Use NoDrop to fix panic safety issues

ArrayVec::drop was not panic safe — if there would be a panic during an
element's drop, the discriminant would never be set to Dropped, and the
array elements would potentially double drop.

Fix this by going back to the old NoDrop composition. The NoDrop struct
thas its own Drop impl, that will trigger too on panic during an element's
drop. This serves to make ArrayVec::drop panic safe.

Also tweak IntoIter::drop to make it panic safe: set inner ArrayVec's
length before dropping any elements.

Thank you to @Stebalien for reporting this bug and providing the
excellent testcases in this commit.

Using NoDrop expands ArrayVec to have two drop flags again, but this
is a temporary tradeoff, drop flags will eventually go away.

Fixes #3

src/lib.rs

@@ -1,4 +1,5 @@
 extern crate odds;
+extern crate nodrop;
 
 use std::iter;
 use std::mem;
@@ -9,13 +10,13 @@ use std::ops::{
 };
 use std::slice;
 
+use nodrop::NoDrop;
+
 // extra traits
 use std::borrow::{Borrow, BorrowMut};
 use std::hash::{Hash, Hasher};
 use std::fmt;
 
-use odds::debug_assert_unreachable;
-
 mod array;
 pub use array::Array;
 pub use odds::IndexRange as RangeArgument;
@@ -30,13 +31,6 @@ unsafe fn new_array<A: Array>() -> A {
     mem::uninitialized()
 }
 
-/// repr(u8) - Make sure the non-nullable pointer optimization does not occur!
-#[repr(u8)]
-enum NoDrop<T> {
-    Alive(T),
-    Dropped,
-}
-
 /// A vector with a fixed capacity.
 ///
 /// The **ArrayVec** is a vector backed by a fixed size array. It keeps track of
@@ -57,12 +51,12 @@ pub struct ArrayVec<A: Array> {
 impl<A: Array> Drop for ArrayVec<A> {
     fn drop(&mut self) {
         // clear all elements
-        while let Some(_) = self.pop() { }
-
-        // inhibit drop
-        unsafe {
-            ptr::write(&mut self.xs, NoDrop::Dropped);
+        while let Some(_) = self.pop() {
         }
+
+        // NoDrop inhibits array's drop
+        // panic safety: NoDrop::drop will trigger on panic, so the inner
+        // array will not drop even after panic.
     }
 }
 
@@ -84,7 +78,7 @@ impl<A: Array> ArrayVec<A> {
     /// ```
     pub fn new() -> ArrayVec<A> {
         unsafe {
-            ArrayVec { xs: NoDrop::Alive(new_array()), len: Index::zero() }
+            ArrayVec { xs: NoDrop::new(new_array()), len: Index::zero() }
         }
     }
 
@@ -375,31 +369,6 @@ impl<A: Array> DerefMut for ArrayVec<A> {
     }
 }
 
-impl<T> Deref for NoDrop<T> {
-    type Target = T;
-
-    // Use type invariant, always Alive.
-    #[inline]
-    fn deref(&self) -> &T {
-        match *self {
-            NoDrop::Alive(ref inner) => inner,
-            _ => unsafe { debug_assert_unreachable() }
-        }
-    }
-}
-
-impl<T> DerefMut for NoDrop<T> {
-    // Use type invariant, always Alive.
-    #[inline]
-    fn deref_mut(&mut self) -> &mut T {
-        match *self {
-            NoDrop::Alive(ref mut inner) => inner,
-            _ => unsafe { debug_assert_unreachable() }
-        }
-    }
-}
-
-
 /// Create an **ArrayVec** from an array.
 ///
 /// ## Examples
@@ -412,7 +381,7 @@ impl<T> DerefMut for NoDrop<T> {
 /// ```
 impl<A: Array> From<A> for ArrayVec<A> {
     fn from(array: A) -> Self {
-        ArrayVec { xs: NoDrop::Alive(array), len: Index::from(A::capacity()) }
+        ArrayVec { xs: NoDrop::new(array), len: Index::from(A::capacity()) }
     }
 }
 
@@ -524,10 +493,16 @@ impl<A: Array> ExactSizeIterator for IntoIter<A> { }
 
 impl<A: Array> Drop for IntoIter<A> {
     fn drop(&mut self) {
-        // exhaust iterator and clear the vector
-        while let Some(_) = self.next() { }
+        // panic safety: Set length to 0 before dropping elements.
+        let index = self.index.to_usize();
+        let len = self.v.len();
         unsafe {
             self.v.set_len(0);
+            let elements = slice::from_raw_parts(self.v.get_unchecked_mut(index),
+                                                 len - index);
+            for elt in elements {
+                ptr::read(elt);
+            }
         }
     }
 }
@@ -588,6 +563,8 @@ impl<'a, A: Array> Drop for Drain<'a, A>
     where A::Item: 'a
 {
     fn drop(&mut self) {
+        // len is currently 0 so panicking while dropping will not cause a double drop.
+
         // exhaust self first
         while let Some(_) = self.next() { }