CoercePointee leaks unstable unsizing impls

[BoxyUwU]

#![feature(derive_coerce_pointee)]
use std::marker::PhantomData;
use std::marker::CoercePointee; 
#[derive(CoercePointee)] #[repr(transparent)]
struct MyPointer<'a, T, #[pointee] U: ?Sized> {
    ptr: &'a (T, U),
}

fn main() {
    let x = MyPointer { ptr: &(1u32, 1u32) };
    let _: MyPointer<u32, dyn Send> = x;
}

Originally posted by @lcnr in #133820 (comment)

[compiler-errors]

I'll take a stab at this.