Introduce AbstractPos

src/libexpr/eval.cc

@@ -775,7 +775,7 @@ 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,
@@ -957,7 +957,7 @@ 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
@@ -969,13 +969,13 @@ 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),
@@ -1171,7 +1171,7 @@ void EvalState::evalFile(const SourcePath & path, Value & v, bool mustBeTrivial)
                 *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.to_string())
             : nullptr;
 

src/libexpr/eval.hh

@@ -76,7 +76,7 @@ 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;

src/libexpr/get-drvs.cc

@@ -150,7 +150,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
         const Value * outTI = queryMeta("outputsToInstall");
         if (!outTI) return outputs;
-        const auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+        auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
             /* ^ this shows during `nix-env -i` right under the bad derivation */
         if (!outTI->isList()) throw errMsg;
         Outputs result;

src/libexpr/nixexpr.cc

@@ -8,6 +8,64 @@
 
 namespace nix {
 
+struct SourcePathAdapter : AbstractPos
+{
+    std::string file;
+
+    std::optional<std::string> getSource() const override
+    {
+        return std::nullopt;
+    }
+
+    void print(std::ostream & out) const override
+    {
+        out << fmt(ANSI_BLUE "at " ANSI_WARNING "%s:%s" ANSI_NORMAL ":", file, showErrPos());
+    }
+};
+
+struct StringPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << fmt(ANSI_BLUE "at " ANSI_WARNING "«string»:%s" ANSI_NORMAL ":", showErrPos());
+    }
+};
+
+struct StdinPosAdapter : AbstractPos
+{
+    void print(std::ostream & out) const override
+    {
+        out << fmt(ANSI_BLUE "at " ANSI_WARNING "«stdin»:%s" ANSI_NORMAL ":", showErrPos());
+    }
+};
+
+Pos::operator std::shared_ptr<AbstractPos>() const
+{
+    if (!line) return nullptr;
+
+    switch (origin) {
+    case foFile: {
+        auto pos = std::make_shared<SourcePathAdapter>();
+        pos->line = line;
+        pos->column = column;
+        pos->file = file;
+        return pos;
+    }
+    case foStdin: {
+        auto pos = std::make_shared<StdinPosAdapter>();
+        pos->line = line;
+        pos->column = column;
+        return pos;
+    }
+    case foString:
+        auto pos = std::make_shared<StringPosAdapter>();
+        pos->line = line;
+        pos->column = column;
+        return pos;
+    }
+    assert(false);
+}
+
 /* Displaying abstract syntax trees. */
 
 static void showString(std::ostream & str, std::string_view s)
@@ -289,7 +347,6 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 }
 
 
-
 /* Computing levels/displacements for variables. */
 
 void Expr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)

src/libexpr/nixexpr.hh

@@ -21,8 +21,14 @@ MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 
-/* Position objects. */
+// FIXME: change this into a variant?
+typedef enum {
+    foFile,
+    foStdin,
+    foString
+} FileOrigin;
 
+/* Position objects. */
 struct Pos
 {
     std::string file;
@@ -31,6 +37,8 @@ struct Pos
     uint32_t column;
 
     explicit operator bool() const { return line > 0; }
+
+    operator std::shared_ptr<AbstractPos>() const;
 };
 
 class PosIdx {

src/libexpr/primops.cc

@@ -799,7 +799,7 @@ 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;
     }
 }