Const generics and transmute: cannot transmute between types of different sizes

[bgourlie]

The following code should compile, but it doesn't:

#![feature(const_generics)]

use std::{
    mem::{self, MaybeUninit},
    ptr,
};

fn main() {
}

pub struct Foo<const SIZE: usize> {
    messages: [Option<String>; SIZE],
}

impl<const SIZE: usize> Default for Foo<{SIZE}> {
    fn default() -> Self {
        let arr = {
            let mut arr: [MaybeUninit<Option<String>>; {SIZE}] =
                unsafe { MaybeUninit::uninit().assume_init() };

            for elem in &mut arr[..] {
                unsafe { ptr::write(elem.as_mut_ptr(), None) }
            }

            unsafe { mem::transmute::<_, [Option<String>; {SIZE}]>(arr) }
        };
        Foo {
            messages: arr,
        }
    }
}

The error produced is

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
  --> src\main.rs:25:22
   |
25 |             unsafe { mem::transmute::<_, [Option<String>; {SIZE}]>(arr) }
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: source type: `[std::mem::MaybeUninit<std::option::Option<std::string::String>>; _]` (this type does not have a fixed size)
   = note: target type: `[std::option::Option<std::string::String>; _]` (this type does not have a fixed size)

[DutchGhost]

Duplicate of #61956 .

To fix this for now, either use transmute_copy, or take a raw pointer to the MaybeUninit array, cast the ptr to the type you want, and ptr::read it, while making sure you mem::forget the MaybeUninit array!

[varkor]

Closing as duplicate of #61956.

[hniksic]

@DutchGhost Why is it necessary to mem::forget() the array of MaybeUninit? AFAIK MaybeUninit doesn't implement Drop so mem::forget-ing it should be no-op.