Auto merge of #60396 - cuviper:ordered-retain, r=scottmcm

Document the order of {Vec,VecDeque,String}::retain

It's natural for `retain` to work in order from beginning to end, but
this wasn't actually documented to be the case. If we actually promise
this, then the caller can do useful things like track the index of each
element being tested, as [discussed in the forum][1]. This is now
documented for `Vec`, `VecDeque`, and `String`.

[1]: https://users.rust-lang.org/t/vec-retain-by-index/27697

`HashMap` and `HashSet` also have `retain`, and the `hashbrown`
implementation does happen to use a plain `iter()` order too, but it's
not certain that this should always be the case for these types.

r? @scottmcm

src/liballoc/collections/vec_deque.rs

@@ -1835,8 +1835,8 @@ impl<T> VecDeque<T> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns false.
-    /// This method operates in place and preserves the order of the retained
-    /// elements.
+    /// This method operates in place, visiting each element exactly once in the
+    /// original order, and preserves the order of the retained elements.
     ///
     /// # Examples
     ///
@@ -1848,6 +1848,20 @@ impl<T> VecDeque<T> {
     /// buf.retain(|&x| x%2 == 0);
     /// assert_eq!(buf, [2, 4]);
     /// ```
+    ///
+    /// The exact order may be useful for tracking external state, like an index.
+    ///
+    /// ```
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.extend(1..6);
+    ///
+    /// let keep = [false, true, true, false, true];
+    /// let mut i = 0;
+    /// buf.retain(|_| (keep[i], i += 1).0);
+    /// assert_eq!(buf, [2, 3, 5]);
+    /// ```
     #[stable(feature = "vec_deque_retain", since = "1.4.0")]
     pub fn retain<F>(&mut self, mut f: F)
         where F: FnMut(&T) -> bool

src/liballoc/string.rs

@@ -1200,8 +1200,8 @@ impl String {
     /// Retains only the characters specified by the predicate.
     ///
     /// In other words, remove all characters `c` such that `f(c)` returns `false`.
-    /// This method operates in place and preserves the order of the retained
-    /// characters.
+    /// This method operates in place, visiting each character exactly once in the
+    /// original order, and preserves the order of the retained characters.
     ///
     /// # Examples
     ///
@@ -1212,6 +1212,16 @@ impl String {
     ///
     /// assert_eq!(s, "foobar");
     /// ```
+    ///
+    /// The exact order may be useful for tracking external state, like an index.
+    ///
+    /// ```
+    /// let mut s = String::from("abcde");
+    /// let keep = [false, true, true, false, true];
+    /// let mut i = 0;
+    /// s.retain(|_| (keep[i], i += 1).0);
+    /// assert_eq!(s, "bce");
+    /// ```
     #[inline]
     #[stable(feature = "string_retain", since = "1.26.0")]
     pub fn retain<F>(&mut self, mut f: F)

src/liballoc/vec.rs

@@ -937,8 +937,8 @@ impl<T> Vec<T> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
-    /// This method operates in place and preserves the order of the retained
-    /// elements.
+    /// This method operates in place, visiting each element exactly once in the
+    /// original order, and preserves the order of the retained elements.
     ///
     /// # Examples
     ///
@@ -947,6 +947,16 @@ impl<T> Vec<T> {
     /// vec.retain(|&x| x%2 == 0);
     /// assert_eq!(vec, [2, 4]);
     /// ```
+    ///
+    /// The exact order may be useful for tracking external state, like an index.
+    ///
+    /// ```
+    /// let mut vec = vec![1, 2, 3, 4, 5];
+    /// let keep = [false, true, true, false, true];
+    /// let mut i = 0;
+    /// vec.retain(|_| (keep[i], i += 1).0);
+    /// assert_eq!(vec, [2, 3, 5]);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn retain<F>(&mut self, mut f: F)
         where F: FnMut(&T) -> bool