refactor(3p/nix): Apply clang-tidy's performance-* fixes r/789

This applies the performance fixes listed here:

https://clang.llvm.org/extra/clang-tidy/checks/list.html

60 files changed, 189 insertions, 166 deletions

diff --git a/third_party/nix/src/libexpr/common-eval-args.cc b/third_party/nix/src/libexpr/common-eval-args.cc
index 7a87841c9b..33319cbabb 100644
--- a/third_party/nix/src/libexpr/common-eval-args.cc
+++ b/third_party/nix/src/libexpr/common-eval-args.cc
@@ -29,7 +29,7 @@ MixEvalArgs::MixEvalArgs() {
           "add a path to the list of locations used to look up <...> file "
           "names")
       .label("path")
-      .handler([&](std::string s) { searchPath.push_back(s); });
+      .handler([&](const std::string& s) { searchPath.push_back(s); });
 }
 
 Bindings* MixEvalArgs::getAutoArgs(EvalState& state) {
diff --git a/third_party/nix/src/libexpr/eval.cc b/third_party/nix/src/libexpr/eval.cc
index 4be301167b..4053dffe51 100644
--- a/third_party/nix/src/libexpr/eval.cc
+++ b/third_party/nix/src/libexpr/eval.cc
@@ -298,7 +298,7 @@ static Strings parseNixPath(const string& s) {
   return res;
 }
 
-EvalState::EvalState(const Strings& _searchPath, ref<Store> store)
+EvalState::EvalState(const Strings& _searchPath, const ref<Store>& store)
     : sWith(symbols.create("<with>")),
       sOutPath(symbols.create("outPath")),
       sDrvPath(symbols.create("drvPath")),
diff --git a/third_party/nix/src/libexpr/eval.hh b/third_party/nix/src/libexpr/eval.hh
index 48c67b6229..48400d5f08 100644
--- a/third_party/nix/src/libexpr/eval.hh
+++ b/third_party/nix/src/libexpr/eval.hh
@@ -105,7 +105,7 @@ class EvalState {
   std::unordered_map<Path, Path> resolvedPaths;
 
  public:
-  EvalState(const Strings& _searchPath, ref<Store> store);
+  EvalState(const Strings& _searchPath, const ref<Store>& store);
   ~EvalState();
 
   void addToSearchPath(const string& s);
diff --git a/third_party/nix/src/libexpr/get-drvs.cc b/third_party/nix/src/libexpr/get-drvs.cc
index c4afc0fb45..39870abcbd 100644
--- a/third_party/nix/src/libexpr/get-drvs.cc
+++ b/third_party/nix/src/libexpr/get-drvs.cc
@@ -15,7 +15,7 @@ namespace nix {
 DrvInfo::DrvInfo(EvalState& state, string attrPath, Bindings* attrs)
     : state(&state), attrs(attrs), attrPath(std::move(attrPath)) {}
 
-DrvInfo::DrvInfo(EvalState& state, ref<Store> store,
+DrvInfo::DrvInfo(EvalState& state, const ref<Store>& store,
                  const std::string& drvPathWithOutputs)
     : state(&state), attrPath("") {
   auto spec = parseDrvPathWithOutputs(drvPathWithOutputs);
diff --git a/third_party/nix/src/libexpr/get-drvs.hh b/third_party/nix/src/libexpr/get-drvs.hh
index ef6ecd253e..f0de0f67b8 100644
--- a/third_party/nix/src/libexpr/get-drvs.hh
+++ b/third_party/nix/src/libexpr/get-drvs.hh
@@ -34,7 +34,7 @@ struct DrvInfo {
 
   DrvInfo(EvalState& state) : state(&state){};
   DrvInfo(EvalState& state, string attrPath, Bindings* attrs);
-  DrvInfo(EvalState& state, ref<Store> store,
+  DrvInfo(EvalState& state, const ref<Store>& store,
           const std::string& drvPathWithOutputs);
 
   string queryName() const;
diff --git a/third_party/nix/src/libexpr/primops.cc b/third_party/nix/src/libexpr/primops.cc
index 7b73bec035..307459396e 100644
--- a/third_party/nix/src/libexpr/primops.cc
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -34,7 +34,7 @@ namespace nix {
    name>. */
 std::pair<string, string> decodeContext(const string& s) {
   if (s.at(0) == '!') {
-    size_t index = s.find("!", 1);
+    size_t index = s.find('!', 1);
     return std::pair<string, string>(string(s, index + 1),
                                      string(s, 1, index - 1));
   }
@@ -2172,7 +2172,7 @@ static void prim_splitVersion(EvalState& state, const Pos& pos, Value** args,
   unsigned int n = 0;
   for (auto& component : components) {
     auto listElem = v.listElems()[n++] = state.allocValue();
-    mkString(*listElem, std::move(component));
+    mkString(*listElem, component);
   }
 }
 
@@ -2246,7 +2246,8 @@ static void prim_fetchTarball(EvalState& state, const Pos& pos, Value** args,
 
 RegisterPrimOp::PrimOps* RegisterPrimOp::primOps;
 
-RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun) {
+RegisterPrimOp::RegisterPrimOp(const std::string& name, size_t arity,
+                               PrimOpFun fun) {
   if (primOps == nullptr) {
     primOps = new PrimOps;
   }
diff --git a/third_party/nix/src/libexpr/primops.hh b/third_party/nix/src/libexpr/primops.hh
index 8e674509c7..6abd0508a0 100644
--- a/third_party/nix/src/libexpr/primops.hh
+++ b/third_party/nix/src/libexpr/primops.hh
@@ -11,7 +11,7 @@ struct RegisterPrimOp {
   /* You can register a constant by passing an arity of 0. fun
      will get called during EvalState initialization, so there
      may be primops not yet added and builtins is not yet sorted. */
-  RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun);
+  RegisterPrimOp(const std::string& name, size_t arity, PrimOpFun fun);
 };
 
 /* These primops are disabled without enableNativeCode, but plugins
diff --git a/third_party/nix/src/libmain/common-args.cc b/third_party/nix/src/libmain/common-args.cc
index 9d6f6445c0..3d9b8ebfae 100644
--- a/third_party/nix/src/libmain/common-args.cc
+++ b/third_party/nix/src/libmain/common-args.cc
@@ -26,7 +26,7 @@ MixCommonArgs::MixCommonArgs(const string& programName)
       .shortName('j')
       .label("jobs")
       .description("maximum number of parallel builds")
-      .handler([=](std::string s) { settings.set("max-jobs", s); });
+      .handler([=](const std::string& s) { settings.set("max-jobs", s); });
 
   std::string cat = "config";
   globalConfig.convertToArgs(*this, cat);
diff --git a/third_party/nix/src/libmain/shared.cc b/third_party/nix/src/libmain/shared.cc
index 5c20846911..f6c80cae30 100644
--- a/third_party/nix/src/libmain/shared.cc
+++ b/third_party/nix/src/libmain/shared.cc
@@ -36,7 +36,7 @@ void printGCWarning() {
   }
 }
 
-void printMissing(ref<Store> store, const PathSet& paths) {
+void printMissing(const ref<Store>& store, const PathSet& paths) {
   unsigned long long downloadSize;
   unsigned long long narSize;
   PathSet willBuild;
@@ -48,7 +48,7 @@ void printMissing(ref<Store> store, const PathSet& paths) {
                narSize);
 }
 
-void printMissing(ref<Store> store, const PathSet& willBuild,
+void printMissing(const ref<Store>& store, const PathSet& willBuild,
                   const PathSet& willSubstitute, const PathSet& unknown,
                   unsigned long long downloadSize, unsigned long long narSize) {
   if (!willBuild.empty()) {
@@ -260,14 +260,15 @@ void parseCmdLine(
     int argc, char** argv,
     std::function<bool(Strings::iterator& arg, const Strings::iterator& end)>
         parseArg) {
-  parseCmdLine(baseNameOf(argv[0]), argvToStrings(argc, argv), parseArg);
+  parseCmdLine(baseNameOf(argv[0]), argvToStrings(argc, argv),
+               std::move(parseArg));
 }
 
 void parseCmdLine(
     const string& programName, const Strings& args,
     std::function<bool(Strings::iterator& arg, const Strings::iterator& end)>
         parseArg) {
-  LegacyArgs(programName, parseArg).parseCmdline(args);
+  LegacyArgs(programName, std::move(parseArg)).parseCmdline(args);
 }
 
 void printVersion(const string& programName) {
@@ -298,7 +299,8 @@ void showManPage(const string& name) {
   throw SysError(format("command 'man %1%' failed") % name.c_str());
 }
 
