Merge pull request #9658 from pennae/env-diet

reduce the size of Env by one pointer

(cherry picked from commit 83f5622545a2fc31eb7e7d5105f64ed6dd3058b3)
Change-Id: I5636290526d0165cfc61aee1e7a5b94db4a26cef

src/libexpr/nixexpr.hh

@@ -139,6 +139,7 @@ std::ostream & operator << (std::ostream & str, const Pos & pos);
 struct Env;
 struct Value;
 class EvalState;
+struct ExprWith;
 struct StaticEnv;
 
 
@@ -221,8 +222,11 @@ struct ExprVar : Expr
     Symbol name;
 
     /* Whether the variable comes from an environment (e.g. a rec, let
-       or function argument) or from a "with". */
-    bool fromWith;
+       or function argument) or from a "with".
+
+       `nullptr`: Not from a `with`.
+       Valid pointer: the nearest, innermost `with` expression to query first. */
+    ExprWith * fromWith;
 
     /* In the former case, the value is obtained by going `level`
        levels up from the current environment and getting the
@@ -380,6 +384,7 @@ struct ExprWith : Expr
     PosIdx pos;
     Expr * attrs, * body;
     size_t prevWith;
+    ExprWith * parentWith;
     ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
     PosIdx getPos() const override { return pos; }
     COMMON_METHODS
@@ -473,14 +478,14 @@ extern ExprBlackHole eBlackHole;
    runtime. */
 struct StaticEnv
 {
-    bool isWith;
+    ExprWith * isWith;
     const StaticEnv * up;
 
     // Note: these must be in sorted order.
     typedef std::vector<std::pair<Symbol, Displacement>> Vars;
     Vars vars;
 
-    StaticEnv(bool isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
+    StaticEnv(ExprWith * isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
         vars.reserve(expectedSize);
     };