std: move str::as_buf into StrSlice

src/libstd/unstable/lang.rs

@@ -56,7 +56,7 @@ pub fn fail_bounds_check(file: *c_char, line: size_t,
                          index: size_t, len: size_t) {
     let msg = fmt!("index out of bounds: the len is %d but the index is %d",
                     len as int, index as int);
-    do str::as_buf(msg) |p, _len| {
+    do msg.as_buf |p, _len| {
         fail_(p as *c_char, file, line);
     }
 }