-int handleExceptions(const string& programName, std::function<void()> fun) {
+int handleExceptions(const string& programName,
+                     const std::function<void()>& fun) {
   ReceiveInterrupts receiveInterrupts;  // FIXME: need better place for this
 
   string error = ANSI_RED "error:" ANSI_NORMAL " ";
diff --git a/third_party/nix/src/libmain/shared.hh b/third_party/nix/src/libmain/shared.hh
index db236a7017..da1c9cc66a 100644
--- a/third_party/nix/src/libmain/shared.hh
+++ b/third_party/nix/src/libmain/shared.hh
@@ -18,7 +18,8 @@ class Exit : public std::exception {
   virtual ~Exit();
 };
 
-int handleExceptions(const string& programName, std::function<void()> fun);
+int handleExceptions(const string& programName,
+                     const std::function<void()>& fun);
 
 /* Don't forget to call initPlugins() after settings are initialized! */
 void initNix();
@@ -40,9 +41,9 @@ void printGCWarning();
 
 class Store;
 
-void printMissing(ref<Store> store, const PathSet& paths);
+void printMissing(const ref<Store>& store, const PathSet& paths);
 
-void printMissing(ref<Store> store, const PathSet& willBuild,
+void printMissing(const ref<Store>& store, const PathSet& willBuild,
                   const PathSet& willSubstitute, const PathSet& unknown,
                   unsigned long long downloadSize, unsigned long long narSize);
 
diff --git a/third_party/nix/src/libstore/binary-cache-store.cc b/third_party/nix/src/libstore/binary-cache-store.cc
index b0d8c81d80..487c12b531 100644
--- a/third_party/nix/src/libstore/binary-cache-store.cc
+++ b/third_party/nix/src/libstore/binary-cache-store.cc
@@ -98,7 +98,7 @@ Path BinaryCacheStore::narInfoFileFor(const Path& storePath) {
   return storePathToHash(storePath) + ".narinfo";
 }
 
-void BinaryCacheStore::writeNarInfo(ref<NarInfo> narInfo) {
+void BinaryCacheStore::writeNarInfo(const ref<NarInfo>& narInfo) {
   auto narInfoFile = narInfoFileFor(narInfo->path);
 
   upsertFile(narInfoFile, narInfo->to_string(), "text/x-nix-narinfo");
diff --git a/third_party/nix/src/libstore/binary-cache-store.hh b/third_party/nix/src/libstore/binary-cache-store.hh
index cd88ccdd84..f5bd66bbd6 100644
--- a/third_party/nix/src/libstore/binary-cache-store.hh
+++ b/third_party/nix/src/libstore/binary-cache-store.hh
@@ -65,7 +65,7 @@ class BinaryCacheStore : public Store {
 
   std::string narInfoFileFor(const Path& storePath);
 
-  void writeNarInfo(ref<NarInfo> narInfo);
+  void writeNarInfo(const ref<NarInfo>& narInfo);
 
  public:
   bool isValidPathUncached(const Path& path) override;
diff --git a/third_party/nix/src/libstore/build.cc b/third_party/nix/src/libstore/build.cc
index 389c99f06c..fe64635847 100644
--- a/third_party/nix/src/libstore/build.cc
+++ b/third_party/nix/src/libstore/build.cc
@@ -145,7 +145,7 @@ class Goal : public std::enable_shared_from_this<Goal> {
  public:
   virtual void work() = 0;
 
-  void addWaitee(GoalPtr waitee);
+  void addWaitee(const GoalPtr& waitee);
 
   virtual void waiteeDone(GoalPtr waitee, ExitCode result);
 
@@ -280,10 +280,10 @@ class Worker {
                                RepairFlag repair = NoRepair);
 
   /* Remove a dead goal. */
-  void removeGoal(GoalPtr goal);
+  void removeGoal(const GoalPtr& goal);
 
   /* Wake up a goal (i.e., there is something for it to do). */
-  void wakeUp(GoalPtr goal);
+  void wakeUp(const GoalPtr& goal);
 
   /* Return the number of local build and substitution processes
      currently running (but not remote builds via the build
@@ -292,7 +292,7 @@ class Worker {
 
   /* Registers a running child process.  `inBuildSlot' means that
      the process counts towards the jobs limit. */
-  void childStarted(GoalPtr goal, const set<int>& fds, bool inBuildSlot,
+  void childStarted(const GoalPtr& goal, const set<int>& fds, bool inBuildSlot,
                     bool respectTimeouts);
 
   /* Unregisters a running child process.  `wakeSleepers' should be
@@ -303,7 +303,7 @@ class Worker {
 
   /* Put `goal' to sleep until a build slot becomes available (which
      might be right away). */
-  void waitForBuildSlot(GoalPtr goal);
+  void waitForBuildSlot(const GoalPtr& goal);
 
   /* Wait for any goal to finish.  Pretty indiscriminate way to
      wait for some resource that some other goal is holding. */
@@ -332,7 +332,7 @@ class Worker {
 
 //////////////////////////////////////////////////////////////////////
 
-void addToWeakGoals(WeakGoals& goals, GoalPtr p) {
+void addToWeakGoals(WeakGoals& goals, const GoalPtr& p) {
   // FIXME: necessary?
   // FIXME: O(n)
   for (auto& i : goals) {
@@ -343,7 +343,7 @@ void addToWeakGoals(WeakGoals& goals, GoalPtr p) {
   goals.push_back(p);
 }
 
-void Goal::addWaitee(GoalPtr waitee) {
+void Goal::addWaitee(const GoalPtr& waitee) {
   waitees.insert(waitee);
   addToWeakGoals(waitee->waiters, shared_from_this());
 }
@@ -445,7 +445,9 @@ void handleDiffHook(uid_t uid, uid_t gid, Path tryA, Path tryB, Path drvPath,
   auto diffHook = settings.diffHook;
   if (diffHook != "" && settings.runDiffHook) {
     try {
-      RunOptions diffHookOptions(diffHook, {tryA, tryB, drvPath, tmpDir});
+      RunOptions diffHookOptions(
+          diffHook, {std::move(tryA), std::move(tryB), std::move(drvPath),
+                     std::move(tmpDir)});
       diffHookOptions.searchPath = true;
       diffHookOptions.uid = uid;
       diffHookOptions.gid = gid;
@@ -979,7 +981,7 @@ class DerivationGoal : public Goal {
 
   void done(BuildResult::Status status, const string& msg = "");
 
-  PathSet exportReferences(PathSet storePaths);
+  PathSet exportReferences(const PathSet& storePaths);
 };
 
 const Path DerivationGoal::homeDir = "/homeless-shelter";
@@ -1520,7 +1522,7 @@ void DerivationGoal::tryToBuild() {
   started();
 }
 
-void replaceValidPath(const Path& storePath, const Path tmpPath) {
+void replaceValidPath(const Path& storePath, const Path& tmpPath) {
   /* We can't atomically replace storePath (the original) with
      tmpPath (the replacement), so we have to move it out of the
      way first.  We'd better not be interrupted here, because if
@@ -1840,7 +1842,7 @@ int childEntry(void* arg) {
   return 1;
 }
 
-PathSet DerivationGoal::exportReferences(PathSet storePaths) {
+PathSet DerivationGoal::exportReferences(const PathSet& storePaths) {
   PathSet paths;
 
   for (auto storePath : storePaths) {
@@ -4455,7 +4457,7 @@ GoalPtr Worker::makeSubstitutionGoal(const Path& path, RepairFlag repair) {
   return goal;
 }
 
-static void removeGoal(GoalPtr goal, WeakGoalMap& goalMap) {
+static void removeGoal(const GoalPtr& goal, WeakGoalMap& goalMap) {
   /* !!! inefficient */
   for (auto i = goalMap.begin(); i != goalMap.end();) {
     if (i->second.lock() == goal) {
@@ -4469,7 +4471,7 @@ static void removeGoal(GoalPtr goal, WeakGoalMap& goalMap) {
   }
 }
 
-void Worker::removeGoal(GoalPtr goal) {
+void Worker::removeGoal(const GoalPtr& goal) {
   nix::removeGoal(goal, derivationGoals);
   nix::removeGoal(goal, substitutionGoals);
   if (topGoals.find(goal) != topGoals.end()) {
@@ -4492,15 +4494,15 @@ void Worker::removeGoal(GoalPtr goal) {
   waitingForAnyGoal.clear();
 }
 
-void Worker::wakeUp(GoalPtr goal) {
+void Worker::wakeUp(const GoalPtr& goal) {
   goal->trace("woken up");
   addToWeakGoals(awake, goal);
 }
 
 unsigned Worker::getNrLocalBuilds() { return nrLocalBuilds; }
 
-void Worker::childStarted(GoalPtr goal, const set<int>& fds, bool inBuildSlot,
-                          bool respectTimeouts) {
+void Worker::childStarted(const GoalPtr& goal, const set<int>& fds,
+                          bool inBuildSlot, bool respectTimeouts) {
   Child child;
   child.goal = goal;
   child.goal2 = goal.get();
@@ -4542,7 +4544,7 @@ void Worker::childTerminated(Goal* goal, bool wakeSleepers) {
   }
 }
 
