evaluate inherit (from) exprs only once per directive

desugaring inherit-from to syntactic duplication of the source expr also
duplicates side effects of the source expr (such as trace calls) and
expensive computations (such as derivationStrict).

(cherry picked from commit cefd0302b55b3360dbca59cfcb4bf6a750d6cdcf)
Change-Id: Iff519f991adef2e51683ba2c552d37a3df7a179e

1 files changed, 16 insertions, 0 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 793e1a707..cb630d201 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -129,6 +129,18 @@ struct ExprVar : Expr
     COMMON_METHODS
 };
 
+struct ExprInheritFrom : ExprVar
+{
+    ExprInheritFrom(PosIdx pos, Displacement displ): ExprVar(pos, {})
+    {
+        this->level = 0;
+        this->displ = displ;
+        this->fromWith = nullptr;
+    }
+
+    void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+};
+
 struct ExprSelect : Expr
 {
     PosIdx pos;
@@ -189,6 +201,7 @@ struct ExprAttrs : Expr
     };
     typedef std::map<Symbol, AttrDef> AttrDefs;
     AttrDefs attrs;
+    std::unique_ptr<std::vector<Expr *>> inheritFromExprs;
     struct DynamicAttrDef {
         Expr * nameExpr, * valueExpr;
         PosIdx pos;
@@ -202,6 +215,9 @@ struct ExprAttrs : Expr
     PosIdx getPos() const override { return pos; }
     COMMON_METHODS
 
+    std::shared_ptr<const StaticEnv> bindInheritSources(
+        EvalState & es, const std::shared_ptr<const StaticEnv> & env);
+    Env * buildInheritFromEnv(EvalState & state, Env & up);
     void showBindings(const SymbolTable & symbols, std::ostream & str) const;
 };