add ExprAttrs::AttrDef::chooseByKind

in place of inherited() — not quite useful yet since we don't
distinguish plain and inheritFrom attr kinds so far.

src/libexpr/eval.cc

@@ -1209,9 +1209,9 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
             Value * vAttr;
             if (hasOverrides && !i.second.inherited()) {
                 vAttr = state.allocValue();
-                mkThunk(*vAttr, env2, i.second.e);
+                mkThunk(*vAttr, *i.second.chooseByKind(&env2, &env, &env2), i.second.e);
             } else
-                vAttr = i.second.e->maybeThunk(state, i.second.inherited() ? env : env2);
+                vAttr = i.second.e->maybeThunk(state, *i.second.chooseByKind(&env2, &env, &env2));
             env2.values[displ++] = vAttr;
             v.attrs->push_back(Attr(i.first, vAttr, i.second.pos));
         }
@@ -1243,9 +1243,14 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         }
     }
 
-    else
+    else {
-        for (auto & i : attrs)
+        for (auto & i : attrs) {
-            v.attrs->push_back(Attr(i.first, i.second.e->maybeThunk(state, env), i.second.pos));
+            v.attrs->push_back(Attr(
+                    i.first,
+                    i.second.e->maybeThunk(state, *i.second.chooseByKind(&env, &env, &env)),
+                    i.second.pos));
+        }
+    }
 
     /* Dynamic attrs apply *after* rec and __overrides. */
     for (auto & i : dynamicAttrs) {
@@ -1281,8 +1286,11 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
        while the inherited attributes are evaluated in the original
        environment. */
     Displacement displ = 0;
-    for (auto & i : attrs->attrs)
+    for (auto & i : attrs->attrs) {
-        env2.values[displ++] = i.second.e->maybeThunk(state, i.second.inherited() ? env : env2);
+        env2.values[displ++] = i.second.e->maybeThunk(
+            state,
+            *i.second.chooseByKind(&env2, &env, &env2));
+    }
 
     auto dts = state.debugRepl
         ? makeDebugTraceStacker(

src/libexpr/nixexpr.cc

@@ -334,16 +334,19 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
         es.exprEnvs.insert(std::make_pair(this, env));
 
     if (recursive) {
-        auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), recursive ? attrs.size() : 0);
+        auto newEnv = [&] () -> std::shared_ptr<const StaticEnv> {
+            auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs.size());
 
-        Displacement displ = 0;
+            Displacement displ = 0;
-        for (auto & i : attrs)
+            for (auto & i : attrs)
-            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+                newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+            return newEnv;
+        }();
 
         // No need to sort newEnv since attrs is in sorted order.
 
         for (auto & i : attrs)
-            i.second.e->bindVars(es, i.second.inherited() ? env : newEnv);
+            i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, newEnv));
 
         for (auto & i : dynamicAttrs) {
             i.nameExpr->bindVars(es, newEnv);
@@ -352,7 +355,7 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
     }
     else {
         for (auto & i : attrs)
-            i.second.e->bindVars(es, env);
+            i.second.e->bindVars(es, i.second.chooseByKind(env, env, env));
 
         for (auto & i : dynamicAttrs) {
             i.nameExpr->bindVars(es, env);
@@ -409,16 +412,19 @@ void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
+    auto newEnv = [&] () -> std::shared_ptr<const StaticEnv> {
+        auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
 
-    Displacement displ = 0;
+        Displacement displ = 0;
-    for (auto & i : attrs->attrs)
+        for (auto & i : attrs->attrs)
-        newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+            newEnv->vars.emplace_back(i.first, i.second.displ = displ++);
+        return newEnv;
+    }();
 
     // No need to sort newEnv since attrs->attrs is in sorted order.
 
     for (auto & i : attrs->attrs)
-        i.second.e->bindVars(es, i.second.inherited() ? env : newEnv);
+        i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, newEnv));
 
     if (es.debugRepl)
         es.exprEnvs.insert(std::make_pair(this, newEnv));

src/libexpr/nixexpr.hh

@@ -178,6 +178,20 @@ struct ExprAttrs : Expr
         AttrDef() { };
 
         bool inherited() const { return kind == Kind::Inherited; }
+
+        template<typename T>
+        const T & chooseByKind(const T & plain, const T & inherited, const T & inheritedFrom) const
+        {
+            switch (kind) {
+            case Kind::Plain:
+                return plain;
+            case Kind::Inherited:
+                return inherited;
+            default:
+            case Kind::InheritedFrom:
+                return inheritedFrom;
+            }
+        }
     };
     typedef std::map<Symbol, AttrDef> AttrDefs;
     AttrDefs attrs;