std: Re-optimize tls access on local allocation path

I did this once but acciddentally undid it in a later patch.

src/libstd/unstable/lang.rs

@@ -13,6 +13,7 @@
 use c_str::ToCStr;
 use cast::transmute;
 use libc::{c_char, c_void, size_t, uintptr_t};
+use option::{Some, None};
 use sys;
 use rt::task::Task;
 use rt::local::Local;
@@ -35,14 +36,13 @@ pub fn fail_bounds_check(file: *c_char, line: size_t,
 
 #[lang="malloc"]
 pub unsafe fn local_malloc(td: *c_char, size: uintptr_t) -> *c_char {
-    let mut alloc = ::ptr::null();
-    do Local::borrow::<Task,()> |task| {
-        rtdebug!("task pointer: %x, heap pointer: %x",
-                 ::borrow::to_uint(task),
-                 ::borrow::to_uint(&task.heap));
-        alloc = task.heap.alloc(td as *c_void, size as uint) as *c_char;
+    // XXX: Unsafe borrow for speed. Lame.
+    match Local::try_unsafe_borrow::<Task>() {
+        Some(task) => {
+            (*task).heap.alloc(td as *c_void, size as uint) as *c_char
+        }
+        None => rtabort!("local malloc outside of task")
     }
-    return alloc;
 }
 
 // NB: Calls to free CANNOT be allowed to fail, as throwing an exception from