LookupContext missing constructor

The title of this message is wrong. LookupContext must have a
constructor:

ActionView::LookupContext.new(nil)
=> #<ActionView::LookupContext:0x007f8e84755d18 @details_key=nil,
@details={:handlers=>[:erb, :builder, :coffee], :formats=>[:html, :text, :js, :css, :ics, :csv, :xml, :rss, :atom, :yaml, :multipart_form, :url_encoded_form, :json], :locale=>[:en, :en]},
@skip_default_locale=false, @frozen_formats=false, @cache=true,
@prefixes=[], @view_paths=[]>

My problem is I can't find it:

https://github.com/rails/rails/blob/master/actionpack/lib/action_view/lookup_context.rb

There's other initialize methods for other classes, but not for
LookupContext. Where is it?

I think I found it on line 170:

    def initialize(view_paths, details = {}, prefixes = [])
      @details, @details_key = {}, nil
      @skip_default_locale = false
      @cache = true
      @prefixes = prefixes
      @rendered_format = nil

      self.view_paths = view_paths
      initialize_details(details)
    end

It's after 3 module definitions of the same class. It seems almost
intentionally designed to be confusing.