这是一个不安全的 Rust 错误还是我错过了一些重要的东西？

Question

我是 Rust 新手，遇到了一个奇怪的问题，程序的成功或失败取决于是否将不安全的 Rust 放入构造函数中。不确定这是否是一个错误，或者我只是在后者中很幸运，或者我只是错过了一些重要的东西。

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")
        }
    }
}

输出：

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)

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")
        }
    }
}

输出：

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

我希望这两种情况都能起作用。

Answer 1

指向

this

的指针在这里

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

指向一个函数本地变量，一旦你从该函数返回，它就会变成悬空！之后取消引用它就是 UB。