diff --git a/third_party/nix/src/libexpr/attr-set.cc b/third_party/nix/src/libexpr/attr-set.cc
index 8b34be6fd..42ebe629b 100644
--- a/third_party/nix/src/libexpr/attr-set.cc
+++ b/third_party/nix/src/libexpr/attr-set.cc
@@ -59,7 +59,7 @@ Bindings::iterator Bindings::end() { return attributes_.end(); }
 
 void Bindings::merge(const Bindings& other) {
   for (auto& [key, value] : other.attributes_) {
-    this->attributes_[key] = value;
+    this->attributes_.insert_or_assign(key, value);
   }
 }
 
diff --git a/third_party/nix/src/libexpr/attr-set.hh b/third_party/nix/src/libexpr/attr-set.hh
index 31e5101d8..d4823cb19 100644
--- a/third_party/nix/src/libexpr/attr-set.hh
+++ b/third_party/nix/src/libexpr/attr-set.hh
@@ -20,7 +20,6 @@ struct Attr {
   Pos* pos;      // TODO(tazjin): Who owns this?
   Attr(Symbol name, Value* value, Pos* pos = &noPos)
       : name(name), value(value), pos(pos){};
-  Attr() : pos(&noPos){};
 };
 
 // Convenience alias for the backing map, with the garbage-collecting
diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index 6df7e66b6..fb6213069 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -6,6 +6,7 @@
 #include <fstream>
 #include <iostream>
 #include <new>
+#include <optional>
 #include <variant>
 
 #include <absl/strings/match.h>
@@ -320,6 +321,7 @@ EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
       sOutputHash(symbols.Create("outputHash")),
       sOutputHashAlgo(symbols.Create("outputHashAlgo")),
       sOutputHashMode(symbols.Create("outputHashMode")),
+      sDerivationNix(std::nullopt),
       repair(NoRepair),
       store(store),
       baseEnv(allocEnv(128)),
@@ -664,9 +666,9 @@ static inline void mkThunk(Value& v, Env& env, Expr* expr) {
 void EvalState::mkThunk_(Value& v, Expr* expr) { mkThunk(v, baseEnv, expr); }
 
 void EvalState::mkPos(Value& v, Pos* pos) {
-  if ((pos != nullptr) && pos->file.set()) {
+  if ((pos != nullptr) && pos->file.has_value() && pos->file.value().set()) {
     mkAttrs(v, 3);
-    mkString(*allocAttr(v, sFile), pos->file);
+    mkString(*allocAttr(v, sFile), pos->file.value());
     mkInt(*allocAttr(v, sLine), pos->line);
     mkInt(*allocAttr(v, sColumn), pos->column);
   } else {
@@ -847,7 +849,6 @@ void ExprAttrs::eval(EvalState& state, Env& env, Value& value) {
                      nameSym, i.pos, *j->second.pos);
     }
 
-    i.valueExpr->setName(nameSym);
     value.attrs->push_back(
         Attr(nameSym, i.valueExpr->maybeThunk(state, *dynamicEnv), &i.pos));
   }
@@ -936,7 +937,13 @@ void ExprSelect::eval(EvalState& state, Env& env, Value& v) {
     state.forceValue(*vAttrs, (pos2 != nullptr ? *pos2 : this->pos));
 
   } catch (Error& e) {
-    if ((pos2 != nullptr) && pos2->file != state.sDerivationNix) {
+    // This code relies on 'sDerivationNix' being correcty mutated at
+    // some prior point (it would previously otherwise have been a
+    // nullptr).
+    //
+    // We haven't seen this fail, so for now the contained value is
+    // just accessed at the risk of potentially crashing.
+    if ((pos2 != nullptr) && pos2->file != state.sDerivationNix.value()) {
       addErrorPrefix(e, "while evaluating the attribute '%1%' at %2%:\n",
                      showAttrPath(state, env, attrPath), *pos2);
     }
@@ -1792,8 +1799,8 @@ void EvalState::printStats() {
         auto list = topObj.list("functions");
         for (auto& i : functionCalls) {
           auto obj = list.object();
-          if (i.first->name.set()) {
-            obj.attr("name", (const std::string&)i.first->name);
+          if (i.first->name.has_value()) {
+            obj.attr("name", (const std::string&)i.first->name.value());
           } else {
             obj.attr("name", nullptr);
           }
diff --git a/third_party/nix/src/libexpr/eval.hh b/third_party/nix/src/libexpr/eval.hh
index 9e4e500b4..11cc295b2 100644
--- a/third_party/nix/src/libexpr/eval.hh
+++ b/third_party/nix/src/libexpr/eval.hh
@@ -62,10 +62,13 @@ class EvalState {
   SymbolTable symbols;
 
