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

为什么这里有借用错误？ now.succ()返回一个新对象，并且看起来succ()调用应该返回该新对象，在使用replace发生可变借用之前结束不可变借用。

Answer 1

参数的顺序很重要。例如，这有效：

#models/resultvalue.rb

class Resultvalue < ApplicationRecord
    serialize :values, Array 
end

（link to playground）

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

当没有借款重叠发生时，为什么会有借款错误？

1 个答案: