transmute!

transmute.rs

#![feature(untagged_unions)]

#![feature(const_generics, const_evaluatable_checked)]
#![allow(incomplete_features)]

use core::mem::ManuallyDrop;

struct Bool<const B: bool>;

trait True {}

impl True for Bool<true> {}

unsafe fn transmute<T, U>(t: T) -> U
where
    Bool<{
        core::mem::size_of::<T>()  == core::mem::size_of::<U>()
    }>: True
{
    union Transmute<T, U> {
        t: ManuallyDrop<T>,
        u: ManuallyDrop<U>
    }
    
    ManuallyDrop::into_inner(Transmute { t: ManuallyDrop::new(t) }.u)
}

fn main() {
    unsafe {
       // let x = transmute::<u8, u16>(0);
        let y = transmute::<u8, i8>(0);
    }
}