core mem replace

• https://doc.rust-lang.org/nightly/core/mem/fn.replace.html
• Moves src into the referenced dest, returning the previous dest value.
• If you want to replace the values of two variables, see core mem swap
• If you want to replace with a default value, see core mem take .
• replace allows consumption of a struct field by replacing it with another value.

내부 구현을 보자

pub const fn replace<T>(dest: &mut T, src: T) -> T {
    // SAFETY: We read from `dest` but directly write `src` into it afterwards,
    // such that the old value is not duplicated. Nothing is dropped and
    // nothing here can panic.
    unsafe {
        let result = ptr::read(dest);
        ptr::write(dest, src);
        result
    }
}

replace를 사용하면 누가봐도 당연하지만 컴파일러가 싫어하는 코드를 안전하게 작성할 수 있다.

use std::mem;

struct Buffer<T>(Vec<T>);

impl<T> Buffer<T> {
    /// This is not ok,
    /// error: cannot move out of dereference of `&mut` pointer
    fn replace_index(&mut self, i: usize, v: T) -> T {
        let t = self.0[i];
        self.0[i] = v;
    }
    /// This is Ok
    fn replace_index(&mut self, i: usize, v: T) -> T {
        mem::replace(&mut self.0[i], v)
    }
}