Add more comments to trans_block

src/comp/middle/trans.rs

@@ -5407,7 +5407,8 @@ fn is_nil(@ty.t r_ty) -> bool {
                 // FIXME: This is a temporary hack to prevent compile
                 // failures. There's some expression variant that claims
                 // to be ty_nil but but does not translate to T_nil. Need
-                // to hunt it down.
+                // to hunt it down. Of course, if we're talking about nil,
+                // do we really want to do this whole business anyway?
                 if (!is_nil(r_ty)) {
                     // This alloca is declared at the function level, above
                     // the block scope