64)));
    Test_Pass("move_field_partial");
    return 0;
}