Merge pull request #7451 from edolstra/abstract-pos

Introduce AbstractPos

src/libcmd/repl.cc

@@ -215,17 +215,15 @@ static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positi
     out << dt.hint.str() << "\n";
 
     // prefer direct pos, but if noPos then try the expr.
-    auto pos = *dt.pos
-        ? *dt.pos
-        : positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
+    auto pos = dt.pos
+        ? dt.pos
+        : static_cast<std::shared_ptr<AbstractPos>>(positions[dt.expr.getPos() ? dt.expr.getPos() : noPos]);
 
     if (pos) {
-        printAtPos(pos, out);
-
-        auto loc = getCodeLines(pos);
-        if (loc.has_value()) {
+        out << pos;
+        if (auto loc = pos->getCodeLines()) {
             out << "\n";
-            printCodeLines(out, "", pos, *loc);
+            printCodeLines(out, "", *pos, *loc);
             out << "\n";
         }
     }
@@ -589,15 +587,17 @@ bool NixRepl::processLine(std::string line)
         Value v;
         evalString(arg, v);
 
-        const auto [file, line] = [&] () -> std::pair<std::string, uint32_t> {
+        const auto [path, line] = [&] () -> std::pair<Path, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
                 PathSet context;
-                auto filename = state->coerceToString(noPos, v, context).toOwned();
-                state->symbols.create(filename);
-                return {filename, 0};
+                auto path = state->coerceToPath(noPos, v, context);
+                return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
-                return {pos.file, pos.line};
+                if (auto path = std::get_if<Path>(&pos.origin))
+                    return {*path, pos.line};
+                else
+                    throw Error("'%s' cannot be shown in an editor", pos);
             } else {
                 // assume it's a derivation
                 return findPackageFilename(*state, v, arg);
@@ -605,7 +605,7 @@ bool NixRepl::processLine(std::string line)
         }();
 
         // Open in EDITOR
-        auto args = editorFor(file, line);
+        auto args = editorFor(path, line);
         auto editor = args.front();
         args.pop_front();
 

src/libexpr/eval.cc

@@ -820,7 +820,7 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
         ? std::make_unique<DebugTraceStacker>(
             *this,
             DebugTrace {
-                .pos = error->info().errPos ? *error->info().errPos : positions[expr.getPos()],
+                .pos = error->info().errPos ? error->info().errPos : static_cast<std::shared_ptr<AbstractPos>>(positions[expr.getPos()]),
                 .expr = expr,
                 .env = env,
                 .hint = error->info().msg,
@@ -1009,7 +1009,7 @@ void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, cons
 
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
 {
-    e.addTrace(std::nullopt, s, s2);
+    e.addTrace(nullptr, s, s2);
 }
 
 void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const
@@ -1021,13 +1021,13 @@ static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
     EvalState & state,
     Expr & expr,
     Env & env,
-    std::optional<ErrPos> pos,
+    std::shared_ptr<AbstractPos> && pos,
     const char * s,
     const std::string & s2)
 {
     return std::make_unique<DebugTraceStacker>(state,
         DebugTrace {
-            .pos = pos,
+            .pos = std::move(pos),
             .expr = expr,
             .env = env,
             .hint = hintfmt(s, s2),
@@ -1133,9 +1133,9 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (!pos.file.empty()) {
+    if (auto path = std::get_if<Path>(&pos.origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(pos.file);
+        attrs.alloc(sFile).mkString(*path);
         attrs.alloc(sLine).mkInt(pos.line);
         attrs.alloc(sColumn).mkInt(pos.column);
         v.mkAttrs(attrs);
@@ -1243,7 +1243,7 @@ void EvalState::cacheFile(
                 *this,
                 *e,
                 this->baseEnv,
-                e->getPos() ? std::optional(ErrPos(positions[e->getPos()])) : std::nullopt,
+                e->getPos() ? static_cast<std::shared_ptr<AbstractPos>>(positions[e->getPos()]) : nullptr,
                 "while evaluating the file '%1%':", resolvedPath)
             : nullptr;
 
@@ -1514,10 +1514,13 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(*vAttrs, (pos2 ? pos2 : this->pos ) );
 
