Add builtins.traceVerbose

Co-Authored-By: Silvan Mosberger <contact@infinisil.com>

Add builtins.traceVerbose tests

src/libexpr/eval.hh

@@ -646,6 +646,9 @@ struct EvalSettings : Config
 
     Setting<bool> useEvalCache{this, true, "eval-cache",
         "Whether to use the flake evaluation cache."};
+
+    Setting<bool> traceVerbose{this, false, "trace-verbose",
+        "Whether `builtins.traceVerbose` should trace its first argument when evaluated."};
 };
 
 extern EvalSettings evalSettings;

src/libexpr/primops.cc

@@ -970,6 +970,15 @@ static RegisterPrimOp primop_trace({
 });
 
 
+/* Takes two arguments and evaluates to the second one. Used as the
+ * builtins.traceVerbose implementation when --trace-verbose is not enabled
+ */
+static void prim_second(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[1], pos);
+    v = *args[1];
+}
+
 /*************************************************************
  * Derivations
  *************************************************************/
@@ -3926,6 +3935,18 @@ void EvalState::createBaseEnv()
         addPrimOp("__exec", 1, prim_exec);
     }
 
+    addPrimOp({
+        .fun = evalSettings.traceVerbose ? prim_trace : prim_second,
+        .arity = 2,
+        .name = symbols.create("__traceVerbose"),
+        .args = { "e1", "e2" },
+        .doc = R"(
+          Evaluate *e1* and print its abstract syntax representation on standard
+          error if `--trace-verbose` is enabled. Then return *e2*. This function
+          is useful for debugging.
+        )",
+    });
+
     /* Add a value containing the current Nix expression search path. */
     mkList(v, searchPath.size());
     int n = 0;

tests/lang.sh

@@ -5,6 +5,8 @@ export NIX_REMOTE=dummy://
 
 nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>&1 | grep -q Hello
 nix-instantiate --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1
+nix-instantiate --trace-verbose --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grep -q Hello
+(! nix-instantiate --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grep -q Hello)
 (! nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1 | grep -q Hello)
 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" (throw "Foo")' 2>&1 | grep -q Hello