+          (copy-recursively
+           (string-append #$(package-source this-package) "/contrib") ".")

I would have expected trivial-build-system to automatically unpack the source, so maybe this can be simplified to (chdir "/contrib") (untested!)

Or even simpler: remove this line, and add (source (file-append (package-source sway) "/contrib")) as a field.

A quick  "git grep -F '(source (file-append" doesn't result in any packages already doing that, but it seems a perfect fit here to me ...  OTOH, this probably interferes with --with-git-url and such, so I'd guess better not.

Greetings,
Maxime.