bux-lang/examples/move_field_nested.bux

// Session 73 — nested field path moves (`outer.inner.items`)
// Moving a deep droppable field skips root Drop and still drops remaining
// fields at every level of the path.
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 Tracked {
    id: int,
    counter: *int
}

func Tracked_Drop(self: *Tracked) {
    if self.counter != null as *int {
        *self.counter = *self.counter + 1;
    }
}

@[Drop]
struct Inner {
    items: Array<int>,
    note: Tracked
}

func Inner_Drop(self: *Inner) {
    Array_Drop<int>(&self.items);
    Tracked_Drop(&self.note);
}

@[Drop]
struct Outer {
    inner: Inner,
    tag: Tracked
}

func Outer_Drop(self: *Outer) {
    Inner_Drop(&self.inner);
    Tracked_Drop(&self.tag);
}

// Move outer.inner.items — must Drop: outer.inner.note + outer.tag
// (not Outer_Drop whole, not Array double-free)
func TakeNestedItems(counter: *int) -> Array<int> {
    var items: Array<int> = Array_New<int>(2);
    Array_Push<int>(&items, 10);
    Array_Push<int>(&items, 20);
    let outer: Outer = Outer {
        inner: Inner {
            items: items,
            note: Tracked { id: 1, counter: counter }
        },
        tag: Tracked { id: 2, counter: counter }
    };
    return outer.inner.items;
}

// let-move nested path
func PeekAfterNestedTake(counter: *int) -> int {
    var items: Array<int> = Array_New<int>(1);
    Array_Push<int>(&items, 7);
    let outer: Outer = Outer {
        inner: Inner {
            items: items,
            note: Tracked { id: 3, counter: counter }
        },
        tag: Tracked { id: 4, counter: counter }
    };
    let moved: Array<int> = outer.inner.items;
    return Array_Len<int>(&moved) as int;
}

func Main() -> int {
    var drops: int = 0;

    let taken: Array<int> = TakeNestedItems(&drops);
    Test_AssertTrue(Array_Len<int>(&taken) == 2);
    Test_AssertTrue(Array_Get<int>(&taken, 0) == 10);
    // TakeNestedItems: note(id1) + tag(id2) → 2 Tracked drops
    Test_AssertTrue(drops == 2);

    let n: int = PeekAfterNestedTake(&drops);
    Test_AssertTrue(n == 1);
    // +2 more Tracked drops
    Test_AssertTrue(drops == 4);

    PrintLine(String_Concat("nested_drops=", String_FromInt(drops as int64)));
    Test_Pass("move_field_nested");
    return 0;
}