Skip to content

Commit

Permalink
Rollup merge of #129748 - RalfJung:box-validity, r=workingjubilee
Browse files Browse the repository at this point in the history
Box validity: update for new zero-sized rules

Fixes rust-lang/unsafe-code-guidelines#529

Cc `@joshlf` `@rust-lang/opsem`
  • Loading branch information
matthiaskrgr authored Sep 2, 2024
2 parents bd53aa3 + 0e5628d commit 003ddec
Showing 1 changed file with 13 additions and 15 deletions.
28 changes: 13 additions & 15 deletions library/alloc/src/boxed.rs
Original file line number Diff line number Diff line change
Expand Up @@ -53,22 +53,20 @@
//!
//! # Memory layout
//!
//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for
//! its allocation. It is valid to convert both ways between a [`Box`] and a
//! raw pointer allocated with the [`Global`] allocator, given that the
//! [`Layout`] used with the allocator is correct for the type. More precisely,
//! a `value: *mut T` that has been allocated with the [`Global`] allocator
//! with `Layout::for_value(&*value)` may be converted into a box using
//! [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut
//! T` obtained from [`Box::<T>::into_raw`] may be deallocated using the
//! [`Global`] allocator with [`Layout::for_value(&*value)`].
//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for its allocation. It is
//! valid to convert both ways between a [`Box`] and a raw pointer allocated with the [`Global`]
//! allocator, given that the [`Layout`] used with the allocator is correct for the type and the raw
//! pointer points to a valid value of the right type. More precisely, a `value: *mut T` that has
//! been allocated with the [`Global`] allocator with `Layout::for_value(&*value)` may be converted
//! into a box using [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut T`
//! obtained from [`Box::<T>::into_raw`] may be deallocated using the [`Global`] allocator with
//! [`Layout::for_value(&*value)`].
//!
//! For zero-sized values, the `Box` pointer still has to be [valid] for reads
//! and writes and sufficiently aligned. In particular, casting any aligned
//! non-zero integer literal to a raw pointer produces a valid pointer, but a
//! pointer pointing into previously allocated memory that since got freed is
//! not valid. The recommended way to build a Box to a ZST if `Box::new` cannot
//! be used is to use [`ptr::NonNull::dangling`].
//! For zero-sized values, the `Box` pointer has to be non-null and sufficiently aligned. The
//! recommended way to build a Box to a ZST if `Box::new` cannot be used is to use
//! [`ptr::NonNull::dangling`].
//!
//! On top of these basic layout requirements, a `Box<T>` must point to a valid value of `T`.
//!
//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
//! as a single pointer and is also ABI-compatible with C pointers
Expand Down

0 comments on commit 003ddec

Please sign in to comment.