Rollup merge of #108257 - fee1-dead-contrib:fixme-1, r=tmiasko

Remove old FIXME that no longer applies

it looks like Encodable was fallible at some point, but that was changed which means that this FIXME is no longer applicable

compiler/rustc_serialize/src/serialize.rs

@@ -430,11 +430,6 @@ impl<D: Decoder, T: Decodable<D> + Copy> Decodable<D> for Cell<T> {
     }
 }
 
-// FIXME: #15036
-// Should use `try_borrow`, returning an
-// `encoder.error("attempting to Encode borrowed RefCell")`
-// from `encode` when `try_borrow` returns `None`.
-
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for RefCell<T> {
     fn encode(&self, s: &mut S) {
         self.borrow().encode(s);