I think there's a path to add support for ERB without a ton of effort. We can try to require erb from the main bundle and provide features only if it is a dependency.

Basically, we can use the ERB gem to parse the .erb file and extract the Ruby code. Then we parse the Ruby code with Prism and provide features for it.

require "erb"
erb = "<% x = 32 %><p><%= x %></p>"
ruby_code = ERB.new(erb).src # => "#coding:UTF-8\n_erbout = +'';  x = 32 ; _erbout.<< \"<p>\".freeze; _erbout.<<(( x ).to_s); _erbout.<< \"</p>\".freeze; _erbout"

Prism.parse(ruby_code) # => AST

In terms of the codebase, I think the best way forward would be turning creating an abstract parent class Document and turning our existing document class into RubyDocument. Then we can create an ErbDocument class to handle the specific details of ERB.

Notes

Diagnostics, formatting and on type formatting need to happen differently on ERB files. For example, if some is using [].each do %> and breaks the line, we need to insert <% end %> and not just end. Similarly, we can see if erb lint is available to provide diagnostics and potentially format the code too.

It might be worth separating these requests into RubyFormatting and ErbFormatting.