     } catch (Error & e) {
-        auto pos2r = state.positions[pos2];
-        if (pos2 && pos2r.file != state.derivationNixPath)
-            state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
-                showAttrPath(state, env, attrPath));
+        if (pos2) {
+            auto pos2r = state.positions[pos2];
+            auto origin = std::get_if<Path>(&pos2r.origin);
+            if (!(origin && *origin == state.derivationNixPath))
+                state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
+                    showAttrPath(state, env, attrPath));
+        }
         throw;
     }
 
@@ -2494,7 +2497,8 @@ void EvalState::printStats()
                     else
                         obj["name"] = nullptr;
                     if (auto pos = positions[fun->pos]) {
-                        obj["file"] = (std::string_view) pos.file;
+                        if (auto path = std::get_if<Path>(&pos.origin))
+                            obj["file"] = *path;
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }
@@ -2508,7 +2512,8 @@ void EvalState::printStats()
                 for (auto & i : attrSelects) {
                     json obj = json::object();
                     if (auto pos = positions[i.first]) {
-                        obj["file"] = (const std::string &) pos.file;
+                        if (auto path = std::get_if<Path>(&pos.origin))
+                            obj["file"] = *path;
                         obj["line"] = pos.line;
                         obj["column"] = pos.column;
                     }

src/libexpr/eval.hh

@@ -78,7 +78,7 @@ struct RegexCache;
 std::shared_ptr<RegexCache> makeRegexCache();
 
 struct DebugTrace {
-    std::optional<ErrPos> pos;
+    std::shared_ptr<AbstractPos> pos;
     const Expr & expr;
     const Env & env;
     hintformat hint;
@@ -457,8 +457,12 @@ private:
     friend struct ExprAttrs;
     friend struct ExprLet;
 
-    Expr * parse(char * text, size_t length, FileOrigin origin, const PathView path,
-        const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv);
+    Expr * parse(
+        char * text,
+        size_t length,
+        Pos::Origin origin,
+        Path basePath,
+        std::shared_ptr<StaticEnv> & staticEnv);
 
 public:
 

src/libexpr/flake/flake.cc

@@ -220,7 +220,7 @@ static Flake getFlake(
     Value vInfo;
     state.evalFile(flakeFile, vInfo, true); // FIXME: symlink attack
 
-    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile, foFile}, 0, 0));
+    expectType(state, nAttrs, vInfo, state.positions.add({flakeFile}, 1, 1));
 
     if (auto description = vInfo.attrs->get(state.sDescription)) {
         expectType(state, nString, *description->value, description->pos);

src/libexpr/nixexpr.cc

@@ -8,6 +8,58 @@
 
 namespace nix {
 
+struct PosAdapter : AbstractPos
+{
+    Pos::Origin origin;
+
+    PosAdapter(Pos::Origin origin)
+        : origin(std::move(origin))
+    {
+    }
+
+    std::optional<std::string> getSource() const override
+    {
+        return std::visit(overloaded {
+            [](const Pos::none_tag &) -> std::optional<std::string> {
+                return std::nullopt;
+            },
+            [](const Pos::Stdin & s) -> std::optional<std::string> {
+                // Get rid of the null terminators added by the parser.
+                return std::string(s.source->c_str());
+            },
+            [](const Pos::String & s) -> std::optional<std::string> {
+                // Get rid of the null terminators added by the parser.
+                return std::string(s.source->c_str());
+            },
+            [](const Path & path) -> std::optional<std::string> {
+                try {
+                    return readFile(path);
+                } catch (Error &) {
+                    return std::nullopt;
+                }
+            }
+        }, origin);
+    }
+
+    void print(std::ostream & out) const override
+    {
+        std::visit(overloaded {
+            [&](const Pos::none_tag &) { out << "«none»"; },
+            [&](const Pos::Stdin &) { out << "«stdin»"; },
+            [&](const Pos::String & s) { out << "«string»"; },
+            [&](const Path & path) { out << path; }
+        }, origin);
+    }
+};
+
+Pos::operator std::shared_ptr<AbstractPos>() const
+{
+    auto pos = std::make_shared<PosAdapter>(origin);
+    pos->line = line;
+    pos->column = column;
+    return pos;
+}
+
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -248,24 +300,10 @@ void ExprPos::show(const SymbolTable & symbols, std::ostream & str) const
 
