The visibility of modules is currently checked incorrectly.

The following module structure illustrates the problem:

               root module (public by default)
                          |
                          |
                      module b  (private)
                    /         \ 
                   /           \ 
       module c (public)     module d 

Currently any item declared in c is invisible to d, because the visibility of b is private. This is incorrect, since d is behind the same visibility "wall" as c, namely the privacy of b.

My understanding is that items from c should also be visible in b, but I'm not 100% sure of that. We should check what Rust does here.