-void Worker::waitForBuildSlot(GoalPtr goal) {
+void Worker::waitForBuildSlot(const GoalPtr& goal) {
   DLOG(INFO) << "wait for build slot";
   if (getNrLocalBuilds() < settings.maxBuildJobs) {
     wakeUp(goal); /* we can do it right away */
@@ -4553,12 +4555,12 @@ void Worker::waitForBuildSlot(GoalPtr goal) {
 
 void Worker::waitForAnyGoal(GoalPtr goal) {
   DLOG(INFO) << "wait for any goal";
-  addToWeakGoals(waitingForAnyGoal, goal);
+  addToWeakGoals(waitingForAnyGoal, std::move(goal));
 }
 
 void Worker::waitForAWhile(GoalPtr goal) {
   DLOG(INFO) << "wait for a while";
-  addToWeakGoals(waitingForAWhile, goal);
+  addToWeakGoals(waitingForAWhile, std::move(goal));
 }
 
 void Worker::run(const Goals& _topGoals) {
diff --git a/third_party/nix/src/libstore/crypto.cc b/third_party/nix/src/libstore/crypto.cc
index 2fbc30fb31..90580b8dc9 100644
--- a/third_party/nix/src/libstore/crypto.cc
+++ b/third_party/nix/src/libstore/crypto.cc
@@ -104,12 +104,12 @@ PublicKeys getDefaultPublicKeys() {
 
   // FIXME: filter duplicates
 
-  for (auto s : settings.trustedPublicKeys.get()) {
+  for (const auto& s : settings.trustedPublicKeys.get()) {
     PublicKey key(s);
     publicKeys.emplace(key.name, key);
   }
 
-  for (auto secretKeyFile : settings.secretKeyFiles.get()) {
+  for (const auto& secretKeyFile : settings.secretKeyFiles.get()) {
     try {
       SecretKey secretKey(readFile(secretKeyFile));
       publicKeys.emplace(secretKey.name, secretKey.toPublicKey());
diff --git a/third_party/nix/src/libstore/derivations.cc b/third_party/nix/src/libstore/derivations.cc
index 029c14640b..9c71d7209e 100644
--- a/third_party/nix/src/libstore/derivations.cc
+++ b/third_party/nix/src/libstore/derivations.cc
@@ -38,7 +38,7 @@ bool BasicDerivation::isBuiltin() const {
   return string(builder, 0, 8) == "builtin:";
 }
 
-Path writeDerivation(ref<Store> store, const Derivation& drv,
+Path writeDerivation(const ref<Store>& store, const Derivation& drv,
                      const string& name, RepairFlag repair) {
   PathSet references;
   references.insert(drv.inputSrcs.begin(), drv.inputSrcs.end());
@@ -355,7 +355,7 @@ Hash hashDerivationModulo(Store& store, Derivation drv) {
 }
 
 DrvPathWithOutputs parseDrvPathWithOutputs(const string& s) {
-  size_t n = s.find("!");
+  size_t n = s.find('!');
   return n == std::string::npos
              ? DrvPathWithOutputs(s, std::set<string>())
              : DrvPathWithOutputs(
diff --git a/third_party/nix/src/libstore/derivations.hh b/third_party/nix/src/libstore/derivations.hh
index 11277c813e..52b951f5e9 100644
--- a/third_party/nix/src/libstore/derivations.hh
+++ b/third_party/nix/src/libstore/derivations.hh
@@ -67,7 +67,7 @@ struct Derivation : BasicDerivation {
 class Store;
 
 /* Write a derivation to the Nix store, and return its path. */
-Path writeDerivation(ref<Store> store, const Derivation& drv,
+Path writeDerivation(const ref<Store>& store, const Derivation& drv,
                      const string& name, RepairFlag repair = NoRepair);
 
 /* Read a derivation from a file. */
diff --git a/third_party/nix/src/libstore/download.cc b/third_party/nix/src/libstore/download.cc
index 752f546a58..7476dd50b5 100644
--- a/third_party/nix/src/libstore/download.cc
+++ b/third_party/nix/src/libstore/download.cc
@@ -130,7 +130,7 @@ struct CurlDownloader : public Downloader {
       }
     }
 
-    void failEx(std::exception_ptr ex) {
+    void failEx(const std::exception_ptr& ex) {
       assert(!done);
       done = true;
       callback.rethrow(ex);
@@ -663,7 +663,7 @@ struct CurlDownloader : public Downloader {
     }
   }
 
-  void enqueueItem(std::shared_ptr<DownloadItem> item) {
+  void enqueueItem(const std::shared_ptr<DownloadItem>& item) {
     if (item->request.data && !hasPrefix(item->request.uri, "http://") &&
         !hasPrefix(item->request.uri, "https://")) {
       throw nix::Error("uploading to '%s' is not supported", item->request.uri);
@@ -858,7 +858,7 @@ void Downloader::download(DownloadRequest&& request, Sink& sink) {
 }
 
 CachedDownloadResult Downloader::downloadCached(
-    ref<Store> store, const CachedDownloadRequest& request) {
+    const ref<Store>& store, const CachedDownloadRequest& request) {
   auto url = resolveUri(request.uri);
 
   auto name = request.name;
diff --git a/third_party/nix/src/libstore/download.hh b/third_party/nix/src/libstore/download.hh
index 5e7bbf4ad6..9ddbdfc159 100644
--- a/third_party/nix/src/libstore/download.hh
+++ b/third_party/nix/src/libstore/download.hh
@@ -108,7 +108,7 @@ struct Downloader {
      and is more recent than ‘tarball-ttl’ seconds. Otherwise,
      use the recorded ETag to verify if the server has a more
      recent version, and if so, download it to the Nix store. */
-  CachedDownloadResult downloadCached(ref<Store> store,
+  CachedDownloadResult downloadCached(const ref<Store>& store,
                                       const CachedDownloadRequest& request);
 
   enum Error { NotFound, Forbidden, Misc, Transient, Interrupted };
diff --git a/third_party/nix/src/libstore/export-import.cc b/third_party/nix/src/libstore/export-import.cc
index 1974773ba1..077b0d5390 100644
--- a/third_party/nix/src/libstore/export-import.cc
+++ b/third_party/nix/src/libstore/export-import.cc
@@ -55,7 +55,8 @@ void Store::exportPath(const Path& path, Sink& sink) {
                    << 0;
 }
 
-Paths Store::importPaths(Source& source, std::shared_ptr<FSAccessor> accessor,
+Paths Store::importPaths(Source& source,
+                         const std::shared_ptr<FSAccessor>& accessor,
                          CheckSigsFlag checkSigs) {
   Paths res;
   while (true) {
diff --git a/third_party/nix/src/libstore/globals.cc b/third_party/nix/src/libstore/globals.cc
index 57ba8cff64..066de44365 100644
--- a/third_party/nix/src/libstore/globals.cc
+++ b/third_party/nix/src/libstore/globals.cc
@@ -149,17 +149,18 @@ void BaseSetting<SandboxMode>::convertToArg(Args& args,
   args.mkFlag()
       .longName(name)
       .description("Enable sandboxing.")
-      .handler([=](std::vector<std::string> ss) { override(smEnabled); })
+      .handler([=](const std::vector<std::string>& ss) { override(smEnabled); })
       .category(category);
   args.mkFlag()
       .longName("no-" + name)
       .description("Disable sandboxing.")
-      .handler([=](std::vector<std::string> ss) { override(smDisabled); })
+      .handler(
+          [=](const std::vector<std::string>& ss) { override(smDisabled); })
       .category(category);
   args.mkFlag()
       .longName("relaxed-" + name)
       .description("Enable sandboxing, but allow builds to disable it.")
-      .handler([=](std::vector<std::string> ss) { override(smRelaxed); })
+      .handler([=](const std::vector<std::string>& ss) { override(smRelaxed); })
       .category(category);
 }
 
diff --git a/third_party/nix/src/libstore/local-fs-store.cc b/third_party/nix/src/libstore/local-fs-store.cc
index 9a5ea15386..2120afc0ce 100644
--- a/third_party/nix/src/libstore/local-fs-store.cc
+++ b/third_party/nix/src/libstore/local-fs-store.cc
@@ -12,7 +12,7 @@ LocalFSStore::LocalFSStore(const Params& params) : Store(params) {}
 struct LocalStoreAccessor : public FSAccessor {
   ref<LocalFSStore> store;
 
-  explicit LocalStoreAccessor(ref<LocalFSStore> store) : store(store) {}
+  explicit LocalStoreAccessor(const ref<LocalFSStore>& store) : store(store) {}
 
