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

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

tvix/eval/src/tests/tvix_tests/eval-okay-import-display.exp

@@ -0,0 +1 @@
+<PRIMOP>

tvix/eval/src/tests/tvix_tests/eval-okay-import-display.nix

@@ -0,0 +1,2 @@
+# In C++ Nix 2.3 this used to be <PRIMOP-APP>
+import

tvix/eval/src/tests/tvix_tests/eval-okay-value-display.exp

@@ -0,0 +1 @@
+[ null true false 42 42 "foo\t\nbar" /home/arthur [ 1 2 3 ] <LAMBDA> <PRIMOP> <PRIMOP-APP> { hello = "world"; } ]

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"; }
+]

tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.exp

@@ -0,0 +1 @@
+[ { car = 42; cdr = «repeated»; } [ «repeated» «repeated» «repeated» ] { val = 42; wal = «repeated»; xal = «repeated»; } { tail1 = «repeated»; tail2 = «repeated»; val = 42; } { tail1 = «repeated»; tail2 = «repeated»; val = 21; } ]

tvix/eval/src/tests/tvix_tests/notyetpassing/eval-okay-cycle-display-cpp-nix-2.13.nix

@@ -0,0 +1,34 @@
+let
+  linkedList = {
+    car = 42;
+    cdr = linkedList;
+  };
+
+  list = [
+    linkedList
+    linkedList
+    linkedList
+  ];
+
+  set = {
+    val = 42;
+    wal = set;
+    xal = set;
+  };
+
+  multiTail = {
+   val = 42;
+   tail1 = multiTail;
+   tail2 = multiTail;
+  };
+in
+
+[
+  linkedList
+  list
+  set
+
+  # In C++ Nix 2.3 these would be displayed differently
+  multiTail
+  (let multiTail = { val = 21; tail1 = multiTail; tail2 = multiTail; }; in multiTail)
+]

tvix/eval/src/value/builtin.rs

@@ -121,9 +121,9 @@ impl Debug for Builtin {
 impl Display for Builtin {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         if !self.0.partials.is_empty() {
-            f.write_str("<<primop-app>>")
+            f.write_str("<PRIMOP-APP>")
         } else {
-            f.write_str("<<primop>>")
+            f.write_str("<PRIMOP>")
         }
     }
 }

tvix/eval/src/value/mod.rs

@@ -758,7 +758,8 @@ impl TotalDisplay for Value {
             Value::Path(p) => p.display().fmt(f),
             Value::Attrs(attrs) => attrs.total_fmt(f, set),
             Value::List(list) => list.total_fmt(f, set),
-            Value::Closure(_) => f.write_str("lambda"), // TODO: print position
+            // TODO: fancy REPL display with position
+            Value::Closure(_) => f.write_str("<LAMBDA>"),
             Value::Builtin(builtin) => builtin.fmt(f),
 
             // Nix prints floats with a maximum precision of 5 digits

tvix/verify-lang-tests/default.nix

@@ -59,6 +59,10 @@ let
     "eval-okay-getattrpos-functionargs.nix" = [ nix ];
     # groupBy appeared (long) after 2.3
     "eval-okay-builtins-groupby-thunk.nix" = [ nix ];
+    # import is no longer considered a curried primop in Nix > 2.3
+    "eval-okay-import-display.nix" = [ nix ];
+    # Cycle detection and formatting changed sometime after Nix 2.3
+    "eval-okay-cycle-display-cpp-nix-2.13.nix" = [ nix ];
   };
 
   runCppNixLangTests = cpp-nix: