libexpr: Add nrExprs to NIX_SHOW_STATS

src/libexpr/eval.cc

@@ -2506,6 +2506,7 @@ void EvalState::printStats()
             {"elements", nrValuesInEnvs},
             {"bytes", bEnvs},
         };
+        topObj["nrExprs"] = Expr::nrExprs;
         topObj["list"] = {
             {"elements", nrListElems},
             {"bytes", bLists},

src/libexpr/nixexpr.hh

@@ -155,6 +155,10 @@ std::string showAttrPath(const SymbolTable & symbols, const AttrPath & attrPath)
 
 struct Expr
 {
+    static unsigned long nrExprs;
+    inline Expr() {
+        nrExprs++;
+    }
     virtual ~Expr() { };
     virtual void show(const SymbolTable & symbols, std::ostream & str) const;
     virtual void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);

src/libexpr/parser.y

@@ -653,6 +653,7 @@ formal
 
 namespace nix {
 
+unsigned long Expr::nrExprs = 0;
 
 Expr * EvalState::parse(
     char * text,