move.rs

#[lang="move"]
struct Move<Sized? T, S: MoveSource<T>> {
    ptr: *mut T,
    src: S
}

#[lang="move_source"]
trait MoveSource<Sized? T> {
    unsafe fn cleanup(self, ptr: *mut T) {}
}

#[lang="move_stack"]
struct Stack<'a>;

impl<'a, T> MoveSource<T> For Stack<'a> {
    // No need to "deallocate" stack values.
    unsafe fn cleanup(self, _: *mut T) {}
}

impl<Sized? T, S: MoveSource<T>> Move<T, S> {
    pub unsafe fn from(ptr: *mut T, src: S) -> Move<T, S> {
        Move {
            ptr: ptr,
            src: src
        }
    }
    
    pub unsafe fn into_parts(self) -> (*mut T, S) {
        let (ptr, src) = (self.ptr, self.src);
        mem::forget(self);
        (ptr, src)
    }
}

impl<Sized? T, S: MoveSource<T>> Drop for Move<T, S> {
    fn drop(&mut self) {
        drop_in_place(self.ptr);
        // Maybe MoveSource should require Self be Copy?
        self.src.cleanup(self.ptr);
    }
}

trait DerefMove<S: MoveSource<Self>> {
    type Out;
    type OutSrc: MoveSource<Out>;
    fn deref_move(self: Move<Self, S>) -> Move<Out, OutSrc>;
}

impl<Sized? T, S> DerefMove<S> for Box<T> {
    type Out = T;
    type OutSrc = Heap;
    fn deref_move(self: Move<Box<T>, S>) -> Move<T, Heap> {
        // The way this works, {**box box foo} will free the outer box,
        // move the value out, and only then it would free the inner box.
        // The outer box can be freed early because it's cheaper and more
        // flexible to pass around Move<Box<Foo>, Heap>, rather than the
        // Move<Box<Box<Foo>>, Stack<'a>> that the compiler creates.
        let the_box: Box<T> = *self;
        unsafe {
            Move::from(transmute::<Box<T>, *mut T>(the_box), Heap)
        }
    }
}

struct Heap;

impl<Sized? T> MoveSource<T> for Heap {
    unsafe fn cleanup(self, ptr: *mut T) {
        heap::deallocate(data_pointer(ptr), size_of_val(&*ptr));
    }
}

impl<T, S> DerefMove<S> for Vec<T> {
    type Out = [T];
    type OutSrc = HeapSized;
    fn deref_move(self: Move<Vec<T>, S>) -> Move<T, HeapSized> {
        let vec: *mut [T] = &mut *self;
        let size = self.capacity * mem::size_of::<T>();
        unsafe {
            mem::forget(self);
            Move::from(vec, HeapSized(size))
        }
    }
}

struct HeapSized(uint);

impl<Sized? T> MoveSource<T> for HeapSized {
    unsafe fn cleanup(self, ptr: *mut T) {
        heap::deallocate(data_pointer(ptr), self.0);
    }
}

struct MoveItems<T, S: MoveSource<[T]>> {
    start: *mut T,
    end: *mut T,
    // This might be better off as a per-type-implementing-MoveSource
    // custom "bundled dtor". Would be zero-sized for Stack and just
    // (*mut T, uint) (w/o the length of *mut [T]) for HeapSized.
    alloc: *mut [T],
    src: S
}

impl<T, S: MoveSource<[T]>> IntoIter for Move<[T], S> {
    type Iter = MoveItems<T, S>;
    fn into_iter(self) -> MoveItems<T, S> {
        unsafe {
            let (start, len) = (self.as_mut_ptr(), self.len());
            let (alloc, src) = Move::into_parts(self);
            MoveItems {
                start: start,
                end: start.offset(len),
                alloc: alloc,
                src: src
            }
        }
    }
}

impl<T, S: MoveSource<[T]>> Drop for MoveItems<T, S> {
    fn drop(&mut self) {
        for _ in self {}
        self.src.cleanup(self.alloc);
    }
}

MoveSource needs to be an RFC19 unsafe impl, in addition to cleanup being an unsafe function.

Bikeshed: I think MoveSource should be called Dealloc or Cleanup and Move be called Root.