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
为什么这里有借用错误? now.succ()
返回一个新对象,看起来succ()
调用应返回新对象,在replace
发生可变借用之前结束不可变借用。
论证的顺序很重要。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);
}
(Qazxswpoi)
但在你的例子中,link to playground首先出现,在评估第二个参数时,它已经被借用了。