feat(tvix/eval): Implement builtins.deepSeq r/5175

This is done via a new `deepForce` function on Value. Since values can
be cyclical (for example, see the test-case), we need to do some extra
work to avoid RefCell borrow errors if we ever hit a graph cycle:

While deep-forcing values, we keep a set of thunks that we have
already seen and avoid doing any work on the same thunk twice. The set
is encapsulated in a separate type to stop potentially invalid
pointers from leaking out.

Finally, since deep_force is conceptually similar to
`VM::force_for_output` (but more suited to usage in eval since it
doesn't clone the values) this removes the latter, replacing it with
the former.

Co-Authored-By: Vincent Ambo <tazjin@tvl.su>
Change-Id: Iefddefcf09fae3b6a4d161a5873febcff54b9157
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7000
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 1 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix b/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix
new file mode 100644
index 000000000000..9baa49b063ec
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-fail-deepseq.nix
@@ -0,0 +1 @@
+builtins.deepSeq { x = abort "foo"; } 456