 std::ostream & operator << (std::ostream & str, const Pos & pos)
 {
-    if (!pos)
+    if (auto pos2 = (std::shared_ptr<AbstractPos>) pos) {
+        str << *pos2;
+    } else
         str << "undefined position";
-    else
-    {
-        auto f = format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%");
-        switch (pos.origin) {
-            case foFile:
-                f % (const std::string &) pos.file;
-                break;
-            case foStdin:
-            case foString:
-                f % "(string)";
-                break;
-            default:
-                throw Error("unhandled Pos origin!");
-        }
-        str << (f % pos.line % pos.column).str();
-    }
 
     return str;
 }

src/libexpr/nixexpr.hh

@@ -23,15 +23,22 @@ MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 
 /* Position objects. */
-
 struct Pos
 {
-    std::string file;
-    FileOrigin origin;
     uint32_t line;
     uint32_t column;
 
+    struct none_tag { };
+    struct Stdin { ref<std::string> source; };
+    struct String { ref<std::string> source; };
+
+    typedef std::variant<none_tag, Stdin, String, Path> Origin;
+
+    Origin origin;
+
     explicit operator bool() const { return line > 0; }
+
+    operator std::shared_ptr<AbstractPos>() const;
 };
 
 class PosIdx {
@@ -47,7 +54,11 @@ public:
 
     explicit operator bool() const { return id > 0; }
 
-    bool operator<(const PosIdx other) const { return id < other.id; }
+    bool operator <(const PosIdx other) const { return id < other.id; }
+
+    bool operator ==(const PosIdx other) const { return id == other.id; }
+
+    bool operator !=(const PosIdx other) const { return id != other.id; }
 };
 
 class PosTable
@@ -61,13 +72,13 @@ public:
         // current origins.back() can be reused or not.
         mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
 
-        explicit Origin(uint32_t idx): idx(idx), file{}, origin{} {}
+        // Used for searching in PosTable::[].
+        explicit Origin(uint32_t idx): idx(idx), origin{Pos::none_tag()} {}
 
     public:
-        const std::string file;
-        const FileOrigin origin;
+        const Pos::Origin origin;
 
-        Origin(std::string file, FileOrigin origin): file(std::move(file)), origin(origin) {}
+        Origin(Pos::Origin origin): origin(origin) {}
     };
 
     struct Offset {
@@ -107,7 +118,7 @@ public:
             [] (const auto & a, const auto & b) { return a.idx < b.idx; });
         const auto origin = *std::prev(pastOrigin);
         const auto offset = offsets[idx];
-        return {origin.file, origin.origin, offset.line, offset.column};
+        return {offset.line, offset.column, origin.origin};
     }
 };
 

src/libexpr/parser.y

@@ -34,11 +34,6 @@ namespace nix {
         Path basePath;
         PosTable::Origin origin;
         std::optional<ErrorInfo> error;
-        ParseData(EvalState & state, PosTable::Origin origin)
-            : state(state)
-            , symbols(state.symbols)
-            , origin(std::move(origin))
-            { };
     };
 
     struct ParserFormals {
@@ -649,24 +644,20 @@ formal
 namespace nix {
 
 
-Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
-    const PathView path, const PathView basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parse(
+    char * text,
+    size_t length,
+    Pos::Origin origin,
+    Path basePath,
+    std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
-    std::string file;
-    switch (origin) {
-        case foFile:
-            file = path;
-            break;
-        case foStdin:
-        case foString:
-            file = text;
-            break;
-        default:
-            assert(false);
-    }
-    ParseData data(*this, {file, origin});
-    data.basePath = basePath;
+    ParseData data {
+        .state = *this,
+        .symbols = symbols,
+        .basePath = std::move(basePath),
+        .origin = {origin},
+    };
 
     yylex_init(&scanner);
     yy_scan_buffer(text, length, scanner);
@@ -718,14 +709,15 @@ Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<StaticEnv
     auto buffer = readFile(path);
     // readFile should have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), foFile, path, dirOf(path), staticEnv);
+    return parse(buffer.data(), buffer.size(), path, dirOf(path), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s_, const Path & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
-    s.append("\0\0", 2);
-    return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
+    auto s = make_ref<std::string>(std::move(s_));
+    s->append("\0\0", 2);
+    return parse(s->data(), s->size(), Pos::String{.source = s}, basePath, staticEnv);
 }
 
