I've not terribly familiar with language servers but just wanted to add that using nil, as mentioned in #274, marks attributes as unused (see inputs and pkgs in the screenshot below). So my guess is that whatever change needs to happen, should happen in rnix-lsp.
I'm wondering if this extension can somehow leverage rnix-lsp to find out which attributes are unused in an expression.
Example:
would mark
a
as unused.