Reference cast to raw pointer

[ztgoto]

How to get a raw pointer to another raw pointer

fn main() {
    
    let i = 123;
    let p1 = &i;
    let p2 = &p1;

    let raw1 = p1 as *const i32;

    // error code
    let raw2 = p2 as **const i32;
    
    println!("{:p}", raw1);
    println!("{:p}", raw2);
}

print

error: expected mut or const in raw pointer type
   --> demo\src\bin\point.rs:128:22
    |
128 |     let raw2 = p2 as **const i32;
    |                      ^ expected mut or const in raw pointer type
    |
    = help: use `*mut T` or `*const T` as appropriate

error[E0606]: casting `&&i32` as `*const *const i32` is invalid
   --> demo\src\bin\point.rs:128:16
    |
128 |     let raw2 = p2 as **const i32;
    |                ^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

[steffahn]

let raw2 = p1 as *const _ as *const *const i32;

This first turns p1 into a *const &i32 and then casts that into a *const *const i32.

When using such a cast pointer (in unsafe code), you’ll have to be cautious in general since it’s possible to cast *const S into *const T for any types S and T. Since *const i32 and &i32 have the same layout, it should be safe to dereference raw2 obtained in this fashion though.

[H2CO3]

It would be more elegant and safer to just do it directly, one step at a time:

let raw1 = p1 as *const _;
let raw2 = &raw1 as *const _;

(That is, unless you specifically want p2 to point to p1 directly, but that seems like a weird requirement, of which the relevance is not apparent from your example code snippet.)