 
@@ -741,7 +733,8 @@ Expr * EvalState::parseStdin()
     auto buffer = drainFD(0);
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), foStdin, "", absPath("."), staticBaseEnv);
+    auto s = make_ref<std::string>(std::move(buffer));
+    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, absPath("."), staticBaseEnv);
 }
 
 

src/libexpr/primops.cc

@@ -368,8 +368,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        auto base = state.positions[pos];
-        parsed = state.parseExprFromString(std::move(output), base.file);
+        parsed = state.parseExprFromString(std::move(output), "/");
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "While parsing the output from '%1%'", program);
         throw;
@@ -798,7 +797,7 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(std::nullopt, state.coerceToString(pos, *args[0], context).toOwned());
+        e.addTrace(nullptr, state.coerceToString(pos, *args[0], context).toOwned());
         throw;
     }
 }
@@ -4018,7 +4017,7 @@ void EvalState::createBaseEnv()
         // the parser needs two NUL bytes as terminators; one of them
         // is implied by being a C string.
         "\0";
-    eval(parse(code, sizeof(code), foFile, derivationNixPath, "/", staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), derivationNixPath, "/", staticBaseEnv), *vDerivation);
 }
 
 

src/libexpr/tests/libexprtests.hh

@@ -123,7 +123,7 @@ namespace nix {
 
     MATCHER_P(IsAttrsOfSize, n, fmt("Is a set of size [%1%]", n)) {
         if (arg.type() != nAttrs) {
-            *result_listener << "Expexted set got " << arg.type();
+            *result_listener << "Expected set got " << arg.type();
             return false;
         } else if (arg.attrs->size() != (size_t)n) {
             *result_listener << "Expected a set with " << n << " attributes but got " << arg.attrs->size();

src/libexpr/tests/primops.cc

@@ -151,20 +151,7 @@ namespace nix {
         // The `y` attribute is at position
         const char* expr = "builtins.unsafeGetAttrPos \"y\" { y = \"x\"; }";
         auto v = eval(expr);
-        ASSERT_THAT(v, IsAttrsOfSize(3));
-
-        auto file = v.attrs->find(createSymbol("file"));
-        ASSERT_NE(file, nullptr);
-        // FIXME: The file when running these tests is the input string?!?
-        ASSERT_THAT(*file->value, IsStringEq(expr));
-
-        auto line = v.attrs->find(createSymbol("line"));
-        ASSERT_NE(line, nullptr);
-        ASSERT_THAT(*line->value, IsIntEq(1));
-
-        auto column = v.attrs->find(createSymbol("column"));
-        ASSERT_NE(column, nullptr);
-        ASSERT_THAT(*column->value, IsIntEq(33));
+        ASSERT_THAT(v, IsNull());
     }
 
     TEST_F(PrimOpTest, hasAttr) {

src/libexpr/value-to-xml.cc

@@ -24,7 +24,8 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
 static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
-    xmlAttrs["path"] = pos.file;
+    if (auto path = std::get_if<Path>(&pos.origin))
+        xmlAttrs["path"] = *path;
     xmlAttrs["line"] = (format("%1%") % pos.line).str();
     xmlAttrs["column"] = (format("%1%") % pos.column).str();
 }

src/libutil/error.cc

@@ -9,9 +9,9 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-void BaseError::addTrace(std::optional<ErrPos> e, hintformat hint)
+void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint)
 {
-    err.traces.push_front(Trace { .pos = e, .hint = hint });
+    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint });
 }
 
 // c++ std::exception descendants must have a 'const char* what()' function.
@@ -30,91 +30,46 @@ const std::string & BaseError::calcWhat() const
 
 std::optional<std::string> ErrorInfo::programName = std::nullopt;
 
-std::ostream & operator<<(std::ostream & os, const hintformat & hf)
+std::ostream & operator <<(std::ostream & os, const hintformat & hf)
 {
     return os << hf.str();
 }
 
