toJSON: Add attribute path to trace

doc/manual/src/release-notes/rl-next.md

@@ -6,3 +6,5 @@
   [`builtins.flakeRefToString`](@docroot@/language/builtins.md#builtins-flakeRefToString),
   have been added.
   These functions are useful for converting between flake references encoded as attribute sets and URLs.
+
+- [`builtins.toJSON`](@docroot@/language/builtins.md#builtins-parseFlakeRef) now prints [--show-trace](@docroot@/command-ref/conf-file.html#conf-show-trace) items for the path in which it finds an evaluation error.

src/libexpr/value-to-json.cc

@@ -43,6 +43,7 @@ json printValueAsJSON(EvalState & state, bool strict,
             break;
 
         case nNull:
+            // already initialized as null
             break;
 
         case nAttrs: {
@@ -59,7 +60,13 @@ json printValueAsJSON(EvalState & state, bool strict,
                     names.emplace(state.symbols[j.name]);
                 for (auto & j : names) {
                     Attr & a(*v.attrs->find(state.symbols.create(j)));
+                    try {
                         out[j] = printValueAsJSON(state, strict, *a.value, a.pos, context, copyToStore);
+                    } catch (Error & e) {
+                        e.addTrace(state.positions[a.pos],
+                            hintfmt("while evaluating attribute '%1%'", j));
+                        throw;
+                    }
                 }
             } else
                 return printValueAsJSON(state, strict, *i->value, i->pos, context, copyToStore);
@@ -68,8 +75,17 @@ json printValueAsJSON(EvalState & state, bool strict,
 
         case nList: {
             out = json::array();
-            for (auto elem : v.listItems())
+            int i = 0;
+            for (auto elem : v.listItems()) {
+                try {
                     out.push_back(printValueAsJSON(state, strict, *elem, pos, context, copyToStore));
+                } catch (Error & e) {
+                    e.addTrace({},
+                        hintfmt("while evaluating list element at index %1%", i));
+                    throw;
+                }
+                i++;
+            }
             break;
         }
 

tests/lang/eval-fail-toJSON.err.exp

@@ -0,0 +1,57 @@
+error:
+       … while calling the 'toJSON' builtin
+
+         at /pwd/lang/eval-fail-toJSON.nix:1:1:
+
+            1| builtins.toJSON {
+             | ^
+            2|   a.b = [
+
+       … while evaluating attribute 'a'
+
+         at /pwd/lang/eval-fail-toJSON.nix:2:3:
+
+            1| builtins.toJSON {
+            2|   a.b = [
+             |   ^
+            3|     true
+
+       … while evaluating attribute 'b'
+
+         at /pwd/lang/eval-fail-toJSON.nix:2:3:
+
+            1| builtins.toJSON {
+            2|   a.b = [
+             |   ^
+            3|     true
+
+       … while evaluating list element at index 3
+
+       … while evaluating attribute 'c'
+
+         at /pwd/lang/eval-fail-toJSON.nix:7:7:
+
+            6|     {
+            7|       c.d = throw "hah no";
+             |       ^
+            8|     }
+
+       … while evaluating attribute 'd'
+
+         at /pwd/lang/eval-fail-toJSON.nix:7:7:
+
+            6|     {
+            7|       c.d = throw "hah no";
+             |       ^
+            8|     }
+
+       … while calling the 'throw' builtin
+
+         at /pwd/lang/eval-fail-toJSON.nix:7:13:
+
+            6|     {
+            7|       c.d = throw "hah no";
+             |             ^
+            8|     }
+
+       error: hah no

tests/lang/eval-fail-toJSON.nix

@@ -0,0 +1,10 @@
+builtins.toJSON {
+  a.b = [
+    true
+    false
+    "it's a bird"
+    {
+      c.d = throw "hah no";
+    }
+  ];
+}