replace more vector + (issue #2719)

src/libcore/vec.rs

@@ -5,6 +5,7 @@ import ptr::addr_of;
 import libc::size_t;
 
 export append;
+export append_one;
 export init_op;
 export is_empty;
 export is_not_empty;
@@ -492,6 +493,13 @@ pure fn append<T: copy>(+lhs: [T]/~, rhs: [const T]/&) -> [T]/~ {
     ret v;
 }
 
+#[inline(always)]
+pure fn append_one<T>(+lhs: [T]/~, +x: T) -> [T]/~ {
+    let mut v <- lhs;
+    unchecked { push(v, x); }
+    v
+}
+
 #[inline(always)]
 pure fn append_mut<T: copy>(lhs: [mut T]/&, rhs: [const T]/&) -> [mut T]/~ {
     let mut v = [mut]/~;