为什么 "capture by reference" 等同于 Rust 中的 "capture a reference by value"？

Question

摘自 Huon Wilson 的Finding Closure in Rust：

Capturing entirely by value is also strictly more general than capturing by reference: the reference types are first-class in Rust, so "capture by reference" is the same as "capture a reference by value". Thus, unlike C++, there’s little fundamental distinction between capture by reference and by value, and the analysis Rust does is not actually necessary: it just makes programmers’ lives easier.

我很难理解这个问题。如果您“按值”捕获引用，那么您是否捕获存储在堆上的数据？还是指向引用的指针值，在栈中找到？

Answer 1

我觉得文章的意思是说效果差不多。 Immutable references implement Copy trait 因此，当您按值捕获引用时，它会被复制。所以基本上你只是创建了一个对相同数据的共享引用。

fn as_ref(_x: &String) {}

fn as_mut(_x: &mut String) {}

fn main() {
    let mut x = String::from("hello world");
    let y = &x;

    let _ = move || {
        as_ref(y); // here you moved y but it
                   // basically created a copy
    };

    let z = y; // can be used later

    // The same cannot be done by mutable
    // refs because they don't
    // implement Copy trait
    
    let y = &mut x;

    let _ = move || {
        as_mut(y); // here you moved y and
                   // it cannot be used outside
    };

    // ERROR! Cannot be used again
    //let z = y;
}

Playground

Answer 2

Or does it refer to the pointer value of the reference, which is found on the stack?

是的，是的。

在 Rust 中，引用是具体化的，它们是您实际操作的东西。因此，当您按值捕获引用时，您捕获的是引用本身（指针，这是 Rust 引用的真正含义），而不是裁判（指针）。

通过引用捕获基本上只是隐式创建一个引用并按值捕获它。

为什么 "capture by reference" 等同于 Rust 中的 "capture a reference by value"？

Why is "capture by reference" equivalent to "capture a reference by value" in Rust?

memory

closures

pass-by-reference

pass-by-value

rust