Rollup merge of #67300 - aloucks:issue-65970, r=rkruppe

Restore original implementation of Vec::retain

This PR reverts #48065, which aimed to optimize `Vec::retain` by making use of `Vec::drain_filter`. Unfortunately at that time, `drain_filter` was unsound.

The soundness hole in `Vec::drain_filter` was fixed in #61224 by guaranteeing that cleanup logic runs via a nested `Drop`, even in the event of a panic. Implementing this nested drop affects codegen (apparently?) and results in slower code.

Fixes #65970

Author: Centril

src/liballoc/vec.rs

@@ -1079,7 +1079,22 @@ impl<T> Vec<T> {
     pub fn retain<F>(&mut self, mut f: F)
         where F: FnMut(&T) -> bool
     {
-        self.drain_filter(|x| !f(x));
+        let len = self.len();
+        let mut del = 0;
+        {
+            let v = &mut **self;
+
+            for i in 0..len {
+                if !f(&v[i]) {
+                    del += 1;
+                } else if del > 0 {
+                    v.swap(i - del, i);
+                }
+            }
+        }
+        if del > 0 {
+            self.truncate(len - del);
+        }
     }
 
     /// Removes all but the first of consecutive elements in the vector that resolve to the same