---

yaml
---
r: 212927
b: refs/heads/master
c: 032804b
h: refs/heads/master
i:
  212925: f6cf9b2
  212923: b83b2c1
  212919: cd5f062
  212911: 586e3c0
  212895: 3473940
  212863: 9addb74
v: v3

Author: mzabaluev

[refs]

@@ -1,5 +1,5 @@
 ---
-refs/heads/master: a118b936faa7f802c41214cb908622a7893bebfd
+refs/heads/master: 032804bf68e2ac13add895206f2173409acff836
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: ba0e1cd8147d452c356aacb29fb87568ca26f111
 refs/heads/try: 1864973ae17213c5a58c4dd3f9af6d1b6c7d2e05

trunk/src/libcollections/vec.rs

@@ -1410,7 +1410,25 @@ impl<T> ops::DerefMut for Vec<T> {
 impl<T> FromIterator<T> for Vec<T> {
     #[inline]
     fn from_iter<I: IntoIterator<Item=T>>(iterable: I) -> Vec<T> {
-        let mut vector = Vec::new();
+        // Unroll the first iteration, as the vector is going to be
+        // expanded on this iteration in every case when the iterable is not
+        // empty, but the loop in extend_desugared() is not going to see the
+        // vector being full in the few subsequent loop iterations.
+        // So we get better branch prediction and the possibility to
+        // construct the vector with initial estimated capacity.
+        let mut iterator = iterable.into_iter();
+        let mut vector = match iterator.next() {
+            None => return Vec::new(),
+            Some(element) => {
+                let (lower, _) = iterator.size_hint();
+                let mut vector = Vec::with_capacity(1 + lower);
+                unsafe {
+                    ptr::write(vector.get_unchecked_mut(0), element);
+                    vector.set_len(1);
+                }
+                vector
+            }
+        };
         vector.extend(iterable);
         vector
     }