[tracking issue] raw ptr to usize cast inside constants

[oli-obk]

This is the tracking issue for the const_raw_ptr_to_usize_cast feature.

Activating the feature allows casting *const T and *mut T to usize.

I did not open an RFC for this because I believe we want to experiment with the feature (and we can already emulate it with union transmute hacks).

[crlf0710]

Just encountered this, when i try to define a constant that uses ::winapi::um::winuser::IDC_ARROW as an integer, which is defined as:

pub const IDC_ARROW: LPCWSTR = 32512 as LPCWSTR;

[oli-obk]

@crlf0710 what is your use case for casting a raw pointer to an integer? Is there any reason you can't use *const () or something similar?

[crlf0710]

@oli-obk actually it is an integer in the first place, however the upstream library (winapi) cast the integer to a raw pointer (maybe for convenience using it together with the raw Windows APIs) when it defined that variable (see above comment). So i can't use the original integer value in constant evaluation context now :(

I can work around this by (re-)defining those integers myself again. (Arguably the winapi crate can change its definitions to avoid this.) I'm not seeking for any changes, just want to record this use case.

[oli-obk]

I can offer you a stable workaround, but it's very unsafe and if you screw up by placing a real pointer in an integer constant we will error, and if we don't, we might so in the future. If you do undefined behaviour at compile-time, it means the behaviour (compilation success) is also undefined.

union Transmuter {
    from: LPCWSTR,
    to: usize,
}
const MY_CONST: usize = unsafe { Transmuter { from: IDC_ARROW }.to };

[crlf0710]

@oli-obk Great to know, thanks a lot!

[MSxDOS]

if you screw up by placing a real pointer in an integer constant we will error, and if we don't, we might so in the future.

I'm confused, what's wrong with putting a real pointer in an integer constant?

Btw, the following code compiles just fine:

use std::ptr::null;

#[repr(C)]
struct Fields {
    field1: u32,
    field2: [u8; 3],
    field3: u64,
    field4: i32,
}

union Transmuter<T: 'static> {
    ptr: *const T,
    reference: &'static T,
    integer: usize,
}

const FIELD4_OFFSET: usize =
    unsafe { Transmuter { reference: &(Transmuter { ptr: null::<Fields>() }.reference).field4 }.integer };

const _STATIC_ASSERT_EQ_: [(); 16] = [(); FIELD4_OFFSET];