From eed51be53ba3137dfcbf77a0d8f4ee9667ca5a80 Mon Sep 17 00:00:00 2001
From: Andreas Molzer <andreas.molzer@gmx.de>
Date: Sat, 18 Jan 2020 16:05:13 +0100
Subject: [PATCH 1/3] Switch to slice swapping for split

The new slice is already initialized with default values for T due to
the constructor used. Instead of using different defaulted values for
replacing the split-off instances we can simply swap slices. This
reduces pressure on the optimizer and uses a probably optimized method
from the core library.
---
 src/arrayvec.rs | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/src/arrayvec.rs b/src/arrayvec.rs
index 608312c..3b86dd5 100644
--- a/src/arrayvec.rs
+++ b/src/arrayvec.rs
@@ -544,11 +544,10 @@ impl<A: Array> ArrayVec<A> {
     }
     let mut new = Self::default();
     let moves = &mut self.as_mut_slice()[at..];
-    let targets = new.data.as_slice_mut();
-    for (m, t) in moves.iter_mut().zip(targets) {
-      replace(t, replace(m, A::Item::default()));
-    }
-    new.len = self.len - at;
+    let split_len = moves.len();
+    let targets = &mut new.data.as_slice_mut()[..split_len];
+    moves.swap_with_slice(targets);
+    new.len = split_len;
     self.len = at;
     new
   }

From 381eacaa461717c73a3d3a4c604ac108cc3fc4d2 Mon Sep 17 00:00:00 2001
From: Andreas Molzer <andreas.molzer@gmx.de>
Date: Sat, 18 Jan 2020 16:10:12 +0100
Subject: [PATCH 2/3] Use mem::take shorthand for defaulted replace

This makes the code more readable by shortening lines and avoid explicit
or redundant type annotations. The core function would require a rust
version of 1.40 so straightforward implementation is added to the crate.
---
 src/arrayvec.rs | 10 ++++------
 src/lib.rs      |  5 +++++
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/src/arrayvec.rs b/src/arrayvec.rs
index 3b86dd5..c9285ca 100644
--- a/src/arrayvec.rs
+++ b/src/arrayvec.rs
@@ -343,8 +343,7 @@ impl<A: Array> ArrayVec<A> {
   pub fn pop(&mut self) -> Option<A::Item> {
     if self.len > 0 {
       self.len -= 1;
-      let out =
-        replace(&mut self.data.as_slice_mut()[self.len], A::Item::default());
+      let out = take(&mut self.data.as_slice_mut()[self.len]);
       Some(out)
     } else {
       None
@@ -759,8 +758,7 @@ impl<A: Array> Iterator for ArrayVecIterator<A> {
   #[inline]
   fn next(&mut self) -> Option<Self::Item> {
     if self.base < self.len {
-      let out =
-        replace(&mut self.data.as_slice_mut()[self.base], A::Item::default());
+      let out = take(&mut self.data.as_slice_mut()[self.base]);
       self.base += 1;
       Some(out)
     } else {
@@ -779,13 +777,13 @@ impl<A: Array> Iterator for ArrayVecIterator<A> {
   }
   #[inline]
   fn last(mut self) -> Option<Self::Item> {
-    Some(replace(&mut self.data.as_slice_mut()[self.len], A::Item::default()))
+    Some(take(&mut self.data.as_slice_mut()[self.len]))
   }
   #[inline]
   fn nth(&mut self, n: usize) -> Option<A::Item> {
     let i = self.base + (n - 1);
     if i < self.len {
-      let out = replace(&mut self.data.as_slice_mut()[i], A::Item::default());
+      let out = take(&mut self.data.as_slice_mut()[i]);
       self.base = i + 1;
       Some(out)
     } else {
diff --git a/src/lib.rs b/src/lib.rs
index 2100b1e..7bc34a1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -84,3 +84,8 @@ pub use arrayvec::*;
 mod tinyvec;
 #[cfg(feature = "alloc")]
 pub use tinyvec::*;
+
+// Replace with mem::take as soon as MSRV allows it
+fn take<T: Default>(from: &mut T) -> T {
+    replace(from, T::default())
+}

From c21132b4953274168ec879a882b1fad9eeadaf14 Mon Sep 17 00:00:00 2001
From: Andreas Molzer <andreas.molzer@gmx.de>
Date: Sat, 18 Jan 2020 16:23:35 +0100
Subject: [PATCH 3/3] Simplify resize with element or function

Avoids a call to Clone of the element by using the provided instance as
the one inserted last. A similar optimization is used in the standard
library. Also changes the iteration to an independent iterator as the
number of insert elements must be known in advance.
---
 src/arrayvec.rs | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/src/arrayvec.rs b/src/arrayvec.rs
index c9285ca..1c007d9 100644
--- a/src/arrayvec.rs
+++ b/src/arrayvec.rs
@@ -414,14 +414,14 @@ impl<A: Array> ArrayVec<A> {
   where
     A::Item: Clone,
   {
-    use core::cmp::Ordering;
-    match new_len.cmp(&self.len) {
-      Ordering::Less => self.truncate(new_len),
-      Ordering::Equal => (),
-      Ordering::Greater => {
-        while self.len < new_len {
+    match new_len.checked_sub(self.len) {
+      None => self.truncate(new_len),
+      Some(0) => (),
+      Some(new_elements) => {
+        for _ in 1..new_elements {
           self.push(new_val.clone());
         }
+        self.push(new_val);
       }
     }
   }
@@ -454,12 +454,10 @@ impl<A: Array> ArrayVec<A> {
     new_len: usize,
     mut f: F,
   ) {
-    use core::cmp::Ordering;
-    match new_len.cmp(&self.len) {
-      Ordering::Less => self.truncate(new_len),
-      Ordering::Equal => (),
-      Ordering::Greater => {
-        while self.len < new_len {
+    match new_len.checked_sub(self.len) {
+      None => self.truncate(new_len),
+      Some(new_elements) => {
+        for _ in 0..new_elements {
           self.push(f());
         }
       }