Tracking Issue for box_as_ptr

[RalfJung]

Feature gate: #![feature(box_as_ptr)]

This is a tracking issue for Box::as_ptr and Box::as_mut_ptr.

Public API

impl<T: ?Sized, A: Allocator> Box<T, A> {
    pub fn as_mut_ptr(b: &mut Self) -> *mut T;
    pub fn as_ptr(b: &Self) -> *const T;
}

Steps / History

• ACP: Add "as raw pointer" methods to Box libs-team#355
• Implementation: add Box::as_ptr and Box::as_mut_ptr methods #129091
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[orlp]

The current implementation of as_mut_ptr using addr_of_mut still suffers from the fact that moving the Box invalidates the pointer, which seems like a pretty big defect:

#![feature(box_as_ptr)]

fn main() {
    let mut b = Box::new(42);
    let p = Box::as_mut_ptr(&mut b);
    let _b2 = b;
    unsafe { *p = 0; };
}

error: Undefined Behavior: attempting deallocation using <1988> at alloc1024, but that tag does not exist in the borrow stack for this location
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1679:17
     |
1679 |                 self.1.deallocate(From::from(ptr.cast()), layout);
     |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempting deallocation using <1988> at alloc1024, but that tag does not exist in the borrow stack for this location
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
     = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <1988> was created by a Unique retag at offsets [0x0..0x4]
    --> src/main.rs:6:15
     |
6    |     let _b2 = b;
     |               ^
help: <1988> was later invalidated at offsets [0x0..0x4] by a write access
    --> src/main.rs:7:14
     |
7    |     unsafe { *p = 0; };
     |              ^^^^^^
     = note: BACKTRACE (of the first span):
     = note: inside `<std::boxed::Box<i32> as std::ops::Drop>::drop` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1679:17: 1679:66
     = note: inside `std::ptr::drop_in_place::<std::boxed::Box<i32>> - shim(Some(std::boxed::Box<i32>))` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:574:1: 574:56
note: inside `main`
    --> src/main.rs:8:1
     |
8    | }
     | ^

[RalfJung]

Yes, that is currently expected. Moving the Box asserts full uniqueness of the Box, just like a mutable reference.

Also see the discussion at rust-lang/unsafe-code-guidelines#326.