Merge pull request #9582 from pennae/misc-opts

a packet of small optimizations

(cherry picked from commit ee439734e924eb337a869ff2e48aff8b989198bc)
Change-Id: I125d870710750a32a0dece48f39a3e9132b0d023

src/libexpr/nixexpr.hh

@@ -22,6 +22,13 @@ MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 MakeError(RestrictedPathError, Error);
 
+class InfiniteRecursionError : public EvalError
+{
+    friend class EvalState;
+public:
+    using EvalError::EvalError;
+};
+
 /**
  * Position objects.
  */
@@ -450,6 +457,16 @@ struct ExprPos : Expr
     COMMON_METHODS
 };
 
+/* only used to mark thunks as black holes. */
+struct ExprBlackHole : Expr
+{
+    void show(const SymbolTable & symbols, std::ostream & str) const override {}
+    void eval(EvalState & state, Env & env, Value & v) override;
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env) override {}
+};
+
+extern ExprBlackHole eBlackHole;
+
 
 /* Static environments are used to map variable names onto (level,
    displacement) pairs used to obtain the value of the variable at