Rollup merge of rust-lang#61447 - scottmcm:vec-vecdeque, r=sfackler

Add some Vec <-> VecDeque documentation

These are more than just `.into_iter().collect()`, so talk about some of their nuances.

For VecDeque -> Vec I'm trying to intentionally not write a guarantee for people making their own `Vec`s, since the rules are more complicated than I think we want to commit to forever.

The "Vec -> VecDeque doesn't reallocate" guarantee seems reasonable, though.  (And I'm intentionally ambiguous about when it's O(1) instead of O(n).)

Author: Centril

src/liballoc/collections/vec_deque.rs

@@ -2709,6 +2709,11 @@ impl<T: fmt::Debug> fmt::Debug for VecDeque<T> {
 
 #[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
 impl<T> From<Vec<T>> for VecDeque<T> {
+    /// Turn a [`Vec<T>`] into a [`VecDeque<T>`].
+    ///
+    /// This avoids reallocating where possible, but the conditions for that are
+    /// strict, and subject to change, and so shouldn't be relied upon unless the
+    /// `Vec<T>` came from `From<VecDeque<T>>` and hasn't been reallocated.
     fn from(mut other: Vec<T>) -> Self {
         unsafe {
             let other_buf = other.as_mut_ptr();
@@ -2735,6 +2740,32 @@ impl<T> From<Vec<T>> for VecDeque<T> {
 
 #[stable(feature = "vecdeque_vec_conversions", since = "1.10.0")]
 impl<T> From<VecDeque<T>> for Vec<T> {
+    /// Turn a [`VecDeque<T>`] into a [`Vec<T>`].
+    ///
+    /// This never needs to re-allocate, but does need to do O(n) data movement if
+    /// the circular buffer doesn't happen to be at the beginning of the allocation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// // This one is O(1).
+    /// let deque: VecDeque<_> = (1..5).collect();
+    /// let ptr = deque.as_slices().0.as_ptr();
+    /// let vec = Vec::from(deque);
+    /// assert_eq!(vec, [1, 2, 3, 4]);
+    /// assert_eq!(vec.as_ptr(), ptr);
+    ///
+    /// // This one needs data rearranging.
+    /// let mut deque: VecDeque<_> = (1..5).collect();
+    /// deque.push_front(9);
+    /// deque.push_front(8);
+    /// let ptr = deque.as_slices().1.as_ptr();
+    /// let vec = Vec::from(deque);
+    /// assert_eq!(vec, [8, 9, 1, 2, 3, 4]);
+    /// assert_eq!(vec.as_ptr(), ptr);
+    /// ```
     fn from(other: VecDeque<T>) -> Self {
         unsafe {
             let buf = other.buf.ptr();