core: adding str::as_slice .. most likely broken

src/libcore/str.rs

@@ -23,6 +23,7 @@ export
    as_buf,
    as_c_str,
    unpack_slice,
+   as_slice,
 
    // Adding things to and removing things from a string
    push_char,
@@ -1620,6 +1621,16 @@ pure fn unpack_slice<T>(s: str/&, f: fn(*u8, uint) -> T) -> T {
     }
 }
 
+pure fn as_slice<T>(s: str,
+                    start: uint, end: uint,
+                    f: fn(str/&) -> T) -> T unsafe {
+    assert (start <= end);
+    assert (end <= len(s));
+    let p = ptr::addr_of(s);
+    f(::unsafe::reinterpret_cast(
+        (ptr::offset(p, start), (end - start) * sys::size_of::<u8>())))
+}
+
 #[doc = "
 Reserves capacity for exactly `n` bytes in the given string, not including
 the null terminator.