   const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue, sSystem,
-      sOverrides, sOutputs, sOutputName, sIgnoreNulls, sFile, sLine, sColumn,
-      sFunctor, sToString, sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
-      sOutputHash, sOutputHashAlgo, sOutputHashMode;
-  Symbol sDerivationNix;
+      sOutputs, sOutputName, sIgnoreNulls, sFile, sLine, sColumn, sFunctor,
+      sToString, sRight, sWrong, sStructuredAttrs, sBuilder, sArgs, sOutputHash,
+      sOutputHashAlgo, sOutputHashMode;
+
+  // Symbol representing the path to the built-in 'derivation.nix'
+  // file, set during primops initialisation.
+  std::optional<Symbol> sDerivationNix;
 
   /* If set, force copying files to the Nix store even if they
      already exist there. */
diff --git a/third_party/nix/src/libexpr/nixexpr.cc b/third_party/nix/src/libexpr/nixexpr.cc
index 2de37f50e..94e7d335c 100644
--- a/third_party/nix/src/libexpr/nixexpr.cc
+++ b/third_party/nix/src/libexpr/nixexpr.cc
@@ -187,11 +187,11 @@ void ExprConcatStrings::show(std::ostream& str) const {
 void ExprPos::show(std::ostream& str) const { str << "__curPos"; }
 
 std::ostream& operator<<(std::ostream& str, const Pos& pos) {
-  if (!pos) {
+  if (!pos || !pos.file.has_value()) {
     str << "undefined position";
   } else {
     str << (format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%") %
-            (std::string)pos.file % pos.line % pos.column)
+            (std::string)pos.file.value() % pos.line % pos.column)
                .str();
   }
   return str;
@@ -401,17 +401,12 @@ void ExprConcatStrings::bindVars(const StaticEnv& env) {
 void ExprPos::bindVars(const StaticEnv& env) {}
 
 /* Storing function names. */
-
-void Expr::setName(Symbol& name) {}
-
-void ExprLambda::setName(Symbol& name) {
-  this->name = name;
-  body->setName(name);
-}
+void ExprLambda::setName(Symbol& name) { this->name = name; }
 
 std::string ExprLambda::showNamePos() const {
   return (format("%1% at %2%") %
-          (name.set() ? "'" + (std::string)name + "'" : "anonymous function") %
+          (name.has_value() ? "'" + (std::string)name.value() + "'"
+                            : "anonymous function") %
           pos)
       .str();
 }
diff --git a/third_party/nix/src/libexpr/nixexpr.hh b/third_party/nix/src/libexpr/nixexpr.hh
index 7ec6f7f17..3bf9e4a1a 100644
--- a/third_party/nix/src/libexpr/nixexpr.hh
+++ b/third_party/nix/src/libexpr/nixexpr.hh
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <map>
+#include <optional>
 #include <variant>
 
 #include "libexpr/symbol-table.hh"
@@ -21,20 +22,28 @@ MakeError(RestrictedPathError, Error);
 /* Position objects. */
 
 struct Pos {
-  Symbol file;
+  std::optional<Symbol> file;
   unsigned int line, column;
-  Pos() : line(0), column(0){};
-  Pos(const Symbol& file, unsigned int line, unsigned int column)
+  Pos(const std::optional<Symbol>& file, unsigned int line, unsigned int column)
       : file(file), line(line), column(column){};
+
+  // TODO(tazjin): remove this - empty pos is never useful
+  Pos() : file(std::nullopt), line(0), column(0){};
+
   operator bool() const { return line != 0; }
+
   bool operator<(const Pos& p2) const {
+    if (!file.has_value()) {
+      return true;
+    }
+
     if (!line) {
       return p2.line;
     }
     if (!p2.line) {
       return false;
     }
-    int d = ((std::string)file).compare((std::string)p2.file);
+    int d = ((std::string)file.value()).compare((std::string)p2.file.value());
     if (d < 0) {
       return true;
     }
@@ -75,7 +84,6 @@ struct Expr {
   virtual void bindVars(const StaticEnv& env);
   virtual void eval(EvalState& state, Env& env, Value& v);
   virtual Value* maybeThunk(EvalState& state, Env& env);
-  virtual void setName(Symbol& name);
 };
 
