libutil: move LengthSink into sole user

this should've been a filter from the start. since filter support in the
old IO model is very bad we just move it into the only use of it instead

Change-Id: Ifb9cfecf07587ae1d2d55072ddf505c86c79cc1b

lix/libutil/references.cc

@@ -155,12 +155,25 @@ size_t RewritingSource::read(char * data, size_t len)
 
 HashResult computeHashModulo(HashType ht, const std::string & modulus, Source & source)
 {
-    HashSink hashSink(ht);
-    LengthSink lengthSink;
-    RewritingSource rewritingSource(modulus, std::string(modulus.size(), 0), source);
+    struct LengthSink : Sink
+    {
+        Sink & inner;
+        uint64_t length = 0;
 
-    TeeSink tee{hashSink, lengthSink};
-    rewritingSource.drainInto(tee);
+        LengthSink(Sink & inner) : inner(inner) {}
+
+        void operator()(std::string_view data) override
+        {
+            length += data.size();
+            inner(data);
+        }
+    };
+
+    HashSink hashSink(ht);
+    RewritingSource rewritingSource(modulus, std::string(modulus.size(), 0), source);
+    LengthSink lengthSink{hashSink};
+
+    rewritingSource.drainInto(lengthSink);
 
     /* Hash the positions of the self-references. This ensures that a
        NAR with self-references and a NAR with some of the

lix/libutil/serialise.hh

@@ -226,19 +226,6 @@ struct TeeSource : Source
     }
 };
 
-/**
- * A sink that that just counts the number of bytes given to it
- */
-struct LengthSink : Sink
-{
-    uint64_t length = 0;
-
-    void operator () (std::string_view data) override
-    {
-        length += data.size();
-    }
-};
-
 /**
  * Convert a function into a sink.
  */