当没有发生借用重叠时，为什么会出现借用错误？

Question

以下code失败，并出现借用错误：

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

为什么这里有一个借用错误？now.succ()返回一个新对象，看起来succ()调用应该返回新对象，在replace发生可变借用之前结束不可变的借用。

Answer 1

参数的顺序很重要。例如，这是可行的：

/// 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)

但在您的示例中，&mut now首先出现，并且在计算第二个参数时，它已经被借用。