fix(tvix/eval): make tvix display values like nix-instantiate(1) r/6308

In order for the test suite we have currently to be comparable to C++
Nix, we need to display values in the same way. This was largely the
case except in some weird cases.

* <CODE> for thunks and <CYCLE> for repeated thunks (?) are already in
  use. <CODE> formatting is tested by the oracle test suite already.

* Instead of lambda, we need to use <LAMBDA>

* <<primop>> and <<primop-app>> (a formatting C++ Nix uses nowhere)
  now are <PRIMOP> and <PRIMOP-APP>.

We'll probably want to have a fancier display of values (in a separate
trait) down the line. This could be used for interactive usage, e.g. the
REPL or a potential debugger.

There is a peculiarity with C++ Nix 2.3 formatting primops: import is
considered a <<PRIMOP-APP>>, since it is internally implemented by means
of scopedImport. This implementation detail no longer leaks in C++ Nix
2.13 nor in Tvix.

<CYCLE> display is untested at the moment, since we exhibit a
discrepancy to C++ Nix 2.3. Our current detection is more similar to C++
Nix 2.13—luckily it is also the more consistent of the two. See also
b/245.

Change-Id: I1d534434b02e470bf5475b3758920ea81e3420dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8760
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

1 files changed, 16 insertions, 0 deletions

diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix
new file mode 100644
index 000000000000..d34ed1697e77
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-value-display.nix
@@ -0,0 +1,16 @@
+# Sanity check of how values are rendered by tvix vs. nix-instantiate(1).
+# Ensures that we can use this test suite to compare against C++ Nix.
+[
+  null
+  true
+  false
+  42
+  42.0
+  "foo\t\nbar"
+  /home/arthur
+  [ 1 2 3 ]
+  (x: x)
+  builtins.add
+  (builtins.substring 0 1)
+  { hello = "world"; }
+]