Revert "Add docs and debug asserts"

This reverts commit 1a1a7f61.

src/liballoc/collections/vec_deque.rs

@@ -202,17 +202,10 @@ unsafe fn copy_nonoverlapping(&self, dst: usize, src: usize, len: usize) {
                                  len);
     }
 
-    /// Copies all values from `src` to the back of `self`, wrapping around if needed.
-    ///
-    /// # Safety
-    ///
-    /// The capacity must be sufficient to hold self.len() + src.len() elements.
-    /// If so, this function never panics.
+    /// Copies all values from `src` to `self`, wrapping around if needed.
+    /// Assumes capacity is sufficient.
     #[inline]
     unsafe fn copy_slice(&mut self, src: &[T]) {
-        let expected_new_len = self.len() + src.len();
-        debug_assert!(self.capacity() >= expected_new_len);
-
         let dst_high_ptr = self.ptr().add(self.head);
         let dst_high_len = self.cap() - self.head;
 
@@ -223,7 +216,6 @@ unsafe fn copy_slice(&mut self, src: &[T]) {
         ptr::copy_nonoverlapping(src_low.as_ptr(), self.ptr(), src_low.len());
 
         self.head = self.wrap_add(self.head, src.len());
-        debug_assert!(self.len() == expected_new_len);
     }
 
     /// Copies a potentially wrapping block of memory len long from src to dest.
@@ -1858,21 +1850,17 @@ pub fn split_off(&mut self, at: usize) -> Self {
     #[inline]
     #[stable(feature = "append", since = "1.4.0")]
     pub fn append(&mut self, other: &mut Self) {
-        unsafe {
-            // Guarantees there is space in `self` for `other`.
-            self.reserve(other.len());
-
-            {
-                let (src_high, src_low) = other.as_slices();
+        // Guarantees there is space in `self` for `other
+        self.reserve(other.len());
 
-                // This is only safe because copy_slice never panics when capacity is sufficient.
-                self.copy_slice(src_low);
-                self.copy_slice(src_high);
-            }
-
-            // Some values now exist in both `other` and `self` but are made inaccessible in `other`.
-            other.tail = other.head;
+        unsafe {
+            let (src_high, src_low) = other.as_slices();
+            self.copy_slice(src_low);
+            self.copy_slice(src_high);
         }
+
+        // Some values now exist in both `other` and `self` but are made inaccessible in `other`.
+        other.tail = other.head;
     }
 
     /// Retains only the elements specified by the predicate.