diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index f12f02543..5646d08c1 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -139,6 +139,21 @@ struct StringSource : Source
 };
 
 
+/* Adapter class of a Source that saves all data read to `s'. */
+struct SavingSourceAdapter : Source
+{
+    Source & orig;
+    string s;
+    SavingSourceAdapter(Source & orig) : orig(orig) { }
+    size_t read(unsigned char * data, size_t len)
+    {
+        size_t n = orig.read(data, len);
+        s.append((const char *) data, n);
+        return n;
+    }
+};
+
+
 void writePadding(size_t len, Sink & sink);
 void writeString(const unsigned char * buf, size_t len, Sink & sink);
 
diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index 90a730187..f3ee0afc1 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -168,21 +168,6 @@ struct RetrieveRegularNARSink : ParseSink
 };
 
 
-/* Adapter class of a Source that saves all data read to `s'. */
-struct SavingSourceAdapter : Source
-{
-    Source & orig;
-    string s;
-    SavingSourceAdapter(Source & orig) : orig(orig) { }
-    size_t read(unsigned char * data, size_t len)
-    {
-        size_t n = orig.read(data, len);
-        s.append((const char *) data, n);
-        return n;
-    }
-};
-
-
 static void performOp(ref<LocalStore> store, bool trusted, unsigned int clientVersion,
     Source & from, Sink & to, unsigned int op)
 {