Introduce AbstractPos

This makes the position object used in exceptions abstract, with a
method getSource() to get the source code of the file in which the
error originated. This is needed for lazy trees because source files
don't necessarily exist in the filesystem, and we don't want to make
libutil depend on the InputAccessor type in libfetcher.

15 files changed, 218 insertions, 266 deletions

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index c704fcfb1..254c14d28 100644
--- a/src/libcmd/repl.cc
+++ b/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
+        : (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();
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 0d9226d3b..b5e0051ac 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -823,7 +823,7 @@ 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 : (std::shared_ptr<AbstractPos>) positions[expr.getPos()],
                 .expr = expr,
                 .env = env,
                 .hint = error->info().msg,
@@ -1012,7 +1012,7 @@ 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
@@ -1024,13 +1024,13 @@ 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),
@@ -1136,9 +1136,9 @@ 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);
@@ -1246,7 +1246,7 @@ void EvalState::cacheFile(
                 *this,
                 *e,
                 this->baseEnv,
-                e->getPos() ? std::optional(ErrPos(positions[e->getPos()])) : std::nullopt,
+                e->getPos() ? (std::shared_ptr<AbstractPos>) positions[e->getPos()] : nullptr,
                 "while evaluating the file '%1%':", resolvedPath)
             : nullptr;
 
@@ -1517,10 +1517,13 @@ 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;
     }
 
@@ -2497,7 +2500,8 @@ 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;
                     }
@@ -2511,7 +2515,8 @@ 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;
                     }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index cf307d820..21666339b 100644
--- a/src/libexpr/eval.hh
+++ b/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:
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 6344fb253..105d32467 100644
--- a/src/libexpr/flake/flake.cc
+++ b/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);
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 2be560d76..bdfb6457a 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -8,6 +8,65 @@
 
 namespace nix {
 
+struct SourcePathAdapter : AbstractPos
+{
+    Path path;
+
+    SourcePathAdapter(Path path)
+        : path(std::move(path))
+    {
+    }
+
+    std::optional<std::string> getSource() const override
+    {
+        try {
+            return readFile(path);
+        } catch (Error &) {
+            return std::nullopt;
+        }
+    }
+
+    void print(std::ostream & out) const override
+    {
+        out << path;
+    }
+};
+
+struct StringPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << "«string»";
+    }
+};
+
+struct StdinPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << "«stdin»";
+    }
+};
+
+Pos::operator std::shared_ptr<AbstractPos>() const
+{
+    std::shared_ptr<AbstractPos> pos;
+
+    if (auto path = std::get_if<Path>(&origin))
+        pos = std::make_shared<SourcePathAdapter>(*path);
+    else if (std::get_if<stdin_tag>(&origin))
+        pos = std::make_shared<StdinPosAdapter>();
+    else if (std::get_if<string_tag>(&origin))
+        pos = std::make_shared<StringPosAdapter>();
+
+    if (pos) {
+        pos->line = line;
+        pos->column = column;
+    }
+
+    return pos;
+}
+
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -248,24 +307,10 @@ 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;
 }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 5eb022770..0338a8c37 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -23,15 +23,21 @@ MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 
 /* Position objects. */
-
 struct Pos
 {
-    std::string file;
-    FileOrigin origin;
     uint32_t line;
     uint32_t column;
 
+    struct stdin_tag {};
+    struct string_tag {};
+
+    typedef std::variant<stdin_tag, string_tag, Path> Origin;
+
+    Origin origin;
+
     explicit operator bool() const { return line > 0; }
+
+    operator std::shared_ptr<AbstractPos>() const;
 };
 
 class PosIdx {
@@ -47,7 +53,11 @@ 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 +71,13 @@ 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::stdin_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 +117,7 @@ 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};
     }
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index fbf865719..ea4e0bfb0 100644
--- a/src/libexpr/parser.y
+++ b/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 +709,14 @@ 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)
 {
     s.append("\0\0", 2);
-    return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
+    return parse(s.data(), s.size(), Pos::string_tag(), basePath, staticEnv);
 }
 
 
@@ -741,7 +732,7 @@ 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);
+    return parse(buffer.data(), buffer.size(), Pos::stdin_tag(), absPath("."), staticBaseEnv);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 283d2746b..7efe50324 100644
--- a/src/libexpr/primops.cc
+++ b/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);
 }
 
 
diff --git a/src/libexpr/tests/libexprtests.hh b/src/libexpr/tests/libexprtests.hh
index 4f6915882..5bb5e66d3 100644
--- a/src/libexpr/tests/libexprtests.hh
+++ b/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();
diff --git a/src/libexpr/tests/primops.cc b/src/libexpr/tests/primops.cc
index 16cf66d2c..49fbc5e98 100644
--- a/src/libexpr/tests/primops.cc
+++ b/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) {
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index 7c3bf9492..3f6222768 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/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();
 }
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 3bb3efb0e..1a1aecea5 100644
--- a/src/libutil/error.cc
+++ b/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 {};
-
-            // 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);
+    if (auto source = getSource()) {
+
+        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);
-
-        auto loc = getCodeLines(*einfo.errPos);
+    if (einfo.errPos) {
+        oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *einfo.errPos << ANSI_NORMAL << ":";
 
-        // 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;
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 3d1479c54..c3bb8c0df 100644
--- a/src/libutil/error.hh
+++ b/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(); }
 };
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index ac86d8ac2..904ba6ebe 100644
--- a/src/libutil/logging.cc
+++ b/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);
             }
 
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 776c5f6db..31823a966 100644
--- a/src/nix-env/nix-env.cc
+++ b/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;
         }
     }