test(tvix/eval): genericClosure (pointer) comparison support

genericClosure has very limited support for pointer equality: It relies
on comparison (not equality!) in C++ Nix, so as soon as C++ Nix supports
comparing lists (langVersion >= 6) we can rely on pointer equality for
key.

Since Tvix uses equality, not comparison for the insert, our behavior is
currently different, as documented by the notyetpassing tests.

Change-Id: Ifcd741ed4fc3ccc3825f7038875d56a9918b786a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8720
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-genericClosure-pointer-equality.nix

@@ -0,0 +1,15 @@
+let
+  foo = x: x;
+in
+
+# key needs to be a list since it uses comparison, not equality checks:
+# lists are comparable in Nix if all non-comparable items in them are equal (e.g.
+# functions, attribute sets).
+builtins.genericClosure {
+  startSet = [
+    { key = [ { inherit foo; } ]; val = null; }
+  ];
+  operator = { val, ... }: if val != null then [] else [
+    { key = [ { inherit foo; } ]; val = throw "no pointer equality? 🥺👉👈"; }
+  ];
+}