auto merge of #19388 : nick29581/rust/rc-show, r=alexcrichto

r? @huonw or @alexcrichton

Apparently, we have previously rejected an RFC like this. However, since then we removed `{:?}` and so without this debugging gets really difficult as soon as there is a RefCell anywhere, so I believe there is more benefit to adding these impls than there was before. By using "try_borrow" we can avoid panicing in `Show` (I think).

@ huon in response to a comment in #19254: I noticed that `drop()` checks for the ptr being null, so I checked here too. Now I am checking for both, if you're confident I can change to only checking `strong()`.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 75d4342083b..bd250938836 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -719,6 +719,13 @@ impl<T> Clone for Weak<T> {
     }
 }
 
+#[experimental = "Show is experimental."]
+impl<T: fmt::Show> fmt::Show for Weak<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "(Weak)")
+    }
+}
+
 #[doc(hidden)]
 trait RcBoxPtr<T> {
     fn inner(&self) -> &RcBox<T>;