diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 4a615c59d..047856abc 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1859,7 +1859,7 @@ static void addPath(
         // be rewritten to the actual output).
         state.realiseContext(context);
 
-        StorePathSet refs = StorePathSet();
+        StorePathSet refs;
 
         if (state.store->isInStore(path)) {
             auto [storePath, subPath] = state.store->toStorePath(path);
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index a039c57b3..065cac8b5 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -308,7 +308,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, Source & narSource
 }
 
 StorePath BinaryCacheStore::addToStoreFromDump(Source & dump, const string & name,
-    FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, StorePathSet references)
+    FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, const StorePathSet & references)
 {
     if (method != FileIngestionMethod::Recursive || hashAlgo != htSHA256)
         unsupported("addToStoreFromDump");
@@ -386,7 +386,7 @@ void BinaryCacheStore::queryPathInfoUncached(const StorePath & storePath,
 }
 
 StorePath BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
-    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair, StorePathSet references)
+    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair, const StorePathSet & references)
 {
     /* FIXME: Make BinaryCacheStore::addToStoreCommon support
        non-recursive+sha256 so we can just use the default
diff --git a/src/libstore/binary-cache-store.hh b/src/libstore/binary-cache-store.hh
index d8f077db2..a703994d3 100644
--- a/src/libstore/binary-cache-store.hh
+++ b/src/libstore/binary-cache-store.hh
@@ -97,11 +97,11 @@ public:
         RepairFlag repair, CheckSigsFlag checkSigs) override;
 
     StorePath addToStoreFromDump(Source & dump, const string & name,
-        FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, StorePathSet references ) override;
+        FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, const StorePathSet & references ) override;
 
     StorePath addToStore(const string & name, const Path & srcPath,
         FileIngestionMethod method, HashType hashAlgo,
-        PathFilter & filter, RepairFlag repair, StorePathSet references) override;
+        PathFilter & filter, RepairFlag repair, const StorePathSet & references) override;
 
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 694c408f3..383c6b990 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/src/libstore/build/local-derivation-goal.cc
@@ -1180,7 +1180,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
     StorePath addToStore(const string & name, const Path & srcPath,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
         PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair,
-        StorePathSet references = StorePathSet()) override
+        const StorePathSet & references = StorePathSet()) override
     { throw Error("addToStore"); }
 
     void addToStore(const ValidPathInfo & info, Source & narSource,
@@ -1200,7 +1200,7 @@ struct RestrictedStore : public virtual RestrictedStoreConfig, public virtual Lo
 
     StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair,
-        StorePathSet references = StorePathSet()) override
+        const StorePathSet & references = StorePathSet()) override
     {
         auto path = next->addToStoreFromDump(dump, name, method, hashAlgo, repair, references);
         goal.addDependency(path);
diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 433ddc8e1..c4fb91364 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -227,7 +227,7 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
 
     StorePath addToStore(const string & name, const Path & srcPath,
         FileIngestionMethod method, HashType hashAlgo,
-        PathFilter & filter, RepairFlag repair, StorePathSet references) override
+        PathFilter & filter, RepairFlag repair, const StorePathSet & references) override
     { unsupported("addToStore"); }
 
     StorePath addTextToStore(const string & name, const string & s,
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 33ac77af5..444308d88 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1302,7 +1302,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
 
 StorePath LocalStore::addToStoreFromDump(Source & source0, const string & name,
-    FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, StorePathSet references)
+    FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, const StorePathSet & references)
 {
     /* For computing the store path. */
     auto hashSink = std::make_unique<HashSink>(hashAlgo);
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 7a7329dd8..c6441ef01 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -140,7 +140,7 @@ public:
         RepairFlag repair, CheckSigsFlag checkSigs) override;
 
     StorePath addToStoreFromDump(Source & dump, const string & name,
-        FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, StorePathSet references) override;
+        FileIngestionMethod method, HashType hashAlgo, RepairFlag repair, const StorePathSet & references) override;
 
     StorePath addTextToStore(const string & name, const string & s,
         const StorePathSet & references, RepairFlag repair) override;
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 632dfcf21..0db442579 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -578,7 +578,7 @@ ref<const ValidPathInfo> RemoteStore::addCAToStore(
 
 
 StorePath RemoteStore::addToStoreFromDump(Source & dump, const string & name,
-      FileIngestionMethod method, HashType hashType, RepairFlag repair, StorePathSet references)
+      FileIngestionMethod method, HashType hashType, RepairFlag repair, const StorePathSet & references)
 {
     return addCAToStore(dump, name, FixedOutputHashMethod{ .fileIngestionMethod = method, .hashType = hashType }, references, repair)->path;
 }
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 31eb1871a..9c3e0bf05 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -73,7 +73,7 @@ public:
 
     /* Add a content-addressable store path. Does not support references. `dump` will be drained. */
     StorePath addToStoreFromDump(Source & dump, const string & name,
-        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair, StorePathSet references = StorePathSet()) override;
+        FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair, const StorePathSet & references = StorePathSet()) override;
 
     void addToStore(const ValidPathInfo & info, Source & nar,
         RepairFlag repair, CheckSigsFlag checkSigs) override;
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 84d6db297..329cf679f 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -237,7 +237,7 @@ StorePath Store::computeStorePathForText(const string & name, const string & s,
 
 
 StorePath Store::addToStore(const string & name, const Path & _srcPath,
-    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair, StorePathSet references)
+    FileIngestionMethod method, HashType hashAlgo, PathFilter & filter, RepairFlag repair, const StorePathSet & references)
 {
     Path srcPath(absPath(_srcPath));
     auto source = sinkToSource([&](Sink & sink) {
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index c4fdb176a..34d4dac3c 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -453,7 +453,7 @@ public:
        libutil/archive.hh). */
     virtual StorePath addToStore(const string & name, const Path & srcPath,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256,
-        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair, StorePathSet references = StorePathSet());
+        PathFilter & filter = defaultPathFilter, RepairFlag repair = NoRepair, const StorePathSet & references = StorePathSet());
 
     /* Copy the contents of a path to the store and register the
        validity the resulting path, using a constant amount of
@@ -470,7 +470,7 @@ public:
     // FIXME: remove?
     virtual StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair,
-        StorePathSet references = StorePathSet())
+        const StorePathSet & references = StorePathSet())
     { unsupported("addToStoreFromDump"); }
 
     /* Like addToStore, but the contents written to the output path is