std: Add cfg(test) to UnsafeArc assertions

This is a ubiquitous type in concurrent code, and the assertions are causing
significant code bloat for simple operations such as reading the pointer
(injecting a failure point, etc).

I am testing executable sizes with no I/O implementations (everything stubbed
out to return nothing), and this took the size of a libnative executable from
328K to 207K (37% reduction in size), so I think that this is one assertion
that's well worth configuring off for now.

Author: alexcrichton

src/libstd/sync/arc.rs

@@ -80,7 +80,8 @@ impl<T: Send> UnsafeArc<T> {
     #[inline]
     pub fn get(&self) -> *mut T {
         unsafe {
-            assert!((*self.data).count.load(Relaxed) > 0);
+            // FIXME(#12049): this needs some sort of debug assertion
+            if cfg!(test) { assert!((*self.data).count.load(Relaxed) > 0); }
             return &mut (*self.data).data as *mut T;
         }
     }
@@ -90,7 +91,8 @@ impl<T: Send> UnsafeArc<T> {
     #[inline]
     pub fn get_immut(&self) -> *T {
         unsafe {
-            assert!((*self.data).count.load(Relaxed) > 0);
+            // FIXME(#12049): this needs some sort of debug assertion
+            if cfg!(test) { assert!((*self.data).count.load(Relaxed) > 0); }
             return &(*self.data).data as *T;
         }
     }
@@ -109,7 +111,8 @@ impl<T: Send> Clone for UnsafeArc<T> {
         unsafe {
             // This barrier might be unnecessary, but I'm not sure...
             let old_count = (*self.data).count.fetch_add(1, Acquire);
-            assert!(old_count >= 1);
+            // FIXME(#12049): this needs some sort of debug assertion
+            if cfg!(test) { assert!(old_count >= 1); }
             return UnsafeArc { data: self.data };
         }
     }
@@ -127,7 +130,8 @@ impl<T> Drop for UnsafeArc<T>{
             // Must be acquire+release, not just release, to make sure this
             // doesn't get reordered to after the unwrapper pointer load.
             let old_count = (*self.data).count.fetch_sub(1, SeqCst);
-            assert!(old_count >= 1);
+            // FIXME(#12049): this needs some sort of debug assertion
+            if cfg!(test) { assert!(old_count >= 1); }
             if old_count == 1 {
                 let _: ~ArcData<T> = cast::transmute(self.data);
             }