RawVec - The Rustonomicon
Description: The Dark Arts of Advanced and Unsafe Rust Programming

# RawVec

We've actually reached an interesting situation here: we've duplicated the logic for specifying a buffer and freeing its memory in Vec and IntoIter. Now that we've implemented it and identified actual logic duplication, this is a good time to perform some logic compression.

We're going to abstract out the `(ptr, cap)` pair and give them the logic for allocating, growing, and freeing:

    struct RawVec<T> {
        ptr: Unique<T>,
        cap: usize,
    }

    impl<T> RawVec<T> {
        fn new() -> Self {
            assert!(mem::size_of::<T>() != 0, "TODO: implement ZST support");
            RawVec { ptr: Unique::dangling(), cap: 0 }
        }

        // unchanged from Vec
        fn grow(&mut self) {
            unsafe {
                let align = mem::align_of::<T>();
                let elem_size = mem::size_of::<T>();

                let (new_cap, ptr) = if self.cap == 0 {
                    let ptr = heap::allocate(elem_size, align);
                    (1, ptr)
                } else {
                    let new_cap = 2 * self.cap;
                    let ptr = heap::reallocate(self.ptr.as_ptr() as *mut _,
                                                self.cap * elem_size,
                                                new_cap * elem_size,
                                                align);
                    (new_cap, ptr)
                };

                // If allocate or reallocate fail, we'll get `null` back
                if ptr.is_null() { oom() }

                self.ptr = Unique::new(ptr as *mut _);
                self.cap = new_cap;
            }
        }
    }

    impl<T> Drop for RawVec<T> {
        fn drop(&mut self) {
            if self.cap != 0 {
                let align = mem::align_of::<T>();
                let elem_size = mem::size_of::<T>();
                let num_bytes = elem_size * self.cap;
                unsafe {
                    heap::deallocate(self.ptr.as_mut() as *mut _, num_bytes, align);
                }
            }
        }
    }

And change Vec as follows:

    pub struct Vec<T> {
        buf: RawVec<T>,
        len: usize,
    }

    impl<T> Vec<T> {
        fn ptr(&self) -> *mut T { self.buf.ptr.as_ptr() }

        fn cap(&self) -> usize { self.buf.cap }

        pub fn new() -> Self {
            Vec { buf: RawVec::new(), len: 0 }
        }

        // push/pop/insert/remove largely unchanged:
        // * `self.ptr -> self.ptr()`
        // * `self.cap -> self.cap()`
        // * `self.grow -> self.buf.grow()`
    }

    impl<T> Drop for Vec<T> {
        fn drop(&mut self) {
            while let Some(_) = self.pop() {}
            // deallocation is handled by RawVec
        }
    }

And finally we can really simplify IntoIter:

    struct IntoIter<T> {
        _buf: RawVec<T>, // we don't actually care about this. Just need it to live.
        start: *const T,
        end: *const T,
    }

    // next and next_back literally unchanged since they never referred to the buf

    impl<T> Drop for IntoIter<T> {
        fn drop(&mut self) {
            // only need to ensure all our elements are read;
            // buffer will clean itself up afterwards.
            for _ in &mut *self {}
        }
    }

    impl<T> Vec<T> {
        pub fn into_iter(self) -> IntoIter<T> {
            unsafe {
                // need to use ptr::read to unsafely move the buf out since it's
                // not Copy, and Vec implements Drop (so we can't destructure it).
                let buf = ptr::read(&self.buf);
                let len = self.len;
                mem::forget(self);

                IntoIter {
                    start: *buf.ptr,
                    end: buf.ptr.offset(len as isize),
                    _buf: buf,
                }
            }
        }
    }

Much better.