Add unimplemented! macro

src/libstd/macros.rs

@@ -123,6 +123,13 @@ macro_rules! unreachable (() => (
     fail!("internal error: entered unreachable code");
 ))
 
+/// A standardised placeholder for marking unfinished code. It fails with the
+/// message `"not yet implemented"` when executed.
+#[macro_export]
+macro_rules! unimplemented(
+    () => (fail!("not yet implemented"))
+)
+
 #[macro_export]
 macro_rules! format(($($arg:tt)*) => (
     format_args!(::std::fmt::format, $($arg)*)

src/test/run-fail/unimplemented-macro-fail.rs

@@ -0,0 +1,12 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern:not yet implemented
+fn main() { unimplemented!() }