split out vec-zsts correctly

Author: Gankra

src/doc/tarpl/SUMMARY.md

@@ -45,5 +45,6 @@
 	* [Insert and Remove](vec-insert-remove.md)
 	* [IntoIter](vec-into-iter.md)
 	* [Drain](vec-drain.md)
+	* [Handling Zero-Sized Types](vec-zsts.md)
 	* [Final Code](vec-final.md)
 * [Implementing Arc and Mutex](arc-and-mutex.md)

src/doc/tarpl/vec-zsts.md

@@ -0,0 +1,176 @@
+% Handling Zero-Sized Types
+
+It's time. We're going to fight the spectre that is zero-sized types. Safe Rust
+*never* needs to care about this, but Vec is very intensive on raw pointers and
+raw allocations, which are exactly the *only* two things that care about
+zero-sized types. We need to be careful of two things:
+
+* The raw allocator API has undefined behaviour if you pass in 0 for an
+  allocation size.
+* raw pointer offsets are no-ops for zero-sized types, which will break our
+  C-style pointer iterator.
+
+Thankfully we abstracted out pointer-iterators and allocating handling into
+RawValIter and RawVec respectively. How mysteriously convenient.
+
+
+
+
+## Allocating Zero-Sized Types
+
+So if the allocator API doesn't support zero-sized allocations, what on earth
+do we store as our allocation? Why, `heap::EMPTY` of course! Almost every operation
+with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
+to be considered to store or load them. This actually extends to `ptr::read` and
+`ptr::write`: they won't actually look at the pointer at all. As such we *never* need
+to change the pointer.
+
+Note however that our previous reliance on running out of memory before overflow is
+no longer valid with zero-sized types. We must explicitly guard against capacity
+overflow for zero-sized types.
+
+Due to our current architecture, all this means is writing 3 guards, one in each
+method of RawVec.
+
+```rust,ignore
+impl<T> RawVec<T> {
+    fn new() -> Self {
+        unsafe {
+            // !0 is usize::MAX. This branch should be stripped at compile time.
+            let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
+
+            // heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
+            RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
+        }
+    }
+
+    fn grow(&mut self) {
+        unsafe {
+            let elem_size = mem::size_of::<T>();
+
+            // since we set the capacity to usize::MAX when elem_size is
+            // 0, getting to here necessarily means the Vec is overfull.
+            assert!(elem_size != 0, "capacity overflow");
+
+            let align = mem::align_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 *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) {
+        let elem_size = mem::size_of::<T>();
+
+        // don't free zero-sized allocations, as they were never allocated.
+        if self.cap != 0 && elem_size != 0 {
+            let align = mem::align_of::<T>();
+
+            let num_bytes = elem_size * self.cap;
+            unsafe {
+                heap::deallocate(*self.ptr as *mut _, num_bytes, align);
+            }
+        }
+    }
+}
+```
+
+That's it. We support pushing and popping zero-sized types now. Our iterators
+(that aren't provided by slice Deref) are still busted, though.
+
+
+
+
+## Iterating Zero-Sized Types
+
+Zero-sized offsets are no-ops. This means that our current design will always
+initialize `start` and `end` as the same value, and our iterators will yield
+nothing. The current solution to this is to cast the pointers to integers,
+increment, and then cast them back:
+
+```rust,ignore
+impl<T> RawValIter<T> {
+    unsafe fn new(slice: &[T]) -> Self {
+        RawValIter {
+            start: slice.as_ptr(),
+            end: if mem::size_of::<T>() == 0 {
+                ((slice.as_ptr() as usize) + slice.len()) as *const _
+            } else if slice.len() == 0 {
+                slice.as_ptr()
+            } else {
+                slice.as_ptr().offset(slice.len() as isize)
+            }
+        }
+    }
+}
+```
+
+Now we have a different bug. Instead of our iterators not running at all, our
+iterators now run *forever*. We need to do the same trick in our iterator impls.
+Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
+basically be treating the two pointers as if they point to bytes, we'll just
+map size 0 to divide by 1.
+
+```rust,ignore
+impl<T> Iterator for RawValIter<T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        if self.start == self.end {
+            None
+        } else {
+            unsafe {
+                let result = ptr::read(self.start);
+                self.start = if mem::size_of::<T>() == 0 {
+                    (self.start as usize + 1) as *const _
+                } else {
+                    self.start.offset(1);
+                }
+                Some(result)
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let elem_size = mem::size_of::<T>();
+        let len = (self.end as usize - self.start as usize)
+                  / if elem_size == 0 { 1 } else { elem_size };
+        (len, Some(len))
+    }
+}
+
+impl<T> DoubleEndedIterator for RawValIter<T> {
+    fn next_back(&mut self) -> Option<T> {
+        if self.start == self.end {
+            None
+        } else {
+            unsafe {
+                self.end = if mem::size_of::<T>() == 0 {
+                    (self.end as usize - 1) as *const _
+                } else {
+                    self.end.offset(-1);
+                }
+                Some(ptr::read(self.end))
+            }
+        }
+    }
+}
+```
+
+And that's it. Iteration works!