simplify typing rule for vector addition: use mutability of LHS

src/libcore/os.rs

@@ -849,7 +849,7 @@ mod tests {
       };
       assert (ostream as uint != 0u);
       let s = "hello";
-      let mut buf = str::bytes(s) + [0 as u8];
+      let mut buf = vec::to_mut(str::bytes(s) + [0 as u8]);
       vec::as_mut_buf(buf) {|b|
          assert (libc::fwrite(b as *c_void, 1u, str::len(s) + 1u, ostream) ==
                  buf.len())};