bux-lang/examples/move_field_remaining.bux

// Session 70 — per-field Drop after partial move
// Moving one droppable field out of a @[Drop] parent skips Type_Drop of the
// parent but still drops the *remaining* droppable fields.
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 {
    n: int,
    counter: *int
}

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

@[Drop]
struct PairBag {
    left: Array<int>,
    right: Tracked
}

func PairBag_Drop(self: *PairBag) {
    Array_Drop<int>(&self.left);
    Tracked_Drop(&self.right);
}

// Move left out via return — right must still Drop (Tracked_Drop)
func TakeLeft(counter: *int) -> Array<int> {
    var left: Array<int> = Array_New<int>(2);
    Array_Push<int>(&left, 10);
    Array_Push<int>(&left, 20);
    let pair: PairBag = PairBag {
        left: left,
        right: Tracked { n: 2, counter: counter }
    };
    return pair.left;
}

// Move left via let; right still drops at scope end
func PeekAfterTake(counter: *int) -> int {
    var left: Array<int> = Array_New<int>(1);
    Array_Push<int>(&left, 7);
    let pair: PairBag = PairBag {
        left: left,
        right: Tracked { n: 2, counter: counter }
    };
    let moved: Array<int> = pair.left;
    return Array_Len<int>(&moved) as int;
}

// Whole-struct return: no PairBag_Drop here (caller owns it)
func MakePair() -> PairBag {
    var left: Array<int> = Array_New<int>(1);
    Array_Push<int>(&left, 1);
    let pair: PairBag = PairBag {
        left: left,
        right: Tracked { n: 9, counter: null as *int }
    };
    return pair;
}

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

    let taken: Array<int> = TakeLeft(&drops);
    Test_AssertTrue(Array_Len<int>(&taken) == 2);
    Test_AssertTrue(Array_Get<int>(&taken, 0) == 10);
    // TakeLeft must have Dropped remaining right field once
    Test_AssertTrue(drops == 1);

    let n: int = PeekAfterTake(&drops);
    Test_AssertTrue(n == 1);
    Test_AssertTrue(drops == 2);

    let p: PairBag = MakePair();
    Test_AssertTrue(Array_Len<int>(&p.left) == 1);
    Test_AssertTrue(p.right.n == 9);

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