From 602e55dc67b04e97dd96f45f9b80e0b0daa13060 Mon Sep 17 00:00:00 2001
From: bluss <bluss@users.noreply.github.com>
Date: Sun, 25 Mar 2018 23:21:18 +0200
Subject: [PATCH] FEAT: Use drop_in_place for truncate and clear (and drop)

This should perform better in both release and debug mode.

NOTE: This is significant because it changes the drop order of the
elements, and how we handle panicking destructors. If just one of the
destructors panic during ArrayVec drop, clear or truncate, the rest of
the elements should still drop. If we encounter another panic during
that process, however, Rust will abort as usual for panic during
unwinding.
---
 src/lib.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 96e7a24..3f16fc9 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -459,12 +459,18 @@ impl<A: Array> ArrayVec<A> {
     /// assert_eq!(&array[..], &[1, 2, 3]);
     /// ```
     pub fn truncate(&mut self, len: usize) {
-        while self.len() > len { self.pop(); }
+        unsafe {
+            if len < self.len() {
+                let tail: *mut [_] = &mut self[len..];
+                self.set_len(len);
+                ptr::drop_in_place(tail);
+            }
+        }
     }
 
     /// Remove all elements in the vector.
     pub fn clear(&mut self) {
-        while let Some(_) = self.pop() { }
+        self.truncate(0)
     }
 
     /// Retains only the elements specified by the predicate.