rustc: Add a vec::alloc_len and fix arena logic to use it

Author: pcwalton

src/libcore/vec.rs

@@ -11,6 +11,7 @@ export reserve;
 export reserve_at_least;
 export capacity;
 export len;
+export alloc_len;
 export from_fn;
 export from_elem;
 export to_mut;
@@ -153,6 +154,13 @@ pure fn len<T>(&&v: [const T]) -> uint unsafe {
     (**repr).fill / sys::size_of::<T>()
 }
 
+#[doc = "Returns the number of bytes allocated for this vector"]
+#[inline(always)]
+pure fn alloc_len<T>(&&v: [const T]) -> uint unsafe {
+    let repr: **unsafe::vec_repr = ::unsafe::reinterpret_cast(addr_of(v));
+    (**repr).alloc
+}
+
 #[doc = "
 Creates and initializes an immutable vector.
 

src/libstd/arena.rs

@@ -30,9 +30,10 @@ impl arena for arena {
         start = (start + alignm1) & !alignm1;
         let mut end = start + n_bytes;
 
-        if end > vec::len(head.data) {
+        if end > vec::alloc_len(head.data) {
             // Allocate a new chunk.
-            let new_min_chunk_size = uint::max(n_bytes, vec::len(head.data));
+            let new_min_chunk_size = uint::max(n_bytes,
+                                               vec::alloc_len(head.data));
             head = chunk(uint::next_power_of_two(new_min_chunk_size));
             self.chunks = list::cons(head, @self.chunks);
             start = 0u;