From 7ea6c46a6d70506e92630f0186f2c6b0386147a0 Mon Sep 17 00:00:00 2001
From: Aaron Loucks <aloucks@cofront.net>
Date: Sat, 14 Dec 2019 12:38:45 -0500
Subject: [PATCH] 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
---
 src/liballoc/vec.rs | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index 6e165ccb91998..e5abfdd52022a 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -1075,7 +1075,22 @@ 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