Provide a way to guarantee the type is !Unpin

[taiki-e]

UPDATE: see #108 (comment).

As mentioned in #102, it seems difficult to guarantee this with stable rust at this time.

However, I think it would be preferable if an option like #[pin_project(!Unpin)] could be provided in the future.

Some ways have been mentioned in #102, but it needs to work correctly for generics and conditional implementations in order to use with pin-project.

[taiki-e]

We can now support #102 without UnsafeUnpin, so I don't think we need negative implementation support.

(It is recommended to always use the default #[pin_project].)

[taiki-e]

FYI: this can be implemented by

pub struct Wrapper <'a, T> (PhantomData<&'a ()>, PhantomData<T>);
impl<T> Unpin for Wrapper<'_, T> where T: Unpin {}
impl Unpin for Foo where Wrapper<'_, PhantomPinned>: Unpin {}

[taiki-e]

This is equivalent to:

#[pin_project]
struct S {
    #[pin]
    _pin: PhantomPinned,
}

And I know that some people are using this: https://github.com/sfackler/rust-postgres/blob/1c843c8b22b37a279172da2ce4ba053506f57802/tokio-postgres/src/copy_out.rs#L46