From 91e88a4976cf4d61026a3ed8d7589cb18f8e9d18 Mon Sep 17 00:00:00 2001
From: bluss <bluss@users.noreply.github.com>
Date: Sun, 25 Nov 2018 16:33:07 +0100
Subject: [PATCH 1/3] FEAT: Simplify stack guard in extend

This simplification -- borrowing self.len instead of self, leads to
an improvement in the extend_from_slice benchmark.

It's also guided by the discussion of stacked borrows; the old code
would be invalid, because the whole self is borrowed while ptr is derived from
self.
---
 src/lib.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 75860dd0..fb40456a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -890,10 +890,10 @@ impl<A: Array> Extend<A::Item> for ArrayVec<A> {
             // We update the length to handle panic in the iteration of the
             // user's iterator, without dropping any elements on the floor.
             let mut guard = ScopeExitGuard {
-                value: self,
+                value: &mut self.len,
                 data: len,
-                f: |&len, self_| {
-                    self_.set_len(len)
+                f: move |&len, self_len| {
+                    **self_len = Index::from(len);
                 }
             };
             for elt in iter.into_iter().take(take) {

From 23128275ee9b4c2fe5f14914c506a823c7685572 Mon Sep 17 00:00:00 2001
From: bluss <bluss@users.noreply.github.com>
Date: Sun, 25 Nov 2018 16:34:47 +0100
Subject: [PATCH 2/3] TEST: Update benchmark for extend_from_slice

The benchmark was optimized out totally. We think of that as a good
sign, the new extend became transparent to the compiler and we had to
get smarter in how to fool it.
---
 benches/extend.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/benches/extend.rs b/benches/extend.rs
index d380a7ed..0ae7ce32 100644
--- a/benches/extend.rs
+++ b/benches/extend.rs
@@ -5,6 +5,7 @@ extern crate arrayvec;
 use arrayvec::ArrayVec;
 
 use bencher::Bencher;
+use bencher::black_box;
 
 fn extend_with_constant(b: &mut Bencher) {
     let mut v = ArrayVec::<[u8; 512]>::new();
@@ -33,7 +34,7 @@ fn extend_with_slice(b: &mut Bencher) {
     let data = [1; 512];
     b.iter(|| {
         v.clear();
-        v.extend(data.iter().cloned());
+        v.extend(black_box(data.iter()).cloned());
         v[0]
     });
     b.bytes = v.capacity() as u64;

From f40e708f7c2afa6fcd4a3fc509916757e053d142 Mon Sep 17 00:00:00 2001
From: bluss <bluss@users.noreply.github.com>
Date: Sun, 25 Nov 2018 16:37:29 +0100
Subject: [PATCH 3/3] FIX: In truncate, don't access self while holding raw
 pointer derived from self

Again, stacked borrows model makes the `self.set_len()` call illegal
because we are holding (and are going to use) another raw pointer
derived from self, `tail`.
---
 src/lib.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index fb40456a..e40b0a5d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -461,11 +461,11 @@ impl<A: Array> ArrayVec<A> {
     /// array.truncate(4);
     /// assert_eq!(&array[..], &[1, 2, 3]);
     /// ```
-    pub fn truncate(&mut self, len: usize) {
+    pub fn truncate(&mut self, new_len: usize) {
         unsafe {
-            if len < self.len() {
-                let tail: *mut [_] = &mut self[len..];
-                self.set_len(len);
+            if new_len < self.len() {
+                let tail: *mut [_] = &mut self[new_len..];
+                self.len = Index::from(new_len);
                 ptr::drop_in_place(tail);
             }
         }