rust - 为什么没有发生借用重叠时会出现借用错误？

Question

以下代码因借用错误而失败：

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

为什么这里会出现借用错误？now.succ()返回一个新对象，看起来succ()调用应该返回新对象，在可变借用发生之前结束不可变借用replace。

Answer 1

论点的顺序很重要。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);
}

（游乐场链接）

但是在您的示例中，&mut now首先出现，并且在评估第二个参数时，它已经被借用了。