Shoot. I was hoping it would be possible to approximate https://www.microsoft.com/en-us/research/wp-content/uploads/2016/11/trees-that-grow.pdf in Rust without HKTs using this technique, but the compiler seems to lack the ability to reason about impossible constructors when calculating object size:
```rust
#![feature(never_type)]
struct L<A, B> {
    v: Result<(A, [u8;32]), (B, [u8; 64])>,
}

impl<A> L<A, !> {
    fn extractLeft(&self) -> [u8;32] {
        match &self.v {
            Ok((a, i)) => *i,
            Err((impossible, _)) => *impossible,
        }
    }
}
pub fn main(){
    let v : L<(),!> = L{v:Ok(((), [0;32]))};
    println!("{}", core::mem::size_of::<L<(),!>>())
}
```

The above prints `65`, so rustc is not pruning the impossible `Err` constructor