为什么没有发生借用重叠时会出现借用错误？

Question

以下 code 因借用错误而失败：

extern crate chrono; // 0.4.6

fn main() {
    let mut now = chrono::Local::today();
    now = std::mem::replace(&mut now, now.succ());
}

错误是：

error[E0502]: cannot borrow `now` as immutable because it is also borrowed as mutable
 --> src/lib.rs:5:39
  |
5 |     now = std::mem::replace(&mut now, now.succ());
  |           ----------------- --------  ^^^ immutable borrow occurs here
  |           |                 |
  |           |                 mutable borrow occurs here
  |           mutable borrow later used by call

这里为什么会出现借位错误？ now.succ() return 是一个新对象，看起来 succ() 调用应该 return 新对象，在可变借用发生之前结束不可变借用 replace.

Answer 1

参数的顺序很重要。例如，这有效：

/// Same as `std::mem::replace`, but with the reversed parameter order.
pub fn replace<T>(src: T, dest: &mut T) -> T {
    std::mem::replace(dest, src)
}

fn main() {
    let mut now = chrono::Local::today();
    now = replace(now.succ(), &mut now);
}

(link to playground)

但是在你的例子中，&mut now先出现，在评估第二个参数时，它已经被借用了。

为什么没有发生借用重叠时会出现借用错误？

Why is there a borrow error when no borrowing overlap is occurring?

rust

borrow