Tracking issue for release notes of #127679: Stabilize raw_ref_op (RFC 2582)

[rustbot]

This issue tracks the release notes text for #127679.

• Issue is nominated for the responsible team (and T-release nomination is removed).
• Proposed text is drafted by team responsible for underlying change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text:

The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.

# Compatibility Notes
- [Stabilize `&raw const` and `&raw mut` operators (RFC 2582)](https://github.com/rust-lang/rust/pull/127679)

Release blog section (if any, leave blank if no section is expected):

[traviscross]

@RalfJung: What do you think?

cc @rust-lang/lang

[RalfJung]

Here's a first version. Maybe this should be a joint blog section with #129578.

Native syntax for creating a raw pointer

Unsafe code sometimes has to deal with pointers that may dangle, may be misaligned, or may not point to valid data. A common case where this comes up are packed structs. In such a case, it is important to avoid creating a reference, as that would cause undefined behavior. This means the usual & and &mut operators cannot be used, as those create a reference -- even if the reference is immediately cast to a raw pointer, it's too late to avoid the undefined behavior.

For several years, the macros std::ptr::addr_of! and std::ptr::addr_of_mut! have served this purpose. Now the time has come to provide a proper native syntax for this operation: addr_of!(expr) becomes &raw const expr, and addr_of_mut!(expr) becomes &raw mut expr. For example:

#[repr(packed)]
struct Packed {
    not_aligned_field: i32,
}

fn main() {
    let p = Packed { not_aligned_field: 1_82 };
    
    // This would be undefined behavior!
    // It is rejected by the compiler.
    //let ptr = &p.not_aligned_field as *const i32;
    
    // This is the old way of creating a pointer.
    let ptr = std::ptr::addr_of!(p.not_aligned_field);
    
    // This is the new way.
    let ptr = &raw const p.not_aligned_field;

    // Accessing the pointer has not changed.
    // Note that `val = *ptr` would be undefined behavior because
    // the pointer is not aligned!
    let val = unsafe { ptr.read_unaligned() };
}

The native syntax makes it more clear that the operand expression of these operators is interpreted as a place expression. It also avoids the term "address-of" when referring to the action of creating a pointer. A pointer is more than just an address, so Rust is moving away from terms like "address-of" that reaffirm a false equivalence of pointers and addresses.

[traviscross]

Sounds good. Where you mention a place expression, let's link to your blog post:

https://www.ralfj.de/blog/2024/08/14/places.html