 std::ostream& operator<<(std::ostream& str, const Expr& e);
@@ -219,8 +227,9 @@ struct Formals {
 };
 
 struct ExprLambda : Expr {
+ public:
   Pos pos;
-  Symbol name;
+  std::optional<Symbol> name;
   Symbol arg;
   bool matchAttrs;
   Formals* formals;
@@ -233,10 +242,11 @@ struct ExprLambda : Expr {
         formals(formals),
         body(body) {
     if (!arg.empty() && formals &&
-        formals->argNames.find(arg) != formals->argNames.end())
+        formals->argNames.find(arg) != formals->argNames.end()) {
       throw ParseError(
           format("duplicate formal function argument '%1%' at %2%") % arg %
           pos);
+    }
   };
   void setName(Symbol& name);
   std::string showNamePos() const;
diff --git a/third_party/nix/src/libexpr/parser.y b/third_party/nix/src/libexpr/parser.y
index eb3f92db7..2ea84fea6 100644
--- a/third_party/nix/src/libexpr/parser.y
+++ b/third_party/nix/src/libexpr/parser.y
@@ -16,6 +16,7 @@
 #ifndef BISON_HEADER
 #define BISON_HEADER
 
+#include <optional>
 #include <variant>
 #include "libutil/util.hh"
 #include "libexpr/nixexpr.hh"
@@ -28,9 +29,9 @@ namespace nix {
     {
         EvalState & state;
         SymbolTable & symbols;
-        Expr * result;
+        Expr* result;
         Path basePath;
-        Symbol path;
+        std::optional<Symbol> path;
         std::string error;
         Symbol sLetBody;
         ParseData(EvalState & state)
@@ -139,7 +140,6 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
         } else {
             // This attr path is not defined. Let's create it.
             attrs->attrs[*sym] = ExprAttrs::AttrDef(e, pos);
-            e->setName(*sym);
         }
     } else {
         // Same caveat as the identical line above.
@@ -502,8 +502,9 @@ attrpath
       if (str) {
           $$->push_back(AttrName(str->s));
           delete str;
-      } else
+      } else {
           $$->push_back(AttrName($3));
+      }
     }
   | attr { $$ = new std::vector<AttrName>; $$->push_back(AttrName(data->symbols.Create($1))); }
   | string_attr
diff --git a/third_party/nix/src/libexpr/primops.cc b/third_party/nix/src/libexpr/primops.cc
index b238da762..d96af55f5 100644
--- a/third_party/nix/src/libexpr/primops.cc
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -234,7 +234,7 @@ void prim_exec(EvalState& state, const Pos& pos, Value** args, Value& v) {
   auto output = runProgram(program, true, commandArgs);
   Expr* parsed;
   try {
-    parsed = state.parseExprFromString(output, pos.file);
+    parsed = state.parseExprFromString(output, pos.file.value());
   } catch (Error& e) {
     e.addPrefix(format("While parsing the output from '%1%', at %2%\n") %
                 program % pos);
diff --git a/third_party/nix/src/libexpr/symbol-table.hh b/third_party/nix/src/libexpr/symbol-table.hh
index efc2314b7..dcb44d32f 100644
--- a/third_party/nix/src/libexpr/symbol-table.hh
+++ b/third_party/nix/src/libexpr/symbol-table.hh
@@ -13,8 +13,6 @@ class Symbol {
   friend class SymbolTable;
 
  public:
-  Symbol() : s(0){};
-
   bool operator==(const Symbol& s2) const { return s == s2.s; }
 
   bool operator!=(const Symbol& s2) const { return s != s2.s; }
diff --git a/third_party/nix/src/libexpr/value-to-xml.cc b/third_party/nix/src/libexpr/value-to-xml.cc
index 2a46ac84b..ea1a85a89 100644
--- a/third_party/nix/src/libexpr/value-to-xml.cc
+++ b/third_party/nix/src/libexpr/value-to-xml.cc
@@ -20,7 +20,7 @@ static void printValueAsXML(EvalState& state, bool strict, bool location,
                             PathSet& drvsSeen);
 
 static void posToXML(XMLAttrs& xmlAttrs, const Pos& pos) {
-  xmlAttrs["path"] = pos.file;
+  xmlAttrs["path"] = pos.file.value();
   xmlAttrs["line"] = (format("%1%") % pos.line).str();
   xmlAttrs["column"] = (format("%1%") % pos.column).str();
 }