libexpr: add list index to deepSeq error traces

When deepSeq encounters an error while evaluating a list element, the
error trace now includes the list index, making it easier to locate
the problematic element.

src/libexpr/eval.cc

@@ -2216,8 +2216,15 @@ void EvalState::forceValueDeep(Value & v)
         }
 
         else if (v.isList()) {
+            size_t index = 0;
             for (auto v2 : v.listView())
-                recurse(*v2);
+                try {
+                    recurse(*v2);
+                    index++;
+                } catch (Error & e) {
+                    state.addErrorTrace(e, "while evaluating list element at index %1%", index);
+                    throw;
+                }
         }
     }(v);
 }

tests/functional/lang/eval-fail-deepseq-list-attr.err.exp

@@ -0,0 +1,25 @@
+error:
+       … while calling the 'deepSeq' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:3:1:
+            2|
+            3| builtins.deepSeq [
+             | ^
+            4|   1
+
+       … while evaluating list element at index 1
+
+       … while evaluating the attribute 'b'
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:5:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |     ^
+            8|   }
+
+       … while calling the 'throw' builtin
+         at /pwd/lang/eval-fail-deepseq-list-attr.nix:7:9:
+            6|     a = 2;
+            7|     b = throw "error in attr in list element";
+             |         ^
+            8|   }
+
+       error: error in attr in list element

tests/functional/lang/eval-fail-deepseq-list-attr.nix

@@ -0,0 +1,10 @@
+# Test that deepSeq reports list index and attribute name in error traces.
+
+builtins.deepSeq [
+  1
+  {
+    a = 2;
+    b = throw "error in attr in list element";
+  }
+  3
+] "unexpected success"