From 180dcc3118998ebbe390f876df51f85f1b33407a Mon Sep 17 00:00:00 2001
From: Nathan West <Lucretiel@gmail.com>
Date: Wed, 5 Dec 2018 14:37:38 -0800
Subject: [PATCH 1/3] Optimized string FromIterator impls

---
 src/liballoc/string.rs | 41 +++++++++++++++++++++++------------------
 1 file changed, 23 insertions(+), 18 deletions(-)

diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index 662f8ae614fcb..ef9a34ec611c6 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1732,18 +1732,31 @@ impl<'a> FromIterator<&'a str> for String {
 #[stable(feature = "extend_string", since = "1.4.0")]
 impl FromIterator<String> for String {
     fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
-        let mut buf = String::new();
-        buf.extend(iter);
-        buf
+        let iterator = iter.into_iter();
+
+        match iterator.next() {
+            None => String::new(),
+            Some(buf) => {
+                buf.extend(iterator);
+                buf
+            }
+        }
     }
 }
 
 #[stable(feature = "herd_cows", since = "1.19.0")]
 impl<'a> FromIterator<Cow<'a, str>> for String {
     fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
-        let mut buf = String::new();
-        buf.extend(iter);
-        buf
+        let iterator = iter.into_iter();
+
+        match iterator.next() {
+            None => String::new(),
+            Some(cow) => {
+                let buf = cow.into_owned();
+                buf.extend(iterator);
+                buf
+            }
+        }
     }
 }
 
@@ -1753,9 +1766,7 @@ impl Extend<char> for String {
         let iterator = iter.into_iter();
         let (lower_bound, _) = iterator.size_hint();
         self.reserve(lower_bound);
-        for ch in iterator {
-            self.push(ch)
-        }
+        iterator.for_each(move |c| self.push(c));
     }
 }
 
@@ -1769,27 +1780,21 @@ impl<'a> Extend<&'a char> for String {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> Extend<&'a str> for String {
     fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
-        for s in iter {
-            self.push_str(s)
-        }
+        iter.into_iter().for_each(move |s| self.push_str(s));
     }
 }
 
 #[stable(feature = "extend_string", since = "1.4.0")]
 impl Extend<String> for String {
     fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
-        for s in iter {
-            self.push_str(&s)
-        }
+        iter.into_iter().for_each(move |s| self.push_str(&s));
     }
 }
 
 #[stable(feature = "herd_cows", since = "1.19.0")]
 impl<'a> Extend<Cow<'a, str>> for String {
     fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
-        for s in iter {
-            self.push_str(&s)
-        }
+        iter.into_iter().for_each(move |s| self.push_str(&s));
     }
 }
 

From 823dd8ca334951962e8b192ca1362c08e33d6bcf Mon Sep 17 00:00:00 2001
From: Nathan West <Lucretiel@gmail.com>
Date: Wed, 5 Dec 2018 15:11:32 -0800
Subject: [PATCH 2/3] Fixed mutability

---
 src/liballoc/string.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index ef9a34ec611c6..6962f2ba85296 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1732,11 +1732,11 @@ impl<'a> FromIterator<&'a str> for String {
 #[stable(feature = "extend_string", since = "1.4.0")]
 impl FromIterator<String> for String {
     fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
-        let iterator = iter.into_iter();
+        let mut iterator = iter.into_iter();
 
         match iterator.next() {
             None => String::new(),
-            Some(buf) => {
+            Some(mut buf) => {
                 buf.extend(iterator);
                 buf
             }
@@ -1747,12 +1747,12 @@ impl FromIterator<String> for String {
 #[stable(feature = "herd_cows", since = "1.19.0")]
 impl<'a> FromIterator<Cow<'a, str>> for String {
     fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
-        let iterator = iter.into_iter();
+        let mut iterator = iter.into_iter();
 
         match iterator.next() {
             None => String::new(),
             Some(cow) => {
-                let buf = cow.into_owned();
+                let mut buf = cow.into_owned();
                 buf.extend(iterator);
                 buf
             }

From 811a2bfe5332081d7145de6c488ea7f6c5cf42a5 Mon Sep 17 00:00:00 2001
From: Nathan West <Lucretiel@users.noreply.github.com>
Date: Wed, 5 Dec 2018 17:46:03 -0800
Subject: [PATCH 3/3] Added explainatory comments

---
 src/liballoc/string.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/liballoc/string.rs b/src/liballoc/string.rs
index 6962f2ba85296..f0a64f4631a00 100644
--- a/src/liballoc/string.rs
+++ b/src/liballoc/string.rs
@@ -1734,6 +1734,9 @@ impl FromIterator<String> for String {
     fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
         let mut iterator = iter.into_iter();
 
+        // Because we're iterating over `String`s, we can avoid at least
+        // one allocation by getting the first string from the iterator
+        // and appending to it all the subsequent strings.
         match iterator.next() {
             None => String::new(),
             Some(mut buf) => {
@@ -1749,6 +1752,9 @@ impl<'a> FromIterator<Cow<'a, str>> for String {
     fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
         let mut iterator = iter.into_iter();
 
+        // Because we're iterating over CoWs, we can (potentially) avoid at least
+        // one allocation by getting the first item and appending to it all the
+        // subsequent items.
         match iterator.next() {
             None => String::new(),
             Some(cow) => {