Always print addErrorContext traces

tests/functional/lang.sh

@@ -68,8 +68,16 @@ done
 for i in lang/eval-fail-*.nix; do
     echo "evaluating $i (should fail)";
     i=$(basename "$i" .nix)
+    flags="$(
+        if [[ -e "lang/$i.flags" ]]; then
+            sed -e 's/#.*//' < "lang/$i.flags"
+        else
+            # note that show-trace is also set by init.sh
+            echo "--eval --strict --show-trace"
+        fi
+    )"
     if
-        expectStderr 1 nix-instantiate --eval --strict --show-trace "lang/$i.nix" \
+        expectStderr 1 nix-instantiate $flags "lang/$i.nix" \
             | sed "s!$(pwd)!/pwd!g" > "lang/$i.err"
     then
         diffAndAccept "$i" err err.exp

tests/functional/lang/eval-fail-addErrorContext-example.err.exp

@@ -0,0 +1,38 @@
+error:
+       … while calling the 'addErrorContext' builtin
+         at /pwd/lang/eval-fail-addErrorContext-example.nix:6:7:
+            5|     else
+            6|       builtins.addErrorContext
+             |       ^
+            7|         "while counting down; n = ${toString n}"
+
+       … while counting down; n = 10
+
+       … while calling the 'addErrorContext' builtin
+         at /pwd/lang/eval-fail-addErrorContext-example.nix:6:7:
+            5|     else
+            6|       builtins.addErrorContext
+             |       ^
+            7|         "while counting down; n = ${toString n}"
+
+       … while counting down; n = 9
+
+       … while counting down; n = 8
+
+       … while counting down; n = 7
+
+       … while counting down; n = 6
+
+       … while counting down; n = 5
+
+       … while counting down; n = 4
+
+       … while counting down; n = 3
+
+       … while counting down; n = 2
+
+       … while counting down; n = 1
+
+       (stack trace truncated; use '--show-trace' to show the full, detailed trace)
+
+       error: kaboom

tests/functional/lang/eval-fail-addErrorContext-example.flags

@@ -0,0 +1 @@
+--eval --strict --no-show-trace

tests/functional/lang/eval-fail-addErrorContext-example.nix

@@ -0,0 +1,9 @@
+let
+  countDown = n:
+    if n == 0
+    then throw "kaboom"
+    else
+      builtins.addErrorContext
+        "while counting down; n = ${toString n}"
+        ("x" + countDown (n - 1));
+in countDown 10