Macro std::ptr::raw_const

macro_rules! raw_const {
    ($e:expr) => { ... };
}

Create a const raw pointer to a place, without creating an intermediate reference.

Creating a reference with &/&mut is only allowed if the pointer is properly aligned and points to initialized data. For cases where those requirements do not hold, raw pointers should be used instead. However, &expr as *const _ creates a reference before casting it to a raw pointer, and that reference is subject to the same rules as all other references. This macro can create a raw pointer without creating a reference first.

Example

#![feature(raw_ref_macros)]
use std::ptr;

#[repr(packed)]
struct Packed {
    f1: u8,
    f2: u16,
}

let packed = Packed { f1: 1, f2: 2 };
// `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
let raw_f2 = ptr::raw_const!(packed.f2);
assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);