See NixOS/nix#13377 requesting a work-around until this is written, and NixOS/nix#13350 for the original format changes. Diffs like

 libplugintest = shared_module(
   'plugintest',
   'plugintest.cc',
-  dependencies : [
-    dependency('nix-expr'),
-  ],
+  dependencies : [ dependency('nix-expr') ],
   build_by_default : false,
 )

I don't like, because it means when lists get longer and shorter, there is more "churn". I rather "pay up front" with a multi-line list that might seem overkill, in case the list gets longer.

FWIW, clang-format does seem to preserve trailing commas, at least with the settings we use for Nix.