docs(tvix/eval): add some notes on recursive attribute sets

Change-Id: I36b826f12854a22e60a27ed1982ab5528c58bdad
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6489
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/docs/recursive-attrs.md

@@ -0,0 +1,60 @@
+Recursive attribute sets
+========================
+
+The construction behaviour of recursive attribute sets is very
+specific, and a bit peculiar.
+
+In essence, there are multiple "phases" of scoping that take place
+during attribute set construction:
+
+1. Every inherited value without an explicit source is inherited only
+   from the **outer** scope in which the attribute set is enclosed.
+
+2. A new scope is opened in which all recursive keys are evaluated.
+   This only considers **statically known keys**, attributes can
+   **not** recurse into dynamic keys in `self`!
+
+   For example, this code is invalid in C++ Nix:
+
+   ```
+   nix-repl> rec { ${"a"+""} = 2; b = a * 10; }
+   error: undefined variable 'a' at (string):1:26
+   ```
+
+3. Finally, a third scope is opened in which dynamic keys are
+   evaluated.
+
+This behaviour, while possibly a bit strange and unexpected, actually
+simplifies the implementation of recursive attribute sets in Tvix as
+well.
+
+Essentially, a recursive attribute set like this:
+
+```nix
+rec {
+  inherit a;
+  b = a * 10;
+  ${"c" + ""} = b * 2;
+}
+```
+
+Can be compiled like the following expression:
+
+```nix
+let
+  inherit a;
+in let
+  b = a * 10;
+  in {
+    inherit a b;
+    ${"c" + ""} = b * 2;
+  }
+```
+
+Completely deferring the resolution of recursive identifiers to the
+existing handling of recursive scopes (i.e. deferred access) in let
+bindings.
+
+In practice, we can further specialise this and compile each scope
+directly into the form expected by `OpAttrs` (that is, leaving
+attribute names on the stack) before each value's position.