fix(tvix/eval): fix thunk borrowing error in force_for_output r/5006

This function previously kept a borrow in the form of the
`Thunk::value` result alive while performing arbitrary actions in the
VM, which caused a borrowing error in the test case attached.

The `Ref` value must never be used in cases where control flow is
passed to other parts of the VM.

Change-Id: I41d10aa1882a2166614b670e8ba77aab0e67deca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6825
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 4 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix
new file mode 100644
index 000000000000..7d037c6b37bc
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-rec-nested-access.nix
@@ -0,0 +1,4 @@
+rec {
+  a.b = 1;
+  a.c = a.b * 2;
+}