diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index bdfb6457a..eb6f062b4 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -8,62 +8,55 @@
 
 namespace nix {
 
-struct SourcePathAdapter : AbstractPos
+struct PosAdapter : AbstractPos
 {
-    Path path;
+    Pos::Origin origin;
 
-    SourcePathAdapter(Path path)
-        : path(std::move(path))
+    PosAdapter(Pos::Origin origin)
+        : origin(std::move(origin))
     {
     }
 
     std::optional<std::string> getSource() const override
     {
-        try {
-            return readFile(path);
-        } catch (Error &) {
-            return std::nullopt;
-        }
+        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
     {
-        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»";
+        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
 {
-    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;
-    }
-
+    auto pos = std::make_shared<PosAdapter>(origin);
+    pos->line = line;
+    pos->column = column;
     return pos;
 }
 
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 0338a8c37..ac7ce021e 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -28,10 +28,11 @@ struct Pos
     uint32_t line;
     uint32_t column;
 
-    struct stdin_tag {};
-    struct string_tag {};
+    struct none_tag { };
+    struct Stdin { ref<std::string> source; };
+    struct String { ref<std::string> source; };
 
-    typedef std::variant<stdin_tag, string_tag, Path> Origin;
+    typedef std::variant<none_tag, Stdin, String, Path> Origin;
 
     Origin origin;
 
@@ -72,7 +73,7 @@ public:
         mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
 
         // Used for searching in PosTable::[].
-        explicit Origin(uint32_t idx): idx(idx), origin{Pos::stdin_tag()} {}
+        explicit Origin(uint32_t idx): idx(idx), origin{Pos::none_tag()} {}
 
     public:
         const Pos::Origin origin;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index ea4e0bfb0..e07909f8e 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -713,10 +713,11 @@ Expr * EvalState::parseExprFromFile(const Path & path, std::shared_ptr<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(), Pos::string_tag(), 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);
 }
 
 
@@ -732,7 +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(), Pos::stdin_tag(), absPath("."), staticBaseEnv);
+    auto s = make_ref<std::string>(std::move(buffer));
+    return parse(s->data(), s->size(), Pos::Stdin{.source = s}, absPath("."), staticBaseEnv);
 }