rustc: Replace intrinsic vec_len with unsafe Rust code

Preparation for #1981

src/libcore/vec.rs

@@ -74,11 +74,6 @@ export vec_len;
 export unsafe;
 export u8;
 
-#[abi = "rust-intrinsic"]
-native mod rusti {
-    fn vec_len<T>(&&v: [const T]) -> libc::size_t;
-}
-
 #[abi = "cdecl"]
 native mod rustrt {
     fn vec_reserve_shared<T>(t: *sys::type_desc,
@@ -122,7 +117,10 @@ fn reserve<T>(&v: [const T], n: uint) {
 
 #[doc = "Returns the length of a vector"]
 #[inline(always)]
-pure fn len<T>(v: [const T]) -> uint { unchecked { rusti::vec_len(v) } }
+pure fn len<T>(&&v: [const T]) -> uint unsafe {
+    let repr: **unsafe::vec_repr = ::unsafe::reinterpret_cast(addr_of(v));
+    (**repr).fill / sys::size_of::<T>()
+}
 
 #[doc = "
 Creates and initializes an immutable vector.