Fixing require LoadError as an example for the matrix gem
Some gems like matrix or rexml were added at some point to ruby becoming so-called default gems. But they may be removed from being default in newer ruby versions, resulting in broken builds like for us with prawn: bin/rails c 'require': cannot load such file -- matrix (LoadError) In most such cases you have to update the used gem (for us prawn) to a newer version, which defines the missing gem as required for this ruby version.