Introduce str_slice runtime function

This reduces the time to execute the new lib-str tests from 1:40ish to a few
seconds and will eventually allow the full lib-sha1 test to run in a
reasonable amount of time. XFAIL lib-str in stage0 - it will run very slowly
until the next snapshot.

src/lib/Str.rs

@@ -12,6 +12,7 @@ native "rust" mod rustrt {
     fn str_from_cstr(sbuf cstr) -> str;
     fn str_from_buf(sbuf buf, uint len) -> str;
     fn str_push_byte(str s, uint byte) -> str;
+    fn str_slice(str s, uint begin, uint end) -> str;
     fn refcount[T](str s) -> uint;
 }
 
@@ -384,13 +385,10 @@ fn substr(str s, uint begin, uint len) -> str {
 }
 
 fn slice(str s, uint begin, uint end) -> str {
-    let str accum = "";
-    let uint i = begin;
-    while (i < end) {
-        push_byte(accum, s.(i));
-        i += 1u;
-    }
-    ret accum;
+    // FIXME: Typestate precondition
+    assert (begin <= end);
+    assert (end <= Str.byte_len(s));
+    ret rustrt.str_slice(s, begin, end);
 }
 
 fn shift_byte(&mutable str s) -> u8 {

src/rt/rust_builtin.cpp

@@ -226,6 +226,24 @@ str_push_byte(rust_task* task, rust_str* v, size_t byte)
     return v;
 }
 
+extern "C" CDECL rust_str*
+str_slice(rust_task* task, rust_str* v, size_t begin, size_t end)
+{
+    size_t len = end - begin;
+    rust_str *st =
+        vec_alloc_with_data(task,
+                            len + 1, // +1 to fit at least '\0'
+                            len,
+                            1,
+                            len ? v->data + begin : NULL);
+    if (!st) {
+        task->fail(2);
+        return NULL;
+    }
+    st->data[st->fill++] = '\0';
+    return st;
+}
+
 extern "C" CDECL char const *
 str_buf(rust_task *task, rust_str *s)
 {

src/rt/rustrt.def.in

@@ -32,6 +32,7 @@ str_from_buf
 str_from_cstr
 str_from_vec
 str_push_byte
+str_slice
 str_vec
 task_sleep
 unsafe_vec_to_mut

src/test/run-pass/lib-str.rs

@@ -1,3 +1,6 @@
+// xfail-boot
+// xfail-stage0
+
 use std;
 import std.Str;
 
@@ -98,6 +101,36 @@ fn test_to_upper() {
   assert (Str.eq(expected, actual));
 }
 
+fn test_slice() {
+  assert (Str.eq("ab", Str.slice("abc", 0u, 2u)));
+  assert (Str.eq("bc", Str.slice("abc", 1u, 3u)));
+  assert (Str.eq("", Str.slice("abc", 1u, 1u)));
+
+  fn a_million_letter_a() -> str {
+    auto i = 0;
+    auto res = "";
+    while (i < 100000) {
+      res += "aaaaaaaaaa";
+      i += 1;
+    }
+    ret res;
+  }
+
+  fn half_a_million_letter_a() -> str {
+    auto i = 0;
+    auto res = "";
+    while (i < 100000) {
+      res += "aaaaa";
+      i += 1;
+    }
+    ret res;
+  }
+
+  assert (Str.eq(half_a_million_letter_a(),
+                 Str.slice(a_million_letter_a(),
+                           0u,
+                           500000u)));
+}
 
 fn main() {
   test_bytes_len();
@@ -108,4 +141,5 @@ fn main() {
   test_concat();
   test_connect();
   test_to_upper();
+  test_slice();
 }