Always print addErrorContext traces

src/libexpr/primops.cc

@@ -826,7 +826,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         auto message = state.coerceToString(pos, *args[0], context,
                 "while evaluating the error message passed to builtins.addErrorContext",
                 false, false).toOwned();
-        e.addTrace(nullptr, HintFmt(message));
+        e.addTrace(nullptr, HintFmt(message), TraceKind::Custom);
         throw;
     }
 }

src/libutil/error.cc

@@ -11,9 +11,9 @@
 
 namespace nix {
 
-void BaseError::addTrace(std::shared_ptr<Pos> && e, HintFmt hint)
+void BaseError::addTrace(std::shared_ptr<Pos> && e, HintFmt hint, TraceKind kind)
 {
-    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint });
+    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .kind = kind });
 }
 
 void throwExceptionSelfCheck(){
@@ -379,19 +379,22 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
         // A consecutive sequence of stack traces that are all in `tracesSeen`.
         std::vector<Trace> skippedTraces;
         size_t count = 0;
+        bool truncate = false;
 
         for (const auto & trace : einfo.traces) {
             if (trace.hint.str().empty()) continue;
 
             if (!showTrace && count > 3) {
-                oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full trace)" ANSI_NORMAL << "\n";
-                break;
+                truncate = true;
             }
 
+            if (!truncate || trace.kind == TraceKind::Custom) {
+
                 if (tracesSeen.count(trace)) {
                     skippedTraces.push_back(trace);
                     continue;
                 }
+
                 tracesSeen.insert(trace);
 
                 printSkippedTracesMaybe(oss, ellipsisIndent, count, skippedTraces, tracesSeen);
@@ -400,8 +403,15 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
                 printTrace(oss, ellipsisIndent, count, trace);
             }
+        }
+
 
         printSkippedTracesMaybe(oss, ellipsisIndent, count, skippedTraces, tracesSeen);
+
+        if (truncate) {
+            oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full, detailed trace)" ANSI_NORMAL << "\n";
+        }
+
         oss << "\n" << prefix;
     }
 

src/libutil/error.hh

@@ -61,9 +61,16 @@ void printCodeLines(std::ostream & out,
     const Pos & errPos,
     const LinesOfCode & loc);
 
+enum struct TraceKind {
+    Other,
+    /** Produced by builtins.addErrorContext. Always printed. */
+    Custom,
+};
+
 struct Trace {
     std::shared_ptr<Pos> pos;
     HintFmt hint;
+    TraceKind kind = TraceKind::Other;
 };
 
 inline bool operator<(const Trace& lhs, const Trace& rhs);
@@ -161,7 +168,7 @@ public:
         addTrace(std::move(e), HintFmt(std::string(fs), args...));
     }
 
-    void addTrace(std::shared_ptr<Pos> && e, HintFmt hint);
+    void addTrace(std::shared_ptr<Pos> && e, HintFmt hint, TraceKind kind = TraceKind::Other);
 
     bool hasTrace() const { return !err.traces.empty(); }
 

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