Auto merge of #103 - mbrubeck:panic, r=jdm

Panic-safety fixes

* Make from_elem panic-safe.  Fixes #101.
* Make insert_many panic-safe.  Fixes #96.

r? @SimonSapin or @jdm.  cc @Vurich

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Author: bors-servo

lib.rs

@@ -771,24 +771,33 @@ impl<A: Array> SmallVec<A> {
         unsafe {
             let old_len = self.len();
             assert!(index <= old_len);
-            let ptr = self.as_mut_ptr().offset(index as isize);
+            let mut ptr = self.as_mut_ptr().offset(index as isize);
+
+            // Move the trailing elements.
             ptr::copy(ptr, ptr.offset(lower_size_bound as isize), old_len - index);
-            for (off, element) in iter.enumerate() {
-                if off < lower_size_bound {
-                    ptr::write(ptr.offset(off as isize), element);
-                    let len = self.len() + 1;
-                    self.set_len(len);
-                } else {
-                    // Iterator provided more elements than the hint.
-                    assert!(index + off >= index);  // Protect against overflow.
-                    self.insert(index + off, element);
+
+            // In case the iterator panics, don't double-drop the items we just copied above.
+            self.set_len(index);
+
+            let mut num_added = 0;
+            for element in iter {
+                let mut cur = ptr.offset(num_added as isize);
+                if num_added >= lower_size_bound {
+                    // Iterator provided more elements than the hint.  Move trailing items again.
+                    self.reserve(1);
+                    ptr = self.as_mut_ptr().offset(index as isize);
+                    cur = ptr.offset(num_added as isize);
+                    ptr::copy(cur, cur.offset(1), old_len - index);
                 }
+                ptr::write(cur, element);
+                num_added += 1;
             }
-            let num_added = self.len() - old_len;
             if num_added < lower_size_bound {
                 // Iterator provided fewer elements than the hint
                 ptr::copy(ptr.offset(lower_size_bound as isize), ptr.offset(num_added as isize), old_len - index);
             }
+
+            self.set_len(old_len + num_added);
         }
     }
 
@@ -937,19 +946,17 @@ impl<A: Array> SmallVec<A> where A::Item: Clone {
         if n > A::size() {
             vec![elem; n].into()
         } else {
+            let mut v = SmallVec::<A>::new();
             unsafe {
-                let mut arr: A = ::std::mem::uninitialized();
-                let ptr = arr.ptr_mut();
+                let (ptr, len_ptr, _) = v.triple_mut();
+                let mut local_len = SetLenOnDrop::new(len_ptr);
 
                 for i in 0..n as isize {
                     ::std::ptr::write(ptr.offset(i), elem.clone());
-                }
-
-                SmallVec {
-                    capacity: n,
-                    data: SmallVecData::from_inline(arr),
+                    local_len.increment_len(1);
                 }
             }
+            v
         }
     }
 }
@@ -1337,6 +1344,33 @@ pub unsafe trait Array {
     fn ptr_mut(&mut self) -> *mut Self::Item;
 }
 
+/// Set the length of the vec when the `SetLenOnDrop` value goes out of scope.
+///
+/// Copied from https://github.com/rust-lang/rust/pull/36355
+struct SetLenOnDrop<'a> {
+    len: &'a mut usize,
+    local_len: usize,
+}
+
+impl<'a> SetLenOnDrop<'a> {
+    #[inline]
+    fn new(len: &'a mut usize) -> Self {
+        SetLenOnDrop { local_len: *len, len: len }
+    }
+
+    #[inline]
+    fn increment_len(&mut self, increment: usize) {
+        self.local_len += increment;
+    }
+}
+
+impl<'a> Drop for SetLenOnDrop<'a> {
+    #[inline]
+    fn drop(&mut self) {
+        *self.len = self.local_len;
+    }
+}
+
 macro_rules! impl_array(
     ($($size:expr),+) => {
         $(
@@ -1645,6 +1679,37 @@ mod tests {
         assert_eq!(&v.iter().map(|v| *v).collect::<Vec<_>>(), &[0, 5, 6, 1, 2, 3]);
     }
 
+    #[test]
+    // https://github.com/servo/rust-smallvec/issues/96
+    fn test_insert_many_panic() {
+        struct PanicOnDoubleDrop {
+            dropped: Box<bool>
+        }
+
+        impl Drop for PanicOnDoubleDrop {
+            fn drop(&mut self) {
+                assert!(!*self.dropped, "already dropped");
+                *self.dropped = true;
+            }
+        }
+
+        struct BadIter;
+        impl Iterator for BadIter {
+            type Item = PanicOnDoubleDrop;
+            fn size_hint(&self) -> (usize, Option<usize>) { (1, None) }
+            fn next(&mut self) -> Option<Self::Item> { panic!() }
+        }
+
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> = vec![
+            PanicOnDoubleDrop { dropped: Box::new(false) },
+            PanicOnDoubleDrop { dropped: Box::new(false) },
+        ].into();
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(0, BadIter);
+        });
+        assert!(result.is_err());
+    }
+
     #[test]
     #[should_panic]
     fn test_invalid_grow() {