This fixes bash completion for me, though it may not be the best way.

Hopefully the pull request will go through or tell me how to do it better

modules/programs/bash/default.nix

@@ -86,6 +86,9 @@ in
             for m in "$p/etc/bash_completion.d/"*; do
               source $m
             done
+            for m in "$p/share/bash-completion/completions/"*; do
+              source $m
+            done
           done
           eval "$nullglobStatus"
           unset nullglobStatus p m