This is a tracking issue for constness of the following stable API:

// core::mem

pub const fn size_of_val<T: ?Sized>(val: &T) -> usize;
pub const fn align_of_val<T: ?Sized>(val: &T) -> usize;

It would be useful for macros to be able to do things like:

static KEY: [u8; size_of_val($byte_string)] = $byte_string;

I believe the rust-objc devs would like something like this to properly set up statics containing messageSend selectors (which are magically processed by the objc runtime by putting them in the right linker sections).