I'm running into a this weird issue where the program succeeds or fails based on if a put my unsafe block in the constructor or not. Not sure if it is a bug or I am just getting lucky in the latter or I'm just missing something important.

我遇到了一个奇怪的问题，程序成功或失败取决于是否将我的不安全块放入构造函数中。不知道这是一个错误，还是我只是在后者运气好，或者我只是错过了一些重要的东西。

Failing case:

失败案例：

fn main() {
    let mut xc = Bob::default();
    let mut xp = Bob::default();
    let mut x = Bob::new(Some(&mut xc), Some(&mut xp));

    unsafe {
        (&mut *x.child.unwrap()).string = String::from("child");
        (&mut *xp.child.unwrap()).string = String::from("middle");
    }
    println!("{:?}", xc);
    println!("{:?}", x);
}

#[derive(Debug)]
struct Bob {
    child: Option<*mut Bob>,
    parent: Option<*mut Bob>,
    string: String,
}
impl Bob {
    fn new(child: Option<*mut Bob>, parent: Option<*mut Bob>) -> Bob {
        let mut this = Bob {
            child,
            parent,
            string: String::from("Hello")
        };
        if child.is_some(){
            unsafe { child.unwrap().as_mut().unwrap().parent = Some(&mut this);}
        }
        if parent.is_some(){
            unsafe { parent.unwrap().as_mut().unwrap().child = Some(&mut this);}
        }
        this
    }
}

impl Default for Bob {
    fn default() -> Bob {
        Bob {
            child: None,
            parent: None,
            string: String::from("Hello")
        }
    }
}

Bob { child: None, parent: Some(0x37b76ff7f8), string: "child" }
Bob { child: Some(0x37b76ff8b8), parent: Some(0x37b76ff8f0), string: "thread 'main' panicked at 'failed printing to stdout: Windows stdio in console mode does not support writing non-UTF-8 byte sequences', library\std\src\io\stdio.rs:1019:9
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library\std\src\panicking.rs:593
   1: core::panicking::panic_fmt
             at /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library\core\src\panicking.rs:67
   2: std::io::stdio::print_to
             at /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library\std\src\io\stdio.rs:1019
   3: std::io::stdio::_print
             at /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library\std\src\io\stdio.rs:1096
   4: rust_allocators::main
             at .\src\main.rs:14
   5: core::ops::function::FnOnce::call_once<void (*)(),tuple$<> >
             at /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225\library\core\src\ops\function.rs:250
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\debug\rust-allocators.exe` (exit code: 0xc0000374, STATUS_HEAP_CORRUPTION)

Process finished with exit code -1073740940 (0xC0000374)

Working alternative:

工作替代方案：

fn main() {
    let mut xc = Bob::default();
    let mut xp = Bob::default();
    let mut x = Bob::new(Some(&mut xc), Some(&mut xp));
    xp.child = Some(&mut x);
    // unsafe { Some(&mut xp as *mut Bob).unwrap().as_mut().unwrap().child = Some(&mut x);} // ALSO WORKS


    unsafe {
        (&mut *x.child.unwrap()).string = String::from("child");
        (&mut *xp.child.unwrap()).string = String::from("middle");
    }

    println!("{:?}", xc);
    println!("{:?}", x);
}

#[derive(Debug)]
struct Bob {
    child: Option<*mut Bob>,
    parent: Option<*mut Bob>,
    string: String,
}
impl Bob {
    fn new(child: Option<*mut Bob>, parent: Option<*mut Bob>) -> Bob {
        let mut this = Bob {
            child,
            parent,
            string: String::from("Hello")
        };
        if child.is_some(){
            unsafe { child.unwrap().as_mut().unwrap().parent = Some(&mut this);}
        }
        //if parent.is_some(){
        //    unsafe { parent.unwrap().as_mut().unwrap().child = Some(&mut this);}
        //}
        this
    }
}

impl Default for Bob {
    fn default() -> Bob {
        Bob {
            child: None,
            parent: None,
            string: String::from("Hello")
        }
    }
}

Bob { child: None, parent: Some(0x990ecff628), string: "child" }
Bob { child: Some(0x990ecff708), parent: Some(0x990ecff740), string: "middle" }

I expected both cases to work.

我原以为这两个案子都能奏效。

The pointer to this taken here

指向这个的指针在这里

unsafe { child.unwrap().as_mut().unwrap().parent = Some(&mut this);}

is pointing to a function local variable, it becomes dangling as soon as you return from that function! Dereferencing it afterwards is undefined behavior. Returning a value from a function moves it, so it's address might change.

指向一个函数局部变量，当您从该函数返回时，它就会变得悬空！事后取消引用它是未定义的行为。从函数返回值会移动它，因此它的地址可能会改变。

And while miri puts it in other words, it does say the same thing:

换句话说，虽然米莉说了这句话，但它确实表达了同样的意思：

error: Undefined Behavior: dereferencing pointer failed: alloc993 has been freed, so this pointer is dangling
  --> src/main.rs:8:9
   |
8  |         (&mut *xp.child.unwrap()).string = String::from("middle");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: alloc993 has been freed, so this pointer is dangling
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
help: alloc993 was allocated here:
  --> src/main.rs:22:13
   |
22 |         let mut this = Bob {
   |             ^^^^^^^^
help: alloc993 was deallocated here:
  --> src/main.rs:34:5
   |
34 |     }
   |     ^
   = note: BACKTRACE (of the first span):
   = note: inside `main` at src/main.rs:8:9: 8:34

The second version works because you create the pointer and don't subsequently move the values so you don't access any dangling pointers, the parent pointer of xc is still dangling, but you don't access it so that's fine.

第二个版本之所以有效，是因为您创建了指针，并且不会随后移动值，因此您不会访问任何悬空指针，XC的父指针仍然悬空，但您不会访问它，所以这很好。

There is a way to pull this off, we have to ensure

有办法做到这一点，我们必须确保

1. the actual value doesn't move in memory when we return — we can Box::pin it


2. it can't be moved afterwards — add a PhantomPinned so it doesn't implement Unpin

So change to this:

因此，请更改为：

use core::marker::PhantomPinned;
use core::pin::Pin;

#[derive(Default, Debug)]
struct Bob {
    child: Option<*mut Bob>,
    parent: Option<*mut Bob>,
    string: String,
    _marker: PhantomPinned,
}

impl Bob {
    fn new(child: Option<*mut Bob>, parent: Option<*mut Bob>) -> Pin<Box<Bob>> {
        let mut this = Box::pin(Bob {
            child,
            parent,
            string: String::from("Hello"),
            _marker: PhantomPinned,
        });

        if child.is_some() {
            let this = this.as_mut();
            unsafe {
                child.unwrap().as_mut().unwrap().parent = Some(this.get_unchecked_mut());
            }
        }
        if parent.is_some() {
            let this = this.as_mut();
            unsafe {
                parent.unwrap().as_mut().unwrap().child = Some(this.get_unchecked_mut());
            }
        }
        this
    }
}