-std::string showErrPos(const ErrPos & errPos)
+std::ostream & operator <<(std::ostream & str, const AbstractPos & pos)
 {
-    if (errPos.line > 0) {
-        if (errPos.column > 0) {
-            return fmt("%d:%d", errPos.line, errPos.column);
-        } else {
-            return fmt("%d", errPos.line);
-        }
-    }
-    else {
-        return "";
-    }
+    pos.print(str);
+    str << ":" << pos.line;
+    if (pos.column > 0)
+        str << ":" << pos.column;
+    return str;
 }
 
-std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
+std::optional<LinesOfCode> AbstractPos::getCodeLines() const
 {
-    if (errPos.line <= 0)
+    if (line == 0)
         return std::nullopt;
 
-    if (errPos.origin == foFile) {
-        LinesOfCode loc;
-        try {
-            // FIXME: when running as the daemon, make sure we don't
-            // open a file to which the client doesn't have access.
-            AutoCloseFD fd = open(errPos.file.c_str(), O_RDONLY | O_CLOEXEC);
-            if (!fd) return {};
+    if (auto source = getSource()) {
 
-            // count the newlines.
-            int count = 0;
-            std::string line;
-            int pl = errPos.line - 1;
-            do
-            {
-                line = readLine(fd.get());
-                ++count;
-                if (count < pl)
-                    ;
-                else if (count == pl)
-                    loc.prevLineOfCode = line;
-                else if (count == pl + 1)
-                    loc.errLineOfCode = line;
-                else if (count == pl + 2) {
-                    loc.nextLineOfCode = line;
-                    break;
-                }
-            } while (true);
-            return loc;
-        }
-        catch (EndOfFile & eof) {
-            if (loc.errLineOfCode.has_value())
-                return loc;
-            else
-                return std::nullopt;
-        }
-        catch (std::exception & e) {
-            return std::nullopt;
-        }
-    } else {
-        std::istringstream iss(errPos.file);
+        std::istringstream iss(*source);
         // count the newlines.
         int count = 0;
-        std::string line;
-        int pl = errPos.line - 1;
+        std::string curLine;
+        int pl = line - 1;
 
         LinesOfCode loc;
 
-        do
-        {
-            std::getline(iss, line);
+        do {
+            std::getline(iss, curLine);
             ++count;
             if (count < pl)
-            {
                 ;
-            }
             else if (count == pl) {
-                loc.prevLineOfCode = line;
+                loc.prevLineOfCode = curLine;
             } else if (count == pl + 1) {
-                loc.errLineOfCode = line;
+                loc.errLineOfCode = curLine;
             } else if (count == pl + 2) {
-                loc.nextLineOfCode = line;
+                loc.nextLineOfCode = curLine;
                 break;
             }
 
@@ -124,12 +79,14 @@ std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos)
 
         return loc;
     }
+
+    return std::nullopt;
 }
 
 // print lines of code to the ostream, indicating the error column.
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const ErrPos & errPos,
+    const AbstractPos & errPos,
     const LinesOfCode & loc)
 {
     // previous line of code.
@@ -176,28 +133,6 @@ void printCodeLines(std::ostream & out,
     }
 }
 
-void printAtPos(const ErrPos & pos, std::ostream & out)
-{
-    if (pos) {
-        switch (pos.origin) {
-            case foFile: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "%s:%s" ANSI_NORMAL ":", pos.file, showErrPos(pos));
-                break;
-            }
-            case foString: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "«string»:%s" ANSI_NORMAL ":", showErrPos(pos));
-                break;
-            }
-            case foStdin: {
-                out << fmt(ANSI_BLUE "at " ANSI_WARNING "«stdin»:%s" ANSI_NORMAL ":", showErrPos(pos));
-                break;
-            }
-            default:
-                throw Error("invalid FileOrigin in errPos");
-        }
-    }
-}
-
 static std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s)
 {
     std::string res;
@@ -263,22 +198,22 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     std::ostringstream oss;
 
+    auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
+
     // traces
     if (showTrace && !einfo.traces.empty()) {
         for (const auto & trace : einfo.traces) {
             oss << "\n" << "… " << trace.hint.str() << "\n";
 
-            if (trace.pos.has_value() && (*trace.pos)) {
-                auto pos = trace.pos.value();
-                oss << "\n";
-                printAtPos(pos, oss);
+            if (trace.pos) {
+                oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
 
