不可变变量， 为什么可以作为可变引用传入方法？

Question

这里有人能帮我解释一下这个问题吗？谢谢了

let ptr = story.as_mut_ptr();
as_mut_ptr的方法签名是：

    pub fn as_mut_ptr(&mut self) -> *mut u8 {
        self as *mut str as *mut u8
    }

story明明是个不可变变量， 为什么可以作为可变引用传入方法？

sorry， 我搞错了！少看了一句
let mut story = mem::ManuallyDrop::new(story);

下面是完整代码

use std::mem;

fn main() {
    let story = String::from("Rust By Practice");

    // Prevent automatically dropping the String's data
    let mut story = mem::ManuallyDrop::new(story);

    let ptr = story.as_mut_ptr();
    let len = story.len();
    let capacity = story.capacity();

    // story has nineteen bytes
    assert_eq!(16, len);

    // We can re-build a String out of ptr, len, and capacity. This is all
    // unsafe because we are responsible for making sure the components are
    // valid:
    let s = unsafe { String::from_raw_parts(ptr, len, capacity) };

    assert_eq!(*story, s);

    println!("Success!")
}

Answer 1

https://play.rust-lang.org/?version=stable&mode=debug&edition...

error[E0596]: cannot borrow `story` as mutable, as it is not declared as mutable
 --> src/main.rs:4:15
  |
3 |     let story = String::from("Rust By Practice");
  |         ----- help: consider changing this to be mutable: `mut story`
4 |     let ptr = story.as_mut_ptr();
  |               ^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

---

经过 let mut story = mem::ManuallyDrop::new(story); ，story 已经不是一个 String ，而是一个 stuct std::mem::ManuallyDrop<String>。它的 new 是这样的：

pub const fn new(value: T) -> ManuallyDrop<T>

原来的 immutable 的 String ，被 move 进了 mut ManuallyDrop<String> 。

而 MuauallyDrop 实现了 trait DerefMut ，于是会发生 Deref Coercion:

If T implements DerefMut<Target = U>, and x is a value of type T, then:

• In mutable contexts, *x (where T is neither a reference nor a raw pointer) is equivalent to *DerefMut::deref_mut(&mut x).
• Values of type &mut T are coerced to values of type &mut U
• T implicitly implements all the (mutable) methods of the type U.

（这里，T 是 ManuallyDrop<String> ，U 是 String）

于是你可以在 mut ManuallyDrop<String> 上调用所有 String 的 mutable methods。

Answer 2

能调用 但是编译不会通过