libexpr/eval-expr: extract makeThunk function that constructs and increments counters

Motivated by needing to remove a Value default construction from
`ExprVar::maybeThunk`, so part of #744

Change-Id: Ia12b5b019e49055ebd7ffa0cc39a59c06a6a6964

lix/libexpr/eval-expr.cc

@@ -1,4 +1,5 @@
 #include "eval.hh"
+#include "libexpr/nixexpr.hh"
 #include "primops.hh"
 #include "gc-small-vector.hh"
 
@@ -10,14 +11,19 @@
 
 namespace nix {
 
+Value Expr::makeThunk(Evaluator & ctx, Env & env)
+{
+    ctx.stats.nrThunks++;
+    return {NewValueAs::thunk, ctx.mem, env, *this};
+}
+
 /* Create a thunk for the delayed computation of the given expression
    in the given environment.  But if the expression is a variable,
    then look it up right away.  This significantly reduces the number
    of thunks allocated. */
 Value Expr::maybeThunk(EvalState & state, Env & env)
 {
-    state.ctx.stats.nrThunks++;
-    return {NewValueAs::thunk, state.ctx.mem, env, *this};
+    return makeThunk(state.ctx, env);
 }
 
 Value ExprVar::maybeThunk(EvalState & state, Env & env)
@@ -99,18 +105,10 @@ Value ExprSet::eval(EvalState & state, Env & env)
            in the original environment. */
         Displacement displ = 0;
         for (auto & i : attrs) {
-            Value vAttr;
-            if (hasOverrides && i.second.kind != ExprAttrs::AttrDef::Kind::Inherited) {
-                vAttr = {
-                    NewValueAs::thunk,
-                    state.ctx.mem,
-                    *i.second.chooseByKind(&env2, &env, inheritEnv),
-                    *i.second.e
-                };
-                state.ctx.stats.nrThunks++;
-            } else {
-                vAttr = i.second.e->maybeThunk(state, *i.second.chooseByKind(&env2, &env, inheritEnv));
-            }
+            Env & thunkEnv = *i.second.chooseByKind(&env2, &env, inheritEnv);
+            Value vAttr = hasOverrides && i.second.kind != ExprAttrs::AttrDef::Kind::Inherited
+                ? i.second.e->makeThunk(state.ctx, thunkEnv)
+                : i.second.e->maybeThunk(state, thunkEnv);
             env2.values[displ++] = vAttr;
             v.attrs()->push_back(Attr(i.first, vAttr, i.second.pos));
         }

lix/libexpr/eval-inline.hh

@@ -6,7 +6,6 @@
 #include "lix/libexpr/eval-error.hh"
 #include "lix/libexpr/gc-alloc.hh"
 #include "value.hh"
-#include <cstdint>
 
 namespace nix {
 

lix/libexpr/eval.cc

@@ -847,8 +847,7 @@ Value::List * EvalMemory::newList(size_t size)
 
 Value Evaluator::evalLazily(Expr & e)
 {
-    stats.nrThunks++;
-    return {NewValueAs::thunk, mem, builtins.env, e};
+    return e.makeThunk(*this, builtins.env);
 }
 
 Value EvalState::mkPos(PosIdx p)

lix/libexpr/nixexpr.hh

@@ -129,6 +129,7 @@ public:
     virtual JSON toJSON(const SymbolTable & symbols) const;
     virtual void accept(ExprVisitor & ev, std::unique_ptr<Expr> & ptr) = 0;
     virtual Value eval(EvalState & state, Env & env);
+    Value makeThunk(Evaluator & ctx, Env & env);
     virtual Value maybeThunk(EvalState & state, Env & env);
     /* Lambdas have a name associated with them, when they are declared in a binding:
      * `identity = x: x` will print the resulting value as `«lambda identity @ «string»:1:14»`.