chore(tvix/eval): move eval docs to tvix/docs

Change-Id: I75b33c43456389de6e521b4f0ad46d68bc9e98f6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11809
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/docs/src/eval/recursive-attrs.md

@@ -0,0 +1,68 @@
+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.
+
+C++ Nix's Implementation
+------------------------
+
+* [`ExprAttrs`](https://github.com/NixOS/nix/blob/2097c30b08af19a9b42705fbc07463bea60dfb5b/src/libexpr/nixexpr.hh#L241-L268)
+  (AST representation of attribute sets)
+* [`ExprAttrs::eval`](https://github.com/NixOS/nix/blob/075bf6e5565aff9fba0ea02f3333c82adf4dccee/src/libexpr/eval.cc#L1333-L1414)
+* [`addAttr`](https://github.com/NixOS/nix/blob/master/src/libexpr/parser.y#L98-L156) (`ExprAttrs` construction in the parser)