Rollup merge of rust-lang#90772 - GuillaumeGomez:vec-retain-mut, r=joshtriplett

Add Vec::retain_mut

This is to continue the discussion started in rust-lang#83218.

Original comment was:

> Take 2 of rust-lang#34265, since I needed this today.

The reason I think why we should add `retain_mut` is for coherency and for discoverability. For example we have `chunks` and `chunks_mut` or `get` and `get_mut` or `iter` and `iter_mut`, etc. When looking for mutable `retain`, I would expect `retain_mut` to exist. It took me a while to find out about `drain_filter`. So even if it provides an API close to `drain_filter`, just for the discoverability, I think it's worth it.

cc ````@m-ou-se```` ````@jonas-schievink```` ````@Mark-Simulacrum````

Author: matthiaskrgr

library/alloc/src/vec/mod.rs

@@ -1444,6 +1444,34 @@ impl<T, A: Allocator> Vec<T, A> {
     pub fn retain<F>(&mut self, mut f: F)
     where
         F: FnMut(&T) -> bool,
+    {
+        self.retain_mut(|elem| f(elem));
+    }
+
+    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
+    ///
+    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
+    /// This method operates in place, visiting each element exactly once in the
+    /// original order, and preserves the order of the retained elements.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_retain_mut)]
+    ///
+    /// let mut vec = vec![1, 2, 3, 4];
+    /// vec.retain_mut(|x| if *x > 3 {
+    ///     false
+    /// } else {
+    ///     *x += 1;
+    ///     true
+    /// });
+    /// assert_eq!(vec, [2, 3, 4]);
+    /// ```
+    #[unstable(feature = "vec_retain_mut", issue = "90829")]
+    pub fn retain_mut<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&mut T) -> bool,
     {
         let original_len = self.len();
         // Avoid double drop if the drop guard is not executed,
@@ -1496,7 +1524,7 @@ impl<T, A: Allocator> Vec<T, A> {
             g: &mut BackshiftOnDrop<'_, T, A>,
         ) -> bool
         where
-            F: FnMut(&T) -> bool,
+            F: FnMut(&mut T) -> bool,
         {
             // SAFETY: Unchecked element must be valid.
             let cur = unsafe { &mut *g.v.as_mut_ptr().add(g.processed_len) };