docs(web/tvl): add draft for Tvix status update post r/4832

This adds an initial version of the post which we can play with for
further layouting. The post is marked as `draft`, so it will become
available at the direct link, but have a banner on it telling people
not to share it yet.

Change-Id: Idac69e56bee027c2b566f50ef123b54aff6ebc3e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6538
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

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+We've now been working on our rewrite of Nix, [Tvix][], on-and-off for
+over a year.
+
+Of course, for many of us, it's been a pretty turbulent time period.
+While steady progress has been made, we haven't really had the
+bandwidth to communicate and publicise what has been going on - this
+blog post aims to rectify that!
+
+## Nix language evaluator
+
+The most significant progress in the last months has been made on our
+Nix language evaluator. To address a big question right away: Yes, you
+can play with it - in [Tvixbolt][]!
+
+We developed the evaluator to the current state by enumerating the
+various problems we were likely to encounter, and writing small-scale
+solutions to them before assembling them into a whole. Due to the
+nature of this process, we briefly ended up with a very large private
+source tree, which we [integrated]() into our monorepo in the last
+couple of weeks.
+
+This process was slow mostly due to code review bandwidth, but
+remember that we are just volunteers and such bottlenecks are to be
+expected!
+
+Most of this code was written or reviewed by [tazjin][], [grfn][]
+and [sterni][].
+
+### So, what's working now?
+
+The answer is *most things*! However, there are some unfinished and
+important feature areas:
+
+1. The majority of Nix's `builtins` are not yet implemented (including
+   fundamental ones such as `import` and `derivation`).
+
+2. Recursive attribute sets (`rec`) are not yet implemented. This is
+   actually not because of the recursion in itself, but because of the
+   handling of nested keys (such as `a.b`), for which we are designing
+   a more efficient solution than what is currently in place.
+
+In both cases we have mostly figured out how to do the remaining work
+and it is simply a question of time until we've done it. Progress is
+steady and can of course be tracked [in the source][src] (viewer
+without Javascript [here][src-noscript]).
+
+At the same time, we've already implemented a variety of basics that
+are hopefully going to have a huge impact further down, such as:
+
+* The Tvix compiler is built to be able to emit warnings & errors
+  without failing early, as well as retaining as much source
+  information as possible. This will enable developer tooling, such as
+  language servers, to be based on Tvix.
+
+* The Tvix compiler performs very in-depth scope analysis, which
+  allows it to both generate efficient bytecode for accessing
+  identifiers, as well as alert users about problems in their code
+  before runtime.
+
+* The runtime supports tail-call optimisation in many (but not all
+  (yet!)) cases, allowing us to evaluate many recursive expressions in
+  constant stack space.
+
+### How does this all work?
+
+Tvix's evaluator is implemented using a custom abstract machine with a
+very Nix-specific instruction set, as well as a compiler that
+traverses a parsed Nix AST to emit this bytecode and perform a set of
+optimisations and other analysis. The most important benefit of this
+is that we can plan and lay out the execution of a program in a way
+that is better suited to an efficient runtime than directly traversing
+the AST.
+
+TIP: You can see the generated bytecode in [Tvixbolt][]!
+
+This is all written in Rust (of course) and is currently made up of
+less than 5000 lines of code (some of which look deceptively simple,
+especially around scope-handling!).
+
+We run the evaluator against many custom tests we have written as part
+of our CI suite (through `cargo test`), as well as against the
+upstream Nix test suite (which we do not yet pass, but are working
+towards).
+
+### What's next for tvix-eval?
+
+Apart from the missing language features outlined above, the next
+steps are:
+
+* Comprehensive benchmarking. We are standing up an infrastructure for
+  continuous benchmarking to measure the impact of changes, and to be
+  able to identify and optimise hotspots.
+
+* Implementing known optimisations. There are some areas of the code
+  where we are aware of (significant) possible speed gains, but we are
+  holding off of implementing them until the evaluator is feature
+  complete and passes the Nix language test suite.
+
+* Finishing our language specification. Based on the behaviours we've
+  learned, we are writing a specification of the Nix language that
+  captures its various (sometimes subtly tricky) behaviours.
+
+Once we can evaluate `nixpkgs`, focus is likely to shift towards the
+other areas of Tvix.
+
+## The Other Areas of Tvix
+
+Speaking of these other areas (most importantly, the builder and store
+implementation), some progress has been made there also.
+
+While we haven't begun piecing together the final implementations,
+[flokli][] and [adisbladis][] have been hard at work on [go-nix][]
+which aims to implement many of the low-level primitives required for
+Nix (hashing and encoding schemes, archive formats, reference scanning
+...).
+
+We're looking forward to being able to tell you more about this in the
+next update!
+
+## Outro ...
+
+We'd be delighted to onboard new contributors to Tvix! Please take a
+look at the main [TVL page](https://tvl.fyi) to find out how to get in
+touch with us if you'd like to join!
+
+Thanks also, of course, to [NLNet](https://nlnet.nl/) for sponsoring
+some of this work!
+
+And finally, we would like to thank and pay our respects to jD91mZM2 -
+the original author of
+[rnix-parser](https://github.com/nix-community/rnix-parser) - who
+sadly passed away. We use `rnix-parser` in our compiler, and its
+well-designed internals (also thanks to its new maintainers!) have
+saved us a lot of time.
+
+That's it for this update, go play with [Tvixbolt][], tell us about
+the weird ways in which you break it, get in touch, and we'll see you
+around!
+
+[Tvix]: https://tvl.fyi/blog/rewriting-nix
+[Tvixbolt]: https://tvixbolt.tvl.su
+[integrated]: https://cl.tvl.fyi/q/status:merged+%2522tvix/eval%2522+mergedbefore:2022-09-09
+[src]: https://cs.tvl.fyi/depot/-/tree/tvix/eval
+[src-noscript]: https://code.tvl.fyi/tree/tvix/eval
+[tazjin]: https://tazj.in
+[grfn]: https://gws.fyi/
+[sterni]: https://github.com/sternenseemann
+[go-nix]: https://github.com/nix-community/go-nix
+[flokli]: https://github.com/flokli
+[adisbladis]: https://github.com/adisbladis