vec::append reuses its left hand side when possible. (issue #2719)

src/libcore/vec.rs

@@ -484,10 +484,9 @@ fn push_all_move<T>(&v: [const T]/~, -rhs: [const T]/~) {
 
 // Appending
 #[inline(always)]
-pure fn append<T: copy>(lhs: [T]/&, rhs: [const T]/&) -> [T]/~ {
-    let mut v = []/~;
+pure fn append<T: copy>(+lhs: [T]/~, rhs: [const T]/&) -> [T]/~ {
+    let mut v <- lhs;
     unchecked {
-        push_all(v, lhs);
         push_all(v, rhs);
     }
     ret v;

src/test/bench/core-std.rs

@@ -22,6 +22,7 @@ fn main(argv: [str]/~) {
     #bench[shift_push];
     #bench[read_line];
     #bench[str_set];
+    #bench[vec_plus];
     #bench[vec_append];
     #bench[vec_push_all];
 }
@@ -75,11 +76,12 @@ fn str_set() {
     }
 }
 
-fn vec_append() {
+fn vec_plus() {
     let r = rand::rng();
 
-    let mut v = []/~;
-    for uint::range(0, 1500) {|i|
+    let mut v = []/~; 
+    let mut i = 0;
+    while i < 1500 {
         let rv = vec::from_elem(r.gen_uint_range(0, i + 1), i);
         if r.gen_bool() {
             v += rv;
@@ -87,6 +89,24 @@ fn vec_append() {
         else {
             v = rv + v;
         }
+        i += 1;
+    }
+}
+
+fn vec_append() {
+    let r = rand::rng();
+
+    let mut v = []/~;
+    let mut i = 0;
+    while i < 1500 {
+        let rv = vec::from_elem(r.gen_uint_range(0, i + 1), i);
+        if r.gen_bool() {
+            v = vec::append(v, rv);
+        }
+        else {
+            v = vec::append(rv, v);
+        }
+        i += 1;
     }
 }