support folding on lower lines, give failure message

Author: schnittchen

spec/spec_helper.rb

@@ -61,12 +61,14 @@ def syntax(content, pattern)
     syngroups.gsub!(/["'\[\]]/, '').split(', ')
   end
 
-  def fold_and_delete(content)
+  def fold_and_delete(content, opts)
+    start_line = opts[:on_line]
     with_file content do
       @vim.command("set foldmethod=syntax")
 
       @vim.normal("zO")
-      @vim.normal("zM")
+      @vim.normal("#{start_line}G")
+      @vim.normal("zc")
       @vim.normal("dd")
     end
   end
@@ -203,14 +205,29 @@ def self.new
   end
 end
 
-RSpec::Matchers.define :fold_lines do |lines|
+RSpec::Matchers.define :fold_lines do |lines, opts|
   buffer = Buffer.new(VIM, :ex)
+  opts ||= {}
+  start_line = opts[:on_line] ||= 1
+
+  after = nil
 
   match do |code|
-    after = buffer.fold_and_delete(code)
+    after = buffer.fold_and_delete(code, opts)
 
     code.lines.count - after.lines.count == lines
   end
+
+  failure_message do |code|
+    <<~EOF
+    expected
+    #{code}
+    to fold #{lines} lines at line #{start_line},
+    but after folding at line #{start_line} and deleting a line I got
+    #{after}
+    back
+    EOF
+  end
 end
 
 Vimrunner::RSpec.configure do |config|