feat(tvix/eval): implement attrset update (`//`) operator r/4475

The underlying implementation does a few tricks based on which pair of
attrset representations is encountered.

Particularly the effect of short-circuiting the empty cases might be
relevant in nixpkgs/NixOS, due to the use of lib.optionalAttrs.

Change-Id: I22b978b1c69af12926489a71087c6a6219c012f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6140
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 1 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix
new file mode 100644
index 0000000000..9596be22b8
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-attrs-update-empty-lhs.nix
@@ -0,0 +1 @@
+{} // { a = "ok"; }