Explain how it works

[vigna]

I think it would be fantastic if you could comment a sample of the generated code in the README and explain why it works. I'm learning a lot from your code but some written explanation would be priceless. :)

[taiki-e]

As mentioned in the readme, it generates a trait implementation with a condition that will never be true.

For example, in the case of Unpin, the generated code is roughly as follows.

impl Unpin for $ty 
where
    PhantomPinned: Unpin, // PhantomPinned will never be Unpin.
{}

The actual generated code is a bit more complex because of hygiene and to work around the limitations of Rust's type system.