Define cfg(rtopt) when optimizing. Turn off runtime sanity checks

Naturally, and sadly, turning off sanity checks in the runtime is
a noticable performance win. The particular test I'm running goes from
~1.5 s to ~1.3s.

Sanity checks are turned *on* when not optimizing, or when cfg
includes `rtdebug` or `rtassert`.

Makefile.in

@@ -96,7 +96,8 @@ ifdef CFG_DISABLE_OPTIMIZE
   $(info cfg: disabling rustc optimization (CFG_DISABLE_OPTIMIZE))
   CFG_RUSTC_FLAGS +=
 else
-  CFG_RUSTC_FLAGS += -O
+  # The rtopt cfg turns off runtime sanity checks
+  CFG_RUSTC_FLAGS += -O --cfg rtopt
 endif
 
 ifdef CFG_ENABLE_DEBUG

src/libstd/macros.rs

@@ -27,8 +27,10 @@ macro_rules! rtdebug (
 
 macro_rules! rtassert (
     ( $arg:expr ) => ( {
-        if !$arg {
-            rtabort!("assertion failed: %s", stringify!($arg));
+        if ::rt::util::ENFORCE_SANITY {
+            if !$arg {
+                rtabort!("assertion failed: %s", stringify!($arg));
+            }
         }
     } )
 )

src/libstd/rt/util.rs

@@ -19,6 +19,9 @@ use unstable::atomics::{AtomicInt, INIT_ATOMIC_INT, SeqCst};
 #[cfg(target_os="macos")]
 use unstable::running_on_valgrind;
 
+// Indicates whether we should perform expensive sanity checks, including rtassert!
+pub static ENFORCE_SANITY: bool = !cfg!(rtopt) || cfg!(rtdebug) || cfg!(rtassert);
+
 /// Get the number of cores available
 pub fn num_cpus() -> uint {
     #[fixed_stack_segment]; #[inline(never)];