有没有一种安全的方法可以从 Rust 中的可变引用中临时检索拥有的值？

Question

我正在使用两个独立的函数。

第一个接受结构的拥有实例，然后 returns 它。
第二个采用可变引用但需要使用第一个函数。

// This structure is not `Clone`.
struct MyStruct;

fn take_owned(s: MyStruct) -> MyStruct {
    // Do things
    s
}

fn take_mut(s: &mut MyStruct) {
    *s = take_owned(s /* problem */);
}

我想过一个解决方案，但不确定是否可行：

use std::ptr;

// Temporarily turns a mutable reference into an owned value.
fn mut_to_owned<F>(val: &mut MyStruct, f: F)
where
    F: FnOnce(MyStruct) -> MyStruct,
{
    // We're the only one able to access the data referenced by `val`.
    // This operation simply takes ownership of the value.
    let owned = unsafe { ptr::read(val) };

    // Do things to the owned value.
    let result = f(owned);

    // Give the ownership of the value back to its original owner.
    // From its point of view, nothing happened to the value because we have
    // an exclusive reference.
    unsafe { ptr::write(val, result) };
}

使用这个功能，我可以做到：

fn take_mut(s: &mut MyStruct) {
    mut_to_owned(s, take_owned);
}

这段代码合理吗？如果没有，是否有安全的方法？

Answer 1

有人已经在名为 take_mut.

的 crate 中实现了您正在寻找的东西

Function take_mut::take
pub fn take<T, F>(mut_ref: &mut T, closure: F) 
where
    F: FnOnce(T) -> T, 
Allows use of a value pointed to by &mut T as though it was owned, as long as a T is made available afterwards.

...

Will abort the program if the closure panics.

Function take_mut::take_or_recover
pub fn take_or_recover<T, F, R>(mut_ref: &mut T, recover: R, closure: F) 
where
    F: FnOnce(T) -> T,
    R: FnOnce() -> T, 
Allows use of a value pointed to by &mut T as though it was owned, as long as a T is made available afterwards.

...

Will replace &mut T with recover if the closure panics, then continues the panic.

实施基本上就是您所拥有的，加上所描述的恐慌处理。

有没有一种安全的方法可以从 Rust 中的可变引用中临时检索拥有的值？

Is there a safe way to temporarily retrieve an owned value from a mutable reference in Rust?

unsafe

ownership

rust

Function take_mut::take

Function take_mut::take_or_recover