-                auto loc = getCodeLines(pos);
-                if (loc.has_value()) {
+                if (auto loc = trace.pos->getCodeLines()) {
                     oss << "\n";
-                    printCodeLines(oss, "", pos, *loc);
+                    printCodeLines(oss, "", *trace.pos, *loc);
                     oss << "\n";
-                }
+                } else
+                    oss << noSource;
             }
         }
         oss << "\n" << prefix;
@@ -286,22 +221,19 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     oss << einfo.msg << "\n";
 
-    if (einfo.errPos.has_value() && *einfo.errPos) {
-        oss << "\n";
-        printAtPos(*einfo.errPos, oss);
+    if (einfo.errPos) {
+        oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *einfo.errPos << ANSI_NORMAL << ":";
 
-        auto loc = getCodeLines(*einfo.errPos);
-
-        // lines of code.
-        if (loc.has_value()) {
+        if (auto loc = einfo.errPos->getCodeLines()) {
             oss << "\n";
             printCodeLines(oss, "", *einfo.errPos, *loc);
             oss << "\n";
-        }
+        } else
+            oss << noSource;
     }
 
     auto suggestions = einfo.suggestions.trim();
-    if (! suggestions.suggestions.empty()){
+    if (!suggestions.suggestions.empty()) {
         oss << "Did you mean " <<
             suggestions.trim() <<
             "?" << std::endl;

src/libutil/error.hh

@@ -54,13 +54,6 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
-/* adjust Pos::origin bit width when adding stuff here */
-typedef enum {
-    foFile,
-    foStdin,
-    foString
-} FileOrigin;
-
 // the lines of code surrounding an error.
 struct LinesOfCode {
     std::optional<std::string> prevLineOfCode;
@@ -68,54 +61,37 @@ struct LinesOfCode {
     std::optional<std::string> nextLineOfCode;
 };
 
-// ErrPos indicates the location of an error in a nix file.
-struct ErrPos {
-    int line = 0;
-    int column = 0;
-    std::string file;
-    FileOrigin origin;
+/* An abstract type that represents a location in a source file. */
+struct AbstractPos
+{
+    uint32_t line = 0;
+    uint32_t column = 0;
 
-    operator bool() const
-    {
-        return line != 0;
-    }
+    /* Return the contents of the source file. */
+    virtual std::optional<std::string> getSource() const
+    { return std::nullopt; };
 
-    // convert from the Pos struct, found in libexpr.
-    template <class P>
-    ErrPos & operator=(const P & pos)
-    {
-        origin = pos.origin;
-        line = pos.line;
-        column = pos.column;
-        file = pos.file;
-        return *this;
-    }
+    virtual void print(std::ostream & out) const = 0;
 
-    template <class P>
-    ErrPos(const P & p)
-    {
-        *this = p;
-    }
+    std::optional<LinesOfCode> getCodeLines() const;
 };
 
-std::optional<LinesOfCode> getCodeLines(const ErrPos & errPos);
+std::ostream & operator << (std::ostream & str, const AbstractPos & pos);
 
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
-    const ErrPos & errPos,
+    const AbstractPos & errPos,
     const LinesOfCode & loc);
 
-void printAtPos(const ErrPos & pos, std::ostream & out);
-
 struct Trace {
-    std::optional<ErrPos> pos;
+    std::shared_ptr<AbstractPos> pos;
     hintformat hint;
 };
 
 struct ErrorInfo {
     Verbosity level;
     hintformat msg;
-    std::optional<ErrPos> errPos;
+    std::shared_ptr<AbstractPos> errPos;
     std::list<Trace> traces;
 
     Suggestions suggestions;
@@ -177,12 +153,12 @@ public:
     const ErrorInfo & info() const { calcWhat(); return err; }
 
     template<typename... Args>
-    void addTrace(std::optional<ErrPos> e, const std::string & fs, const Args & ... args)
+    void addTrace(std::shared_ptr<AbstractPos> && e, const std::string & fs, const Args & ... args)
     {
-        addTrace(e, hintfmt(fs, args...));
+        addTrace(std::move(e), hintfmt(fs, args...));
     }
 