   Path toRealPath(const Path& path) {
     Path storePath = store->toStorePath(path);
diff --git a/third_party/nix/src/libstore/machines.cc b/third_party/nix/src/libstore/machines.cc
index 1b8bc4cf75..5b0c7e72dd 100644
--- a/third_party/nix/src/libstore/machines.cc
+++ b/third_party/nix/src/libstore/machines.cc
@@ -9,12 +9,12 @@
 
 namespace nix {
 
-Machine::Machine(decltype(storeUri) storeUri, decltype(systemTypes) systemTypes,
-                 decltype(sshKey) sshKey, decltype(maxJobs) maxJobs,
-                 decltype(speedFactor) speedFactor,
-                 decltype(supportedFeatures) supportedFeatures,
-                 decltype(mandatoryFeatures) mandatoryFeatures,
-                 decltype(sshPublicHostKey) sshPublicHostKey)
+Machine::Machine(decltype(storeUri)& storeUri,
+                 decltype(systemTypes)& systemTypes, decltype(sshKey)& sshKey,
+                 decltype(maxJobs) maxJobs, decltype(speedFactor) speedFactor,
+                 decltype(supportedFeatures)& supportedFeatures,
+                 decltype(mandatoryFeatures)& mandatoryFeatures,
+                 decltype(sshPublicHostKey)& sshPublicHostKey)
     : storeUri(
           // Backwards compatibility: if the URI is a hostname,
           // prepend ssh://.
diff --git a/third_party/nix/src/libstore/machines.hh b/third_party/nix/src/libstore/machines.hh
index a5013fe4f1..aef8054c25 100644
--- a/third_party/nix/src/libstore/machines.hh
+++ b/third_party/nix/src/libstore/machines.hh
@@ -19,12 +19,12 @@ struct Machine {
 
   bool mandatoryMet(const std::set<string>& features) const;
 
-  Machine(decltype(storeUri) storeUri, decltype(systemTypes) systemTypes,
-          decltype(sshKey) sshKey, decltype(maxJobs) maxJobs,
+  Machine(decltype(storeUri)& storeUri, decltype(systemTypes)& systemTypes,
+          decltype(sshKey)& sshKey, decltype(maxJobs) maxJobs,
           decltype(speedFactor) speedFactor,
-          decltype(supportedFeatures) supportedFeatures,
-          decltype(mandatoryFeatures) mandatoryFeatures,
-          decltype(sshPublicHostKey) sshPublicHostKey);
+          decltype(supportedFeatures)& supportedFeatures,
+          decltype(mandatoryFeatures)& mandatoryFeatures,
+          decltype(sshPublicHostKey)& sshPublicHostKey);
 };
 
 typedef std::vector<Machine> Machines;
diff --git a/third_party/nix/src/libstore/misc.cc b/third_party/nix/src/libstore/misc.cc
index 076c691b00..f95c4bec8b 100644
--- a/third_party/nix/src/libstore/misc.cc
+++ b/third_party/nix/src/libstore/misc.cc
@@ -171,8 +171,9 @@ void Store::queryMissing(const PathSet& targets, PathSet& willBuild_,
     }
   };
 
-  auto checkOutput = [&](const Path& drvPath, ref<Derivation> drv,
-                         const Path& outPath, ref<Sync<DrvState>> drvState_) {
+  auto checkOutput = [&](const Path& drvPath, const ref<Derivation>& drv,
+                         const Path& outPath,
+                         const ref<Sync<DrvState>>& drvState_) {
     if (drvState_->lock()->done) {
       return;
     }
diff --git a/third_party/nix/src/libstore/nar-accessor.cc b/third_party/nix/src/libstore/nar-accessor.cc
index 3a8d6b8841..7f6e0377ae 100644
--- a/third_party/nix/src/libstore/nar-accessor.cc
+++ b/third_party/nix/src/libstore/nar-accessor.cc
@@ -94,7 +94,7 @@ struct NarAccessor : public FSAccessor {
     }
   };
 
-  explicit NarAccessor(ref<const std::string> nar) : nar(nar) {
+  explicit NarAccessor(const ref<const std::string>& nar) : nar(nar) {
     NarIndexer indexer(*this, *nar);
     parseDump(indexer, indexer);
   }
@@ -111,7 +111,7 @@ struct NarAccessor : public FSAccessor {
       if (type == "directory") {
         member.type = FSAccessor::Type::tDirectory;
         for (auto i = v["entries"].begin(); i != v["entries"].end(); ++i) {
-          std::string name = i.key();
+          const std::string& name = i.key();
           recurse(member.children[name], i.value());
         }
       } else if (type == "regular") {
@@ -225,8 +225,8 @@ ref<FSAccessor> makeLazyNarAccessor(const std::string& listing,
   return make_ref<NarAccessor>(listing, getNarBytes);
 }
 
-void listNar(JSONPlaceholder& res, ref<FSAccessor> accessor, const Path& path,
-             bool recurse) {
+void listNar(JSONPlaceholder& res, const ref<FSAccessor>& accessor,
+             const Path& path, bool recurse) {
   auto st = accessor->stat(path);
 
   auto obj = res.object();
diff --git a/third_party/nix/src/libstore/nar-accessor.hh b/third_party/nix/src/libstore/nar-accessor.hh
index d4bd7366f8..8b18d4a36e 100644
--- a/third_party/nix/src/libstore/nar-accessor.hh
+++ b/third_party/nix/src/libstore/nar-accessor.hh
@@ -23,7 +23,7 @@ class JSONPlaceholder;
 
 /* Write a JSON representation of the contents of a NAR (except file
    contents). */
-void listNar(JSONPlaceholder& res, ref<FSAccessor> accessor, const Path& path,
-             bool recurse);
+void listNar(JSONPlaceholder& res, const ref<FSAccessor>& accessor,
+             const Path& path, bool recurse);
 
 }  // namespace nix
diff --git a/third_party/nix/src/libstore/nar-info.cc b/third_party/nix/src/libstore/nar-info.cc
index 6190840ea6..91cf471554 100644
--- a/third_party/nix/src/libstore/nar-info.cc
+++ b/third_party/nix/src/libstore/nar-info.cc
@@ -122,7 +122,7 @@ std::string NarInfo::to_string() const {
     res += "System: " + system + "\n";
   }
 
-  for (auto sig : sigs) {
+  for (const auto& sig : sigs) {
     res += "Sig: " + sig + "\n";
   }
 
diff --git a/third_party/nix/src/libstore/profiles.cc b/third_party/nix/src/libstore/profiles.cc
index 7d16d9f623..bdb90b7388 100644
--- a/third_party/nix/src/libstore/profiles.cc
+++ b/third_party/nix/src/libstore/profiles.cc
@@ -35,7 +35,7 @@ static int parseName(const string& profileName, const string& name) {
   return -1;
 }
 
-Generations findGenerations(Path profile, int& curGen) {
+Generations findGenerations(const Path& profile, int& curGen) {
   Generations gens;
 
   Path profileDir = dirOf(profile);
@@ -68,7 +68,8 @@ static void makeName(const Path& profile, unsigned int num, Path& outLink) {
   outLink = prefix + "-link";
 }
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath) {
+Path createGeneration(const ref<LocalFSStore>& store, const Path& profile,
+                      const Path& outPath) {
   /* The new generation number should be higher than old the
      previous ones. */
   int dummy;
@@ -226,7 +227,7 @@ void deleteGenerationsOlderThan(const Path& profile, const string& timeSpec,
   deleteGenerationsOlderThan(profile, oldTime, dryRun);
 }
 
-void switchLink(Path link, Path target) {
+void switchLink(const Path& link, Path target) {
   /* Hacky. */
   if (dirOf(target) == dirOf(link)) {
     target = baseNameOf(target);
diff --git a/third_party/nix/src/libstore/profiles.hh b/third_party/nix/src/libstore/profiles.hh
index b1c3f09487..ab31cc993a 100644
--- a/third_party/nix/src/libstore/profiles.hh
+++ b/third_party/nix/src/libstore/profiles.hh
@@ -19,11 +19,12 @@ typedef list<Generation> Generations;
 
 /* Returns the list of currently present generations for the specified
    profile, sorted by generation number. */
-Generations findGenerations(Path profile, int& curGen);
+Generations findGenerations(const Path& profile, int& curGen);
 
 class LocalFSStore;
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath);
+Path createGeneration(const ref<LocalFSStore>& store, const Path& profile,
+                      const Path& outPath);
 
 void deleteGeneration(const Path& profile, unsigned int gen);
 
@@ -40,7 +41,7 @@ void deleteGenerationsOlderThan(const Path& profile, time_t t, bool dryRun);
 void deleteGenerationsOlderThan(const Path& profile, const string& timeSpec,
                                 bool dryRun);
 
-void switchLink(Path link, Path target);
+void switchLink(const Path& link, Path target);
 
