A couple of notes of interest for the work over the past few days:
Git is holding up surprisingly well. I've done daily merges from rails/rails/master into wycats/rails/master and then into wycats/rails/action_view, and have so far only run into a single conflict, which was easy to resolve. Moving Rails to git made the work we're doing possible!
As far as I can tell, all current cases of using render() as an API now use render_for_parts(). This is a huge milestone. Now that the four-element tuple is the standard across the board, we can push back the path lookup to the callers and unify behind something like _renderfor_template(). There's still quite a bit of work to do in general (most notably around partials and layouts), but things are coming along quite nicely.
I was purposely holding off keeping ActionMailer in sync as I did the refactor, because things were too much in flux for that to really be viable. As it turns out, it was relatively simple to move ActionMailer over to the new approach after everything was done. The only real irritation is the repeated use of the render() options to do mostly the same thing. I've moved the code that takes a partial path and converts it into the requisite paths into action_pack/common.rb so it can be used in both ActionController and ActionView. I'm not sure if this will be the final resting place but this refactor definitely pulls out some AC/AV common functionality.
I am more convinced than ever that a common AbstractController superclass for both ActionController and ActionMailer is the way to go. This was actually the thing that originally brought me to Merb (that firm conviction), and I'm pretty convinced it's the right approach for Rails as well. Of course, there's still quite a bit of work to do before it's possible to really evaluate it, so stay tuned.
Lori has been working hard on the ORM adapter for Rails. Effectively, the adapter will work like: ActiveORM.for(@sequel_obj). If the object is compatible with the ActiveORM API (note that the names et al are still very much in flux), it will be passed through. For instance, ActiveORM.for(@ar_obj) will just return the ActiveRecord object. ActiveORM.for(@sequel_obj), however, will return a wrapper around the object that is compatible with Rails expectations. Example: ActiveORM.for(@sequel_obj).new_record?. Note that this paragraph is speculative and represents work that is still very much in flux.
I'm really looking forward to finishing the work that _render_for_parts was a placeholder for. As I go through the codebase, I'm definitely noticing some obvious perf opportunities (non-trivial perf that should show up very clearly on macro-benches) and I'm looking forward to finishing the initial refactor so I can get into some perf optimizations. I should have some initial numbers within a week or so.