addTrace

src/libexpr/primops.cc

@@ -239,13 +239,13 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         parsed = state.parseExprFromString(output, pos.file);
     } catch (Error & e) {
-        e.addPrefix(fmt("While parsing the output from '%1%', at %2%\n", program, pos));
+        e.addTrace(pos, hintfmt("While parsing the output from '%1%'", program));
         throw;
     }
     try {
         state.eval(parsed, v);
     } catch (Error & e) {
-        e.addPrefix(fmt("While evaluating the output from '%1%', at %2%\n", program, pos));
+        e.addTrace(pos, hintfmt("While evaluating the output from '%1%'", program));
         throw;
     }
 }

src/libutil/error.cc

@@ -18,6 +18,14 @@ BaseError & BaseError::addPrefix(const FormatOrString & fs)
     return *this;
 }
 
+// addPrefix is used for show-trace.  Strings added with addPrefix
+// will print ahead of the error itself.
+BaseError & BaseError::addTrace(hintformat hint, ErrPos e)
+{
+    err.traces.push_front(Trace { .pos = e, .hint = hint});
+    return *this;
+}
+
 // c++ std::exception descendants must have a 'const char* what()' function.
 // This stringifies the error and caches it for use by what(), or similarly by msg().
 const string& BaseError::calcWhat() const

src/libutil/error.hh

@@ -93,12 +93,18 @@ struct NixCode {
     std::optional<string> nextLineOfCode;
 };
 
+struct Trace {
+    ErrPos pos;
+    hintformat hint;
+};
+
 struct ErrorInfo {
     Verbosity level;
     string name;
     string description;
     std::optional<hintformat> hint;
     std::optional<NixCode> nixCode;
+    std::list<Trace> traces;
 
     static std::optional<string> programName;
 };
@@ -160,6 +166,7 @@ public:
     const string & msg() const { return calcWhat(); }
     const string & prefix() const { return prefix_; }
     BaseError & addPrefix(const FormatOrString & fs);
+    BaseError & addTrace(ErrPos e, hintformat hint);
 
     const ErrorInfo & info() { calcWhat(); return err; }
 };