 /* Ensure exclusive access to a profile.  Any command that modifies
    the profile first acquires this lock. */
diff --git a/third_party/nix/src/libstore/remote-fs-accessor.cc b/third_party/nix/src/libstore/remote-fs-accessor.cc
index 9aceaa9815..ca478c213f 100644
--- a/third_party/nix/src/libstore/remote-fs-accessor.cc
+++ b/third_party/nix/src/libstore/remote-fs-accessor.cc
@@ -9,7 +9,8 @@
 
 namespace nix {
 
-RemoteFSAccessor::RemoteFSAccessor(ref<Store> store, const Path& cacheDir)
+RemoteFSAccessor::RemoteFSAccessor(const ref<Store>& store,
+                                   const Path& cacheDir)
     : store(store), cacheDir(cacheDir) {
   if (!cacheDir.empty()) {
     createDirs(cacheDir);
@@ -23,7 +24,7 @@ Path RemoteFSAccessor::makeCacheFile(const Path& storePath,
 }
 
 void RemoteFSAccessor::addToCache(const Path& storePath, const std::string& nar,
-                                  ref<FSAccessor> narAccessor) {
+                                  const ref<FSAccessor>& narAccessor) {
   nars.emplace(storePath, narAccessor);
 
   if (!cacheDir.empty()) {
diff --git a/third_party/nix/src/libstore/remote-fs-accessor.hh b/third_party/nix/src/libstore/remote-fs-accessor.hh
index 08985de529..32c729f50d 100644
--- a/third_party/nix/src/libstore/remote-fs-accessor.hh
+++ b/third_party/nix/src/libstore/remote-fs-accessor.hh
@@ -20,10 +20,10 @@ class RemoteFSAccessor : public FSAccessor {
   Path makeCacheFile(const Path& storePath, const std::string& ext);
 
   void addToCache(const Path& storePath, const std::string& nar,
-                  ref<FSAccessor> narAccessor);
+                  const ref<FSAccessor>& narAccessor);
 
  public:
-  RemoteFSAccessor(ref<Store> store,
+  RemoteFSAccessor(const ref<Store>& store,
                    const /* FIXME: use std::optional */ Path& cacheDir = "");
 
   Stat stat(const Path& path) override;
diff --git a/third_party/nix/src/libstore/store-api.cc b/third_party/nix/src/libstore/store-api.cc
index 39cb5ca9f6..50e9b3b6ea 100644
--- a/third_party/nix/src/libstore/store-api.cc
+++ b/third_party/nix/src/libstore/store-api.cc
@@ -1,6 +1,7 @@
 #include "store-api.hh"
 
 #include <future>
+#include <utility>
 
 #include <glog/logging.h>
 
@@ -570,7 +571,7 @@ void Store::buildPaths(const PathSet& paths, BuildMode buildMode) {
   }
 }
 
-void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
+void copyStorePath(ref<Store> srcStore, const ref<Store>& dstStore,
                    const Path& storePath, RepairFlag repair,
                    CheckSigsFlag checkSigs) {
   auto srcUri = srcStore->getUri();
@@ -693,7 +694,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore,
       });
 }
 
-void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
+void copyClosure(const ref<Store>& srcStore, const ref<Store>& dstStore,
                  const PathSet& storePaths, RepairFlag repair,
                  CheckSigsFlag checkSigs, SubstituteFlag substitute) {
   PathSet closure;
@@ -824,14 +825,14 @@ void Store::addToStore(const ValidPathInfo& info, Source& narSource,
                        RepairFlag repair, CheckSigsFlag checkSigs,
                        std::shared_ptr<FSAccessor> accessor) {
   addToStore(info, make_ref<std::string>(narSource.drain()), repair, checkSigs,
-             accessor);
+             std::move(accessor));
 }
 
 void Store::addToStore(const ValidPathInfo& info, const ref<std::string>& nar,
                        RepairFlag repair, CheckSigsFlag checkSigs,
                        std::shared_ptr<FSAccessor> accessor) {
   StringSource source(*nar);
-  addToStore(info, source, repair, checkSigs, accessor);
+  addToStore(info, source, repair, checkSigs, std::move(accessor));
 }
 
 }  // namespace nix
@@ -851,7 +852,7 @@ std::pair<std::string, Store::Params> splitUriAndParams(
   Store::Params params;
   auto q = uri.find('?');
   if (q != std::string::npos) {
-    for (auto s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
+    for (const auto& s : tokenizeString<Strings>(uri.substr(q + 1), "&")) {
       auto e = s.find('=');
       if (e != std::string::npos) {
         auto value = s.substr(e + 1);
@@ -885,7 +886,7 @@ ref<Store> openStore(const std::string& uri_,
   auto params = extraParams;
   params.insert(uriParams.begin(), uriParams.end());
 
-  for (auto fun : *RegisterStoreImplementation::implementations) {
+  for (const auto& fun : *RegisterStoreImplementation::implementations) {
     auto store = fun(uri, params);
     if (store) {
       store->warnUnknownSettings();
@@ -952,11 +953,11 @@ std::list<ref<Store>> getDefaultSubstituters() {
       }
     };
 
-    for (auto uri : settings.substituters.get()) {
+    for (const auto& uri : settings.substituters.get()) {
       addStore(uri);
     }
 
-    for (auto uri : settings.extraSubstituters.get()) {
+    for (const auto& uri : settings.extraSubstituters.get()) {
       addStore(uri);
     }
 
diff --git a/third_party/nix/src/libstore/store-api.hh b/third_party/nix/src/libstore/store-api.hh
index 17299585ed..242a3e6562 100644
--- a/third_party/nix/src/libstore/store-api.hh
+++ b/third_party/nix/src/libstore/store-api.hh
@@ -564,7 +564,7 @@ class Store : public std::enable_shared_from_this<Store>, public Config {
      the Nix store. Optionally, the contents of the NARs are
      preloaded into the specified FS accessor to speed up subsequent
      access. */
-  Paths importPaths(Source& source, std::shared_ptr<FSAccessor> accessor,
+  Paths importPaths(Source& source, const std::shared_ptr<FSAccessor>& accessor,
                     CheckSigsFlag checkSigs = CheckSigs);
 
   struct Stats {
@@ -671,7 +671,7 @@ string storePathToHash(const Path& path);
 void checkStoreName(const string& name);
 
 /* Copy a path from one store to another. */
-void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
+void copyStorePath(ref<Store> srcStore, const ref<Store>& dstStore,
                    const Path& storePath, RepairFlag repair = NoRepair,
                    CheckSigsFlag checkSigs = CheckSigs);
 
@@ -686,7 +686,7 @@ void copyPaths(ref<Store> srcStore, ref<Store> dstStore,
                SubstituteFlag substitute = NoSubstitute);
 
 /* Copy the closure of the specified paths from one store to another. */
-void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
+void copyClosure(const ref<Store>& srcStore, const ref<Store>& dstStore,
                  const PathSet& storePaths, RepairFlag repair = NoRepair,
                  CheckSigsFlag checkSigs = CheckSigs,
                  SubstituteFlag substitute = NoSubstitute);
diff --git a/third_party/nix/src/libutil/archive.cc b/third_party/nix/src/libutil/archive.cc
index 6583dc20b8..32bad07f22 100644
--- a/third_party/nix/src/libutil/archive.cc
+++ b/third_party/nix/src/libutil/archive.cc
@@ -145,7 +145,7 @@ void dumpString(const std::string& s, Sink& sink) {
        << "contents" << s << ")";
 }
 
-static SerialisationError badArchive(string s) {
+static SerialisationError badArchive(const string& s) {
   return SerialisationError("bad archive: " + s);
 }
 
diff --git a/third_party/nix/src/libutil/args.cc b/third_party/nix/src/libutil/args.cc
index 2b44074802..48fa715fdf 100644
--- a/third_party/nix/src/libutil/args.cc
+++ b/third_party/nix/src/libutil/args.cc
@@ -175,7 +175,7 @@ Args::FlagMaker& Args::FlagMaker::mkHashTypeFlag(HashType* ht) {
   arity(1);
   label("type");
   description("hash algorithm ('md5', 'sha1', 'sha256', or 'sha512')");
-  handler([ht](std::string s) {
+  handler([ht](const std::string& s) {
     *ht = parseHashType(s);
     if (*ht == htUnknown) {
       throw UsageError("unknown hash type '%1%'", s);
diff --git a/third_party/nix/src/libutil/config.cc b/third_party/nix/src/libutil/config.cc
index ef47c167ed..828ee1811b 100644
--- a/third_party/nix/src/libutil/config.cc
+++ b/third_party/nix/src/libutil/config.cc
@@ -246,12 +246,12 @@ void BaseSetting<bool>::convertToArg(Args& args, const std::string& category) {
   args.mkFlag()
       .longName(name)
       .description(description)
-      .handler([=](std::vector<std::string> ss) { override(true); })
+      .handler([=](const std::vector<std::string>& ss) { override(true); })
       .category(category);
   args.mkFlag()
       .longName("no-" + name)
       .description(description)
-      .handler([=](std::vector<std::string> ss) { override(false); })
+      .handler([=](const std::vector<std::string>& ss) { override(false); })
       .category(category);
 }
 
diff --git a/third_party/nix/src/libutil/serialise.cc b/third_party/nix/src/libutil/serialise.cc
index 4b12367196..34af4e840a 100644
--- a/third_party/nix/src/libutil/serialise.cc
+++ b/third_party/nix/src/libutil/serialise.cc
@@ -157,8 +157,8 @@ size_t StringSource::read(unsigned char* data, size_t len) {
 #error Coroutines are broken in this version of Boost!
 #endif
 
