Implement `replace()` method to move out of struct/enum about pin-project HOT 7 CLOSED

#[pin_project]
enum Example {
    A(#[pin] PinnedValue),
    B(Option<UnpinnedValue>),
    C,
}

#[project]
match self.as_mut().project() {
    Example::A(_) => { /* ... */ },
    Example::B(value @ Some(_)) => { 
        let value = value.take().unwrap();
        // ... 
    },
    Example::B(None) | Example::C => {},
}
self.set(Example::C); // When using this way, whether you should replace self depends on the situation.

struct DropInPlaceHelper<T>(*mut T);

impl<T> DropInPlaceHelper<T> {
    fn drop(&mut self) {
        ptr::drop_in_place(self.0);
    }
}

struct OverwriteHelper<T>(*mut T, MaybeUninit<T>);

impl<T> OverwriteHelper<T> {
    fn drop(&mut self) {
        ptr::write(self.0, self.1.assume_init());
    }
}

fn partial_replace(ptr: *mut Foo, new_value: Foo) -> Y {
    let _helper0 = OverwriteHelper(ptr, MaybeUninit::new(new_value));
    let _helper1 = DropInPlaceHelper(&mut (*ptr).pinned_field1);
    let _helper2 = DropInPlaceHelper(&mut (*ptr).pinned_field2);
    let _helper3 = DropInPlaceHelper(&mut (*ptr).pinned_field3);
    ptr::read(&mut (*ptr).unpinned_field)
}