-    void addTrace(std::optional<ErrPos> e, hintformat hint);
+    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint);
 
     bool hasTrace() const { return !err.traces.empty(); }
 };

src/libutil/logging.cc

@@ -131,6 +131,21 @@ Activity::Activity(Logger & logger, Verbosity lvl, ActivityType type,
     logger.startActivity(id, lvl, type, s, fields, parent);
 }
 
+void to_json(nlohmann::json & json, std::shared_ptr<AbstractPos> pos)
+{
+    if (pos) {
+        json["line"] = pos->line;
+        json["column"] = pos->column;
+        std::ostringstream str;
+        pos->print(str);
+        json["file"] = str.str();
+    } else {
+        json["line"] = nullptr;
+        json["column"] = nullptr;
+        json["file"] = nullptr;
+    }
+}
+
 struct JSONLogger : Logger {
     Logger & prevLogger;
 
@@ -177,27 +192,14 @@ struct JSONLogger : Logger {
         json["level"] = ei.level;
         json["msg"] = oss.str();
         json["raw_msg"] = ei.msg.str();
-
-        if (ei.errPos.has_value() && (*ei.errPos)) {
-            json["line"] = ei.errPos->line;
-            json["column"] = ei.errPos->column;
-            json["file"] = ei.errPos->file;
-        } else {
-            json["line"] = nullptr;
-            json["column"] = nullptr;
-            json["file"] = nullptr;
-        }
+        to_json(json, ei.errPos);
 
         if (loggerSettings.showTrace.get() && !ei.traces.empty()) {
             nlohmann::json traces = nlohmann::json::array();
             for (auto iter = ei.traces.rbegin(); iter != ei.traces.rend(); ++iter) {
                 nlohmann::json stackFrame;
                 stackFrame["raw_msg"] = iter->hint.str();
-                if (iter->pos.has_value() && (*iter->pos)) {
-                    stackFrame["line"] = iter->pos->line;
-                    stackFrame["column"] = iter->pos->column;
-                    stackFrame["file"] = iter->pos->file;
-                }
+                to_json(stackFrame, iter->pos);
                 traces.push_back(stackFrame);
             }
 

src/nix-env/nix-env.cc

@@ -647,7 +647,7 @@ static void upgradeDerivations(Globals & globals,
                 } else newElems.push_back(i);
 
             } catch (Error & e) {
-                e.addTrace(std::nullopt, "while trying to find an upgrade for '%s'", i.queryName());
+                e.addTrace(nullptr, "while trying to find an upgrade for '%s'", i.queryName());
                 throw;
             }
         }
@@ -958,7 +958,7 @@ static void queryJSON(Globals & globals, std::vector<DrvInfo> & elems, bool prin
         } catch (AssertionError & e) {
             printMsg(lvlTalkative, "skipping derivation named '%1%' which gives an assertion failure", i.queryName());
         } catch (Error & e) {
-            e.addTrace(std::nullopt, "while querying the derivation named '%1%'", i.queryName());
+            e.addTrace(nullptr, "while querying the derivation named '%1%'", i.queryName());
             throw;
         }
     }
@@ -1262,7 +1262,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         } catch (AssertionError & e) {
             printMsg(lvlTalkative, "skipping derivation named '%1%' which gives an assertion failure", i.queryName());
         } catch (Error & e) {
-            e.addTrace(std::nullopt, "while querying the derivation named '%1%'", i.queryName());
+            e.addTrace(nullptr, "while querying the derivation named '%1%'", i.queryName());
             throw;
         }
     }

tests/function-trace.sh

@@ -32,40 +32,40 @@ expect_trace() {
 
 # failure inside a tryEval
 expect_trace 'builtins.tryEval (throw "example")' "
-function-trace entered (string):1:1 at
-function-trace entered (string):1:19 at
-function-trace exited (string):1:19 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace entered «string»:1:19 at
+function-trace exited «string»:1:19 at
+function-trace exited «string»:1:1 at
 "
 
 # Missing argument to a formal function
 expect_trace '({ x }: x) { }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a formal function
 expect_trace '({ x }: x) { x = "x"; y = "y"; }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not enough arguments to a lambda
 expect_trace '(x: y: x + y) 1' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a lambda
 expect_trace '(x: x) 1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not a function
 expect_trace '1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 set -e