-std::unique_ptr<Source> sinkToSource(std::function<void(Sink&)> fun,
-                                     std::function<void()> eof) {
+std::unique_ptr<Source> sinkToSource(const std::function<void(Sink&)>& fun,
+                                     const std::function<void()>& eof) {
   struct SinkToSource : Source {
     using coro_t = boost::coroutines2::coroutine<std::string>;
 
diff --git a/third_party/nix/src/libutil/serialise.hh b/third_party/nix/src/libutil/serialise.hh
index d8c8b181f4..a5992b50ec 100644
--- a/third_party/nix/src/libutil/serialise.hh
+++ b/third_party/nix/src/libutil/serialise.hh
@@ -209,7 +209,8 @@ struct LambdaSource : Source {
 /* Convert a function that feeds data into a Sink into a Source. The
    Source executes the function as a coroutine. */
 std::unique_ptr<Source> sinkToSource(
-    std::function<void(Sink&)> fun, std::function<void()> eof = []() {
+    const std::function<void(Sink&)>& fun,
+    const std::function<void()>& eof = []() {
       throw EndOfFile("coroutine has finished");
     });
 
diff --git a/third_party/nix/src/libutil/util.cc b/third_party/nix/src/libutil/util.cc
index 21da06b8ab..b13dd7e4f9 100644
--- a/third_party/nix/src/libutil/util.cc
+++ b/third_party/nix/src/libutil/util.cc
@@ -76,9 +76,9 @@ void clearEnv() {
   }
 }
 
-void replaceEnv(std::map<std::string, std::string> newEnv) {
+void replaceEnv(const std::map<std::string, std::string>& newEnv) {
   clearEnv();
-  for (auto newEnvVar : newEnv) {
+  for (const auto& newEnvVar : newEnv) {
     setenv(newEnvVar.first.c_str(), newEnvVar.second.c_str(), 1);
   }
 }
@@ -888,9 +888,9 @@ void killUser(uid_t uid) {
 
 /* Wrapper around vfork to prevent the child process from clobbering
    the caller's stack frame in the parent. */
-static pid_t doFork(bool allowVfork, std::function<void()> fun)
+static pid_t doFork(bool allowVfork, const std::function<void()>& fun)
     __attribute__((noinline));
-static pid_t doFork(bool allowVfork, std::function<void()> fun) {
+static pid_t doFork(bool allowVfork, const std::function<void()>& fun) {
 #ifdef __linux__
   pid_t pid = allowVfork ? vfork() : fork();
 #else
@@ -944,7 +944,7 @@ std::vector<char*> stringsToCharPtrs(const Strings& ss) {
   return res;
 }
 
-string runProgram(Path program, bool searchPath, const Strings& args,
+string runProgram(const Path& program, bool searchPath, const Strings& args,
                   const std::optional<std::string>& input) {
   RunOptions opts(program, args);
   opts.searchPath = searchPath;
@@ -1425,7 +1425,7 @@ string base64Decode(const string& s) {
 }
 
 void callFailure(const std::function<void(std::exception_ptr exc)>& failure,
-                 std::exception_ptr exc) {
+                 const std::exception_ptr& exc) {
   try {
     failure(exc);
   } catch (std::exception& e) {
@@ -1516,7 +1516,7 @@ struct InterruptCallbackImpl : InterruptCallback {
 };
 
 std::unique_ptr<InterruptCallback> createInterruptCallback(
-    std::function<void()> callback) {
+    const std::function<void()>& callback) {
   auto interruptCallbacks(_interruptCallbacks.lock());
   interruptCallbacks->push_back(callback);
 
diff --git a/third_party/nix/src/libutil/util.hh b/third_party/nix/src/libutil/util.hh
index 737c4aa77f..5ce12f2ede 100644
--- a/third_party/nix/src/libutil/util.hh
+++ b/third_party/nix/src/libutil/util.hh
@@ -246,7 +246,7 @@ pid_t startProcess(std::function<void()> fun,
 
 /* Run a program and return its stdout in a string (i.e., like the
    shell backtick operator). */
-string runProgram(Path program, bool searchPath = false,
+string runProgram(const Path& program, bool searchPath = false,
                   const Strings& args = Strings(),
                   const std::optional<std::string>& input = {});
 
@@ -453,7 +453,7 @@ struct InterruptCallback {
 /* Register a function that gets called on SIGINT (in a non-signal
    context). */
 std::unique_ptr<InterruptCallback> createInterruptCallback(
-    std::function<void()> callback);
+    const std::function<void()>& callback);
 
 void triggerInterrupt();
 
diff --git a/third_party/nix/src/nix-build/nix-build.cc b/third_party/nix/src/nix-build/nix-build.cc
index 5090cbe513..921fc2ba22 100644
--- a/third_party/nix/src/nix-build/nix-build.cc
+++ b/third_party/nix/src/nix-build/nix-build.cc
@@ -299,7 +299,7 @@ static void _main(int argc, char** argv) {
   if (readStdin) {
     exprs = {state->parseStdin()};
   } else {
-    for (auto i : left) {
+    for (const auto& i : left) {
       if (fromArgs) {
         exprs.push_back(state->parseExprFromString(i, absPath(".")));
       } else {
diff --git a/third_party/nix/src/nix-collect-garbage/nix-collect-garbage.cc b/third_party/nix/src/nix-collect-garbage/nix-collect-garbage.cc
index c3ff068887..0050bdda15 100644
--- a/third_party/nix/src/nix-collect-garbage/nix-collect-garbage.cc
+++ b/third_party/nix/src/nix-collect-garbage/nix-collect-garbage.cc
@@ -18,7 +18,7 @@ bool dryRun = false;
  * Of course, this makes rollbacks to before this point in time
  * impossible. */
 
-void removeOldGenerations(std::string dir) {
+void removeOldGenerations(const std::string& dir) {
   if (access(dir.c_str(), R_OK) != 0) {
     return;
   }
diff --git a/third_party/nix/src/nix-daemon/nix-daemon.cc b/third_party/nix/src/nix-daemon/nix-daemon.cc
index 060cc10497..9f5e8b7cbe 100644
--- a/third_party/nix/src/nix-daemon/nix-daemon.cc
+++ b/third_party/nix/src/nix-daemon/nix-daemon.cc
@@ -194,9 +194,9 @@ struct RetrieveRegularNARSink : ParseSink {
   }
 };
 
-static void performOp(TunnelLogger* logger, ref<Store> store, bool trusted,
-                      unsigned int clientVersion, Source& from, Sink& to,
-                      unsigned int op) {
+static void performOp(TunnelLogger* logger, const ref<Store>& store,
+                      bool trusted, unsigned int clientVersion, Source& from,
+                      Sink& to, unsigned int op) {
   switch (op) {
     case wopIsValidPath: {
       /* 'readStorePath' could raise an error leading to the connection
diff --git a/third_party/nix/src/nix-env/nix-env.cc b/third_party/nix/src/nix-env/nix-env.cc
index ec2da8f835..95a395a96f 100644
--- a/third_party/nix/src/nix-env/nix-env.cc
+++ b/third_party/nix/src/nix-env/nix-env.cc
@@ -171,8 +171,8 @@ static void loadSourceExpr(EvalState& state, const Path& path, Value& v) {
   }
 }
 
-static void loadDerivations(EvalState& state, Path nixExprPath,
-                            string systemFilter, Bindings& autoArgs,
+static void loadDerivations(EvalState& state, const Path& nixExprPath,
+                            const string& systemFilter, Bindings& autoArgs,
                             const string& pathPrefix, DrvInfos& elems) {
   Value vRoot;
   loadSourceExpr(state, nixExprPath, vRoot);
diff --git a/third_party/nix/src/nix-store/dotgraph.cc b/third_party/nix/src/nix-store/dotgraph.cc
index 259ecbd74a..0186a7b22e 100644
--- a/third_party/nix/src/nix-store/dotgraph.cc
+++ b/third_party/nix/src/nix-store/dotgraph.cc
@@ -79,7 +79,7 @@ void printClosure(const Path & nePath, const StoreExpr & fs)
 }
 #endif
 
