syntax: add debug_assert! that is only active with --cfg debug.

src/libsyntax/ext/expand.rs

@@ -544,6 +544,20 @@ pub fn std_macros() -> @str {
         }
     )
 
+    // A assert that is only enabled with --cfg debug, but like with
+    // debug! itself, it still expands to something similar without
+    // that cfg, to stop the asserts bitrotting.
+    #[cfg(debug)]
+    #[macro_escape]
+    mod debug_assert_macro {
+        macro_rules! debug_assert { ($($x:expr),*) => { assert!($($x),*) } }
+    }
+    #[cfg(not(debug))]
+    #[macro_escape]
+    mod debug_assert_macro {
+        macro_rules! debug_assert { ($($x:expr),*) => { if false { assert!($($x),*) } } }
+    }
+
     macro_rules! assert_eq (
         ($given:expr , $expected:expr) => (
             {

src/test/run-fail/debug-assert-fail.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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.
+
+// xfail-fast compile-flags directive doesn't work for check-fast
+// compile-flags: --cfg debug
+// error-pattern:assertion failed
+
+fn main() {
+    debug_assert!(false);
+}

src/test/run-pass/debug-assert-pass.rs

@@ -0,0 +1,13 @@
+// Copyright 2013 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.
+
+fn main() {
+    debug_assert!(false);
+}