`Unpin` bound in `impl<P> Body for Pin<P>` is unnecessarily restrictive

the current code is :

impl<P> Body for Pin<P>
where
    P: Unpin + ops::DerefMut,
    P::Target: Body,
{
// ....
    fn poll_frame(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Frame<Self::Data>, Self::Error>>> {
        Pin::get_mut(self).as_mut().poll_frame(cx)
    }
// ....
}

th correct code should be

impl<P> Body for Pin<P>
where
    P: ops::DerefMut,
    P::Target: Body,
{
// ....
    fn poll_frame(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Frame<Self::Data>, Self::Error>>> {
        self.as_deref_mut().poll_frame(cx)
    }
// ....
}

without the Unpin bound. this likely has gone unnoticed as a !Unpin pinning pointer is a very strange idea, and all pinning pointers in std are Unpin.

but it is possible for such a type to exist, and thus this bound is making the api unnecessarily restrictive.

edit : i just saw as_deref_mut requires rust 1.84, which would be a significant bumb(prob unaceptably so) .
while there is no doubt this is sound in in earlier versions, i'm unsure wether the addition of unsafe code would be warranted.

maybe something to keep in mind for later , when/if http-body's msrv increases

Start at the impl<P> Body for Pin<P> implementation shown in the issue and review how its Unpin bound affects !Unpin pointer types. Check the proposed as_deref_mut approach against the project's Rust MSRV and determine whether the bound can be removed without introducing unsupported or unjustified unsafe code.