Add an sCurrentOutput member to EvalState

src/libexpr/eval.cc

@@ -133,6 +133,7 @@ string showType(const Value & v)
 EvalState::EvalState()
     : sWith(symbols.create("<with>"))
     , sOutPath(symbols.create("outPath"))
+    , sCurrentOutput(symbols.create("currentOutput"))
     , sDrvPath(symbols.create("drvPath"))
     , sType(symbols.create("type"))
     , sMeta(symbols.create("meta"))

src/libexpr/eval.hh

@@ -204,7 +204,7 @@ public:
     SymbolTable symbols;
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName,
-        sSystem, sOverrides;
+        sSystem, sOverrides, sCurrentOutput;
 
 private:
     SrcToStore srcToStore;