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()`.

diff --cc src/librustc_resolve/lib.rs
Simple merge