syntax: Don't use unstable fn to convert single element to a slice

src/libsyntax/util/small_vector.rs

@@ -64,7 +64,10 @@ pub fn as_slice<'a>(&'a self) -> &'a [T] {
                 let result: &[T] = &[];
                 result
             }
-            One(ref v) => slice::ref_slice(v),
+            One(ref v) => {
+                // FIXME: Could be replaced with `slice::ref_slice(v)` when it is stable.
+                unsafe { slice::from_raw_parts(v, 1) }
+            }
             Many(ref vs) => vs
         }
     }