Is there a way to tell the Rust's drop checker we effectively own a `T` without it being in generic parameters?

Question

Let's say I want to write code like this:

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

Currently, the Rust compiler let me do this as long as I don't add a Drop implementation for Inspector.

If I add one, the following compile-time error occurs:

foo.value dropped while still borrowed.

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

And it is obviously right. In fact, I have taken this example from the nomicon.

Now, here is my problem. Let's say I have a weird implementation of a Box that do not have any T in its type parameters.

/// 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) };
    }
}

But this time, Rust's drop checker does not know that MyBox will drop a T when its destructor is called. This enables me to write this unsound code:

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

From this, my question is simple: is there a way to tell the drop checker that it is not ok to have a lifetime bounded to the object when it gets dropped? Because what I basically need is something like PhantomData<T> without having a T.

Answer 1

I can do other nasty things with this MyBox that have nothing to do with Drop...

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);
    }
}

The problem is that MyBox completely erases any information about the borrows in T. There are two ways to fix this:

• The strict option is to change <T> to <T: 'static> in MyBox::new and MyBox::trust_mut. This will prevent the value in T from borrowing any non-'static data.
• The flexible option is to add a lifetime parameter to MyBox, and then change <T> to <T: 'a> in MyBox::new and MyBox::trust_mut. If you want the effects of the strict option, just use 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()
    }
}