Add a _vec.slice function that'll hold us over until .(a,b) syntax is

implemented.  This could actually replace .(a,b) syntax if the language grows
optional function parameters.

Author: jyasskin

Diff for: src/lib/_vec.rs

@@ -48,6 +48,21 @@ fn buf[T](vec[T] v) -> vbuf {
   ret rustrt.vec_buf[T](v);
 }
 
+// Returns elements from [start..end) from v.
+fn slice[T](vec[T] v, int start, int end) -> vec[T] {
+  check(0 <= start);
+  check(start <= end);
+  // FIXME #108: This doesn't work yet.
+  //check(end <= int(len[T](v)));
+  auto result = alloc[T](uint(end - start));
+  let mutable int i = start;
+  while (i < end) {
+    result += vec(v.(i));
+    i += 1;
+  }
+  ret result;
+}
+
 // Ought to take mutable &vec[T] v and just mutate it instead of copy
 // and return.  Blocking on issue #89 for this.
 fn grow[T](mutable vec[T] v, int n, T initval) -> vec[T] {

Diff for: src/test/run-pass/vec-lib.rs

@@ -24,7 +24,18 @@ fn test_init_fn() {
   check (v.(4) == uint(4));
 }
 
+fn test_slice() {
+  let vec[int] v = vec(1,2,3,4,5);
+  auto v2 = std._vec.slice[int](v, 2, 4);
+  // FIXME #108: Can't call templated function twice in the same
+  // program, at the moment.
+  //check (std._vec.len[int](v2) == uint(2));
+  check (v2.(0) == 3);
+  check (v2.(1) == 4);
+}
+
 fn main() {
   test_init_elt();
   //XFAIL: test_init_fn();  // Segfaults.
-}
+  test_slice();
+}