rust - 有没有办法告诉 Rust 的 drop checker 我们有效地拥有一个 `T` 而没有它在通用参数中？

Question

假设我想写这样的代码:

struct Inspector<'a>(&'a u8);

struct Foo<'a> {
    value: Box<u8>,
    inspector: Option<Inspector<'a>>,
}

fn main() {
    let mut foo = Foo { value: Box::new(0), inspector: None };
    foo.inspector = Some(Inspector(&foo.value));
}

playground
目前，Rust 编译器允许我这样做，只要我不添加 Drop Inspector 的实现.
如果我添加一个，则会出现以下编译时错误:

foo.value dropped while still borrowed.

Borrow might be used when foo is dropped and runs the destructor for type Foo<'_>

这显然是正确的。事实上，我从 the nomicon 中取了这个例子。 .
现在，这是我的问题。假设我有一个 Box 的奇怪实现没有任何 T在其类型参数中。

/// An heap-allocated `T` without generic parameters.
struct MyBox {
    data: NonNull<u8>,

    /// SAFETY:
    /// Caller must ensure the value will not be
    /// used again.
    drop_fn: unsafe fn(*mut u8),

    layout: Layout,
}

impl MyBox {
    fn new<T>(val: T) -> Self {
        if mem::size_of::<T>() == 0 {
            panic!("T is a ZST");
        }

        let layout = Layout::new::<T>();
        let data = NonNull::new(unsafe { alloc(layout) })
            .unwrap_or_else(|| handle_alloc_error(layout));

        // pointer refers to uninit owned memory
        unsafe { data.cast::<T>().as_ptr().write(val) };

        Self {
            data,
            // SAFETY: See `drop_fn` field for safety guarantees
            drop_fn: |data| unsafe { drop_in_place(data as *mut T) },
            layout,
        }
    }

    /// Caller must ensure that this box owns a `T`.
    unsafe fn trust_mut<T>(&mut self) -> &mut T {
        &mut *self.data.cast().as_ptr()
    }
}

impl Drop for MyBox {
    fn drop(&mut self) {
        // SAFETY: Value will not be used again
        unsafe { (self.drop_fn)(self.data.as_ptr()) }
        unsafe { dealloc(self.data.as_ptr(), self.layout) };
    }
}

但这一次，Rust 的 drop checker 不知道 MyBox会掉一个 T当它的析构函数被调用时。这使我能够编写这个不健全的代码:

pub struct Inspector<'a>(&'a u8);

impl Drop for Inspector<'_> {
    fn drop(&mut self) {
        /* Could try to inspect `self.0` here which might have been dropped */
    }
}

pub struct Foo<'a> {
    value: Box<u8>,
    inspector: Option<Inspector<'a>>,
}

fn main() {
    let mut b = MyBox::new(Foo {
        value: Box::new(0),
        inspector: None,
    });

    let foo: &mut Foo = unsafe { b.trust_mut() };
    foo.inspector = Some(Inspector(&foo.value)); // No error occurs here
}

playground
由此，我的问题很简单:有没有办法告诉丢弃检查器，当它被丢弃时，将生命周期限制在对象上是不行的？因为我基本上需要的是类似 PhantomData<T> 的东西没有 T .

Answer 1

我可以用这个做其他讨厌的事情 MyBox与Drop无关的...

fn main() {
    let mut vec: Vec<i32> = vec![42];
    let mut b = MyBox::new(&vec[0]); // T is &i32
    {
        let val: &mut &i32 = unsafe { b.trust_mut() };
        println!("{}", *val);
    }
    vec[0] = 1337; // I can mutate `vec[0]` even though `vec[0]` is borrowed by `b`
    {
        let val: &mut &i32 = unsafe { b.trust_mut() };
        println!("{}", *val);
    }
}

问题是 MyBox完全删除有关 T 中借用的任何信息.有两种方法可以解决这个问题:

严格的选项是更改 <T>至 <T: 'static>在 MyBox::new和 MyBox::trust_mut .这将阻止 T 中的值借用任何非 'static数据。

灵活的选项是向 MyBox 添加生命周期参数。 ，然后更改 <T>至 <T: 'a>在 MyBox::new和 MyBox::trust_mut .如果你想要严格选项的效果，只需使用 MyBox<'static> .

use std::alloc::{Layout, alloc, dealloc, handle_alloc_error};
use std::marker::PhantomData;
use std::mem::size_of;
use std::ptr::{self, NonNull};

/// An heap-allocated `T` without generic parameters.
struct MyBox<'a> {
    data: NonNull<u8>,

    /// SAFETY:
    /// Caller must ensure the value will not be
    /// used again.
    drop_fn: unsafe fn(*mut u8),

    layout: Layout,

    _borrow: PhantomData<&'a ()>,
}

impl<'a> MyBox<'a> {
    fn new<T: 'a>(val: T) -> Self {
        if size_of::<T>() == 0 {
            panic!("T is a ZST");
        }

        let layout = Layout::new::<T>();
        let data = NonNull::new(unsafe { alloc(layout) })
            .unwrap_or_else(|| handle_alloc_error(layout));

        // pointer refers to uninit owned memory
        unsafe { data.cast::<T>().as_ptr().write(val) };

        Self {
            data,
            // SAFETY: The caller must ensure that the value will not
            // be using the value again.
            drop_fn: |data| unsafe { ptr::drop_in_place(data as *mut T) },
            layout,
            _borrow: PhantomData,
        }
    }

    /// Caller must ensure that this box owns a `T`.
    unsafe fn trust_mut<T: 'a>(&mut self) -> &mut T {
        &mut *self.data.cast().as_ptr()
    }
}