-void printDotGraph(ref<Store> store, const PathSet& roots) {
+void printDotGraph(const ref<Store>& store, const PathSet& roots) {
   PathSet workList(roots);
   PathSet doneSet;
 
diff --git a/third_party/nix/src/nix-store/dotgraph.hh b/third_party/nix/src/nix-store/dotgraph.hh
index c7069451e1..20fc357b1b 100644
--- a/third_party/nix/src/nix-store/dotgraph.hh
+++ b/third_party/nix/src/nix-store/dotgraph.hh
@@ -6,6 +6,6 @@ namespace nix {
 
 class Store;
 
-void printDotGraph(ref<Store> store, const PathSet& roots);
+void printDotGraph(const ref<Store>& store, const PathSet& roots);
 
 }  // namespace nix
diff --git a/third_party/nix/src/nix-store/graphml.cc b/third_party/nix/src/nix-store/graphml.cc
index 6510b32881..a1a16cc8f2 100644
--- a/third_party/nix/src/nix-store/graphml.cc
+++ b/third_party/nix/src/nix-store/graphml.cc
@@ -37,7 +37,7 @@ static string makeNode(const ValidPathInfo& info) {
       (isDerivation(info.path) ? "derivation" : "output-path"));
 }
 
-void printGraphML(ref<Store> store, const PathSet& roots) {
+void printGraphML(const ref<Store>& store, const PathSet& roots) {
   PathSet workList(roots);
   PathSet doneSet;
   std::pair<PathSet::iterator, bool> ret;
diff --git a/third_party/nix/src/nix-store/graphml.hh b/third_party/nix/src/nix-store/graphml.hh
index c330dc6abc..199421acb2 100644
--- a/third_party/nix/src/nix-store/graphml.hh
+++ b/third_party/nix/src/nix-store/graphml.hh
@@ -6,6 +6,6 @@ namespace nix {
 
 class Store;
 
-void printGraphML(ref<Store> store, const PathSet& roots);
+void printGraphML(const ref<Store>& store, const PathSet& roots);
 
 }  // namespace nix
diff --git a/third_party/nix/src/nix-store/nix-store.cc b/third_party/nix/src/nix-store/nix-store.cc
index 802ebd4207..40d6f01cd9 100644
--- a/third_party/nix/src/nix-store/nix-store.cc
+++ b/third_party/nix/src/nix-store/nix-store.cc
@@ -28,6 +28,9 @@ using namespace nix;
 using std::cin;
 using std::cout;
 
+// TODO(tazjin): clang-tidy's performance lints don't like this, but
+// the automatic fixes fail (it seems that some of the ops want to own
+// the args for whatever reason)
 using Operation = void (*)(Strings, Strings);
 
 static Path gcRoot;
diff --git a/third_party/nix/src/nix/cat.cc b/third_party/nix/src/nix/cat.cc
index 9b2da929ac..2d662a453f 100644
--- a/third_party/nix/src/nix/cat.cc
+++ b/third_party/nix/src/nix/cat.cc
@@ -8,7 +8,7 @@ using namespace nix;
 struct MixCat : virtual Args {
   std::string path;
 
-  void cat(ref<FSAccessor> accessor) {
+  void cat(const ref<FSAccessor>& accessor) {
     auto st = accessor->stat(path);
     if (st.type == FSAccessor::Type::tMissing) {
       throw Error(format("path '%1%' does not exist") % path);
diff --git a/third_party/nix/src/nix/command.hh b/third_party/nix/src/nix/command.hh
index 829ce080d3..4122bdd0e5 100644
--- a/third_party/nix/src/nix/command.hh
+++ b/third_party/nix/src/nix/command.hh
@@ -171,21 +171,23 @@ struct RegisterCommand {
 };
 
 std::shared_ptr<Installable> parseInstallable(SourceExprCommand& cmd,
-                                              ref<Store> store,
+                                              const ref<Store>& store,
                                               const std::string& installable,
                                               bool useDefaultInstallables);
 
-Buildables build(ref<Store> store, RealiseMode mode,
-                 std::vector<std::shared_ptr<Installable>> installables);
+Buildables build(const ref<Store>& store, RealiseMode mode,
+                 const std::vector<std::shared_ptr<Installable>>& installables);
 
-PathSet toStorePaths(ref<Store> store, RealiseMode mode,
-                     std::vector<std::shared_ptr<Installable>> installables);
+PathSet toStorePaths(
+    const ref<Store>& store, RealiseMode mode,
+    const std::vector<std::shared_ptr<Installable>>& installables);
 
-Path toStorePath(ref<Store> store, RealiseMode mode,
-                 std::shared_ptr<Installable> installable);
+Path toStorePath(const ref<Store>& store, RealiseMode mode,
+                 const std::shared_ptr<Installable>& installable);
 
-PathSet toDerivations(ref<Store> store,
-                      std::vector<std::shared_ptr<Installable>> installables,
-                      bool useDeriver = false);
+PathSet toDerivations(
+    const ref<Store>& store,
+    const std::vector<std::shared_ptr<Installable>>& installables,
+    bool useDeriver = false);
 
 }  // namespace nix
diff --git a/third_party/nix/src/nix/doctor.cc b/third_party/nix/src/nix/doctor.cc
index 95bf8a922e..44b4bd605d 100644
--- a/third_party/nix/src/nix/doctor.cc
+++ b/third_party/nix/src/nix/doctor.cc
@@ -64,7 +64,7 @@ struct CmdDoctor : StoreCommand {
     return true;
   }
 
-  static bool checkProfileRoots(ref<Store> store) {
+  static bool checkProfileRoots(const ref<Store>& store) {
     PathSet dirs;
 
     for (auto& dir : tokenizeString<Strings>(getEnv("PATH"), ":")) {
diff --git a/third_party/nix/src/nix/hash.cc b/third_party/nix/src/nix/hash.cc
index da4b64fe75..74ff45d0d8 100644
--- a/third_party/nix/src/nix/hash.cc
+++ b/third_party/nix/src/nix/hash.cc
@@ -34,7 +34,7 @@ struct CmdHash : Command {
   }
 
   void run() override {
-    for (auto path : paths) {
+    for (const auto& path : paths) {
       Hash h = mode == mFile ? hashFile(ht, path) : hashPath(ht, path).first;
       if (truncate && h.hashSize > 20) {
         h = compressHash(h, 20);
@@ -73,7 +73,7 @@ struct CmdToBase : Command {
   }
 
   void run() override {
-    for (auto s : args) {
+    for (const auto& s : args) {
       std::cout << fmt("%s\n", Hash(s, ht).to_string(base, base == SRI));
     }
   }
diff --git a/third_party/nix/src/nix/installables.cc b/third_party/nix/src/nix/installables.cc
index f6cba6d3b6..8d2f932568 100644
--- a/third_party/nix/src/nix/installables.cc
+++ b/third_party/nix/src/nix/installables.cc
@@ -198,8 +198,8 @@ std::string attrRegex = R"([A-Za-z_][A-Za-z0-9-_+]*)";
 static std::regex attrPathRegex(fmt(R"(%1%(\.%1%)*)", attrRegex));
 
 static std::vector<std::shared_ptr<Installable>> parseInstallables(
-    SourceExprCommand& cmd, ref<Store> store, std::vector<std::string> ss,
-    bool useDefaultInstallables) {
+    SourceExprCommand& cmd, const ref<Store>& store,
+    std::vector<std::string> ss, bool useDefaultInstallables) {
   std::vector<std::shared_ptr<Installable>> result;
 
   if (ss.empty() && useDefaultInstallables) {
@@ -213,7 +213,7 @@ static std::vector<std::shared_ptr<Installable>> parseInstallables(
     if (s.compare(0, 1, "(") == 0) {
       result.push_back(std::make_shared<InstallableExpr>(cmd, s));
 
-    } else if (s.find("/") != std::string::npos) {
+    } else if (s.find('/') != std::string::npos) {
       auto path = store->toStorePath(store->followLinksToStore(s));
 
       if (store->isStorePath(path)) {
@@ -233,7 +233,7 @@ static std::vector<std::shared_ptr<Installable>> parseInstallables(
 }
 
 std::shared_ptr<Installable> parseInstallable(SourceExprCommand& cmd,
-                                              ref<Store> store,
+                                              const ref<Store>& store,
                                               const std::string& installable,
                                               bool useDefaultInstallables) {
   auto installables = parseInstallables(cmd, store, {installable}, false);
@@ -241,8 +241,9 @@ std::shared_ptr<Installable> parseInstallable(SourceExprCommand& cmd,
   return installables.front();
 }
 
-Buildables build(ref<Store> store, RealiseMode mode,
-                 std::vector<std::shared_ptr<Installable>> installables) {
+Buildables build(
+    const ref<Store>& store, RealiseMode mode,
+    const std::vector<std::shared_ptr<Installable>>& installables) {
   if (mode != Build) {
     settings.readOnlyMode = true;
   }
@@ -278,8 +279,9 @@ Buildables build(ref<Store> store, RealiseMode mode,
   return buildables;
 }
 
