动态 – Rust中的所有权跟踪：Box(堆)和T(堆栈)之间的差异

尝试使用编程语言Rust,我发现编译器能够非常准确地跟踪堆栈中某些结构的字段的移动(它确切地知道哪个字段已移动).
但是,当我将结构的一部分放入Box(即将其放入堆中)时,编译器不再能够确定在取消引用该框之后发生的所有事件的字段级移动.它将假设“盒子内部”的整个结构已经移动.让我们首先看一下一切都在堆栈上的例子：

struct OuterContainer {
    inner: InnerContainer
}

struct InnerContainer {
    val_a: ValContainer,val_b: ValContainer
}

struct ValContainer {
    i: i32
}


fn main() {
    // Note that the whole structure lives on the stack.
    let structure = OuterContainer {
        inner: InnerContainer {
            val_a: ValContainer { i: 42 },val_b: ValContainer { i: 100 }
        }
    };

    // Move just one field (val_a) of the inner container.
    let move_me = structure.inner.val_a;

    // We can still borrow the other field (val_b).
    let borrow_me = &structure.inner.val_b;
}

现在是相同的例子但只有一个小改动：我们将InnerContainer放入一个盒子(Box< InnerContainer>).

struct OuterContainer {
    inner: Box<InnerContainer>
}

struct InnerContainer {
    val_a: ValContainer,val_b: ValContainer
}

struct ValContainer {
    i: i32
}


fn main() {
    // Note that the whole structure lives on the stack.
    let structure = OuterContainer {
        inner: Box::new(InnerContainer {
            val_a: ValContainer { i: 42 },val_b: ValContainer { i: 100 }
        })
    };

    // Move just one field (val_a) of the inner container.
    // Note that now,the inner container lives on the heap.
    let move_me = structure.inner.val_a;

    // We can no longer borrow the other field (val_b).
    let borrow_me = &structure.inner.val_b; // error: "value used after move"
}

我怀疑它与堆栈的性质和堆的性质有关,前者是静态的(至少每个堆栈帧),而后者是动态的.也许编译器需要安全地使用它,因为我无法清楚地表达/识别.