-PathSet toStorePaths(ref<Store> store, RealiseMode mode,
-                     std::vector<std::shared_ptr<Installable>> installables) {
+PathSet toStorePaths(
+    const ref<Store>& store, RealiseMode mode,
+    const std::vector<std::shared_ptr<Installable>>& installables) {
   PathSet outPaths;
 
   for (auto& b : build(store, mode, installables)) {
@@ -291,8 +293,8 @@ PathSet toStorePaths(ref<Store> store, RealiseMode mode,
   return outPaths;
 }
 
-Path toStorePath(ref<Store> store, RealiseMode mode,
-                 std::shared_ptr<Installable> installable) {
+Path toStorePath(const ref<Store>& store, RealiseMode mode,
+                 const std::shared_ptr<Installable>& installable) {
   auto paths = toStorePaths(store, mode, {installable});
 
   if (paths.size() != 1) {
@@ -303,9 +305,10 @@ Path toStorePath(ref<Store> store, RealiseMode mode,
   return *paths.begin();
 }
 
-PathSet toDerivations(ref<Store> store,
-                      std::vector<std::shared_ptr<Installable>> installables,
-                      bool useDeriver) {
+PathSet toDerivations(
+    const ref<Store>& store,
+    const std::vector<std::shared_ptr<Installable>>& installables,
+    bool useDeriver) {
   PathSet drvPaths;
 
   for (auto& i : installables) {
diff --git a/third_party/nix/src/nix/ls.cc b/third_party/nix/src/nix/ls.cc
index 13b7bbbbb9..5b052f8fcc 100644
--- a/third_party/nix/src/nix/ls.cc
+++ b/third_party/nix/src/nix/ls.cc
@@ -74,7 +74,7 @@ struct MixLs : virtual Args, MixJSON {
            showDirectory);
   }
 
-  void list(ref<FSAccessor> accessor) {
+  void list(const ref<FSAccessor>& accessor) {
     if (path == "/") {
       path = "";
     }
diff --git a/third_party/nix/src/nix/repl.cc b/third_party/nix/src/nix/repl.cc
index 81e940dd3a..78991645ac 100644
--- a/third_party/nix/src/nix/repl.cc
+++ b/third_party/nix/src/nix/repl.cc
@@ -3,6 +3,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <iostream>
+#include <utility>
 
 #include <glog/logging.h>
 
@@ -58,10 +59,10 @@ struct NixRepl {
 
   const Path historyFile;
 
-  NixRepl(const Strings& searchPath, nix::ref<Store> store);
+  NixRepl(const Strings& searchPath, const nix::ref<Store>& store);
   ~NixRepl();
   void mainLoop(const std::vector<std::string>& files);
-  StringSet completePrefix(string prefix);
+  StringSet completePrefix(const string& prefix);
   static bool getLine(string& input, const std::string& prompt);
   Path getDerivationPath(Value& v);
   bool processLine(string line);
@@ -70,7 +71,7 @@ struct NixRepl {
   void reloadFiles();
   void addAttrsToScope(Value& attrs);
   void addVarToScope(const Symbol& name, Value& v);
-  Expr* parseString(string s);
+  Expr* parseString(const string& s);
   void evalString(string s, Value& v);
 
   using ValuesSeen = set<Value*>;
@@ -129,7 +130,7 @@ string removeWhitespace(string s) {
   return s;
 }
 
-NixRepl::NixRepl(const Strings& searchPath, nix::ref<Store> store)
+NixRepl::NixRepl(const Strings& searchPath, const nix::ref<Store>& store)
     : state(searchPath, store),
       staticEnv(false, &state.staticBaseEnv),
       historyFile(getDataDir() + "/nix/repl-history") {
@@ -332,7 +333,7 @@ bool NixRepl::getLine(string& input, const std::string& prompt) {
   return true;
 }
 
-StringSet NixRepl::completePrefix(string prefix) {
+StringSet NixRepl::completePrefix(const string& prefix) {
   StringSet completions;
 
   size_t start = prefix.find_last_of(" \n\r\t(){}[]");
@@ -641,13 +642,13 @@ void NixRepl::addVarToScope(const Symbol& name, Value& v) {
   varNames.insert((string)name);
 }
 
-Expr* NixRepl::parseString(string s) {
+Expr* NixRepl::parseString(const string& s) {
   Expr* e = state.parseExprFromString(s, curDir, staticEnv);
   return e;
 }
 
 void NixRepl::evalString(string s, Value& v) {
-  Expr* e = parseString(s);
+  Expr* e = parseString(std::move(s));
   e->eval(state, *env, v);
   state.forceValue(v);
 }
diff --git a/third_party/nix/src/nix/run.cc b/third_party/nix/src/nix/run.cc
index 9827360f0a..fa471bd542 100644
--- a/third_party/nix/src/nix/run.cc
+++ b/third_party/nix/src/nix/run.cc
@@ -30,7 +30,7 @@ struct CmdRun : InstallablesCommand {
         .description("command and arguments to be executed; defaults to 'bash'")
         .labels({"command", "args"})
         .arity(ArityAny)
-        .handler([&](std::vector<std::string> ss) {
+        .handler([&](const std::vector<std::string>& ss) {
           if (ss.empty()) {
             throw UsageError("--command requires at least one argument");
           }
@@ -227,7 +227,7 @@ void chrootHelper(int argc, char** argv) {
       throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);
     }
 
-    for (auto entry : readDirectory("/")) {
+    for (const auto& entry : readDirectory("/")) {
       auto src = "/" + entry.name;
       auto st = lstat(src);
       if (!S_ISDIR(st.st_mode)) {
diff --git a/third_party/nix/src/nix/search.cc b/third_party/nix/src/nix/search.cc
index 5f6def2202..7fe81adb69 100644
--- a/third_party/nix/src/nix/search.cc
+++ b/third_party/nix/src/nix/search.cc
@@ -16,12 +16,12 @@
 
 using namespace nix;
 
-std::string wrap(std::string prefix, std::string s) {
+std::string wrap(const std::string& prefix, const std::string& s) {
   return prefix + s + ANSI_NORMAL;
 }
 
 std::string hilite(const std::string& s, const std::smatch& m,
-                   std::string postfix) {
+                   const std::string& postfix) {
   return m.empty() ? s
                    : std::string(m.prefix()) + ANSI_RED + std::string(m.str()) +
                          postfix + std::string(m.suffix());
@@ -99,7 +99,7 @@ struct CmdSearch : SourceExprCommand, MixJSON {
 
     std::function<void(Value*, std::string, bool, JSONObject*)> doExpr;
 
-    doExpr = [&](Value* v, std::string attrPath, bool toplevel,
+    doExpr = [&](Value* v, const std::string& attrPath, bool toplevel,
                  JSONObject* cache) {
       DLOG(INFO) << "at attribute '" << attrPath << "'";
 
@@ -262,7 +262,7 @@ struct CmdSearch : SourceExprCommand, MixJSON {
     }
 
     RunPager pager;
-    for (auto el : results) {
+    for (const auto& el : results) {
       std::cout << el.second << "\n";
     }
   }
diff --git a/third_party/nix/src/nix/upgrade-nix.cc b/third_party/nix/src/nix/upgrade-nix.cc
index b225aac2d7..9b060a7f19 100644
--- a/third_party/nix/src/nix/upgrade-nix.cc
+++ b/third_party/nix/src/nix/upgrade-nix.cc
@@ -97,7 +97,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand {
   }
 
   /* Return the profile in which Nix is installed. */
-  static Path getProfileDir(ref<Store> store) {
+  static Path getProfileDir(const ref<Store>& store) {
     Path where;
 
     for (auto& dir : tokenizeString<Strings>(getEnv("PATH"), ":")) {
@@ -143,7 +143,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand {
   }
 
   /* Return the store path of the latest stable Nix. */
-  Path getLatestNix(ref<Store> store) {
+  Path getLatestNix(const ref<Store>& store) {
     // FIXME: use nixos.org?
     auto req = DownloadRequest(storePathsUrl);
     auto res = getDownloader()->download(req);
diff --git a/third_party/nix/src/nix/verify.cc b/third_party/nix/src/nix/verify.cc
index 63dd181e07..a773b3a2c8 100644
--- a/third_party/nix/src/nix/verify.cc
+++ b/third_party/nix/src/nix/verify.cc
@@ -102,8 +102,8 @@ struct CmdVerify : StorePathsCommand {
             size_t actualSigsNeeded = std::max(sigsNeeded, (size_t)1);
             size_t validSigs = 0;
 
-            auto doSigs = [&](StringSet sigs) {
-              for (auto sig : sigs) {
+            auto doSigs = [&](const StringSet& sigs) {
+              for (const auto& sig : sigs) {
                 if (sigsSeen.count(sig) != 0u) {
                   continue;
                 }