Merge pull request #10251 from edolstra/list-builder

Add a ListBuilder helper for constructing list values

src/libexpr/eval.cc

@@ -435,7 +435,8 @@ EvalState::EvalState(
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 
-    vEmptyList.mkList(0);
+    vEmptyList.mkList(buildList(0));
+    vNull.mkNull();
 
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
@@ -923,12 +924,11 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
     }
 }
 
-void EvalState::mkList(Value & v, size_t size)
+ListBuilder::ListBuilder(EvalState & state, size_t size)
+    : size(size)
+    , elems(size <= 2 ? inlineElems : (Value * *) allocBytes(size * sizeof(Value *)))
 {
-    v.mkList(size);
+    state.nrListElems += size;
-    if (size > 2)
-        v.bigList.elems = (Value * *) allocBytes(size * sizeof(Value *));
-    nrListElems += size;
 }
 
 
@@ -1353,9 +1353,10 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
 
 void ExprList::eval(EvalState & state, Env & env, Value & v)
 {
-    state.mkList(v, elems.size());
+    auto list = state.buildList(elems.size());
-    for (auto [n, v2] : enumerate(v.listItems()))
+    for (const auto & [n, v2] : enumerate(list))
-        const_cast<Value * &>(v2) = elems[n]->maybeThunk(state, env);
+        v2 = elems[n]->maybeThunk(state, env);
+    v.mkList(list);
 }
 
 
@@ -1945,7 +1946,7 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
 }
 
 
-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx)
 {
     nrListConcats++;
 
@@ -1963,14 +1964,15 @@ void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Po
         return;
     }
 
-    mkList(v, len);
+    auto list = buildList(len);
-    auto out = v.listElems();
+    auto out = list.elems;
     for (size_t n = 0, pos = 0; n < nrLists; ++n) {
         auto l = lists[n]->listSize();
         if (l)
             memcpy(out + pos, lists[n]->listElems(), l * sizeof(Value *));
         pos += l;
     }
+    v.mkList(list);
 }
 
 

src/libexpr/eval.hh

@@ -186,6 +186,11 @@ public:
      */
     Value vEmptyList;
 
+    /**
+     * Null constant.
+     */
+    Value vNull;
+
     /**
      * The accessor for the root filesystem.
      */
@@ -615,7 +620,11 @@ public:
         return BindingsBuilder(*this, allocBindings(capacity));
     }
 
-    void mkList(Value & v, size_t length);
+    ListBuilder buildList(size_t size)
+    {
+        return ListBuilder(*this, size);
+    }
+
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
@@ -662,7 +671,7 @@ public:
         const SingleDerivedPath & p,
         Value & v);
 
-    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
+    void concatLists(Value & v, size_t nrLists, Value * const * lists, const PosIdx pos, std::string_view errorCtx);
 
     /**
      * Print statistics, if enabled.
@@ -756,6 +765,7 @@ private:
     friend void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
     friend struct Value;
+    friend class ListBuilder;
 };
 
 struct DebugTraceStacker {

src/libexpr/json-to-value.cc

@@ -57,11 +57,10 @@ class JSONSax : nlohmann::json_sax<json> {
         ValueVector values;
         std::unique_ptr<JSONState> resolve(EvalState & state) override
         {
-            Value & v = parent->value(state);
+            auto list = state.buildList(values.size());
-            state.mkList(v, values.size());
+            for (const auto & [n, v2] : enumerate(list))
-            for (size_t n = 0; n < values.size(); ++n) {
+                v2 = values[n];
-                v.listElems()[n] = values[n];
+            parent->value(state).mkList(list);
-            }
             return std::move(parent);
         }
         void add() override {

src/libexpr/primops.cc

@@ -187,13 +187,13 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
             NixStringContextElem::DrvDeep { .drvPath = *storePath },
         });
         attrs.alloc(state.sName).mkString(drv.env["name"]);
-        auto & outputsVal = attrs.alloc(state.sOutputs);
-        state.mkList(outputsVal, drv.outputs.size());
 
+        auto list = state.buildList(drv.outputs.size());
         for (const auto & [i, o] : enumerate(drv.outputs)) {
             mkOutputString(state, attrs, *storePath, o);
-            (outputsVal.listElems()[i] = state.allocValue())->mkString(o.first);
+            (list[i] = state.allocValue())->mkString(o.first);
         }
+        attrs.alloc(state.sOutputs).mkList(list);
 
         auto w = state.allocValue();
         w->mkAttrs(attrs);
@@ -694,10 +694,10 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
     }
 
     /* Create the result list. */
-    state.mkList(v, res.size());
+    auto list = state.buildList(res.size());
-    unsigned int n = 0;
+    for (const auto & [n, i] : enumerate(res))
-    for (auto & i : res)
+        list[n] = i;
-        v.listElems()[n++] = i;
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_genericClosure(PrimOp {
@@ -2423,14 +2423,15 @@ static void prim_attrNames(EvalState & state, const PosIdx pos, Value * * args,
 {
     state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrNames");
 
-    state.mkList(v, args[0]->attrs->size());
+    auto list = state.buildList(args[0]->attrs->size());
 
-    size_t n = 0;
+    for (const auto & [n, i] : enumerate(*args[0]->attrs))
-    for (auto & i : *args[0]->attrs)
+        (list[n] = state.allocValue())->mkString(state.symbols[i.name]);
-        (v.listElems()[n++] = state.allocValue())->mkString(state.symbols[i.name]);
 
-    std::sort(v.listElems(), v.listElems() + n,
+    std::sort(list.begin(), list.end(),
               [](Value * v1, Value * v2) { return strcmp(v1->c_str(), v2->c_str()) < 0; });
+
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_attrNames({
@@ -2450,21 +2451,22 @@ static void prim_attrValues(EvalState & state, const PosIdx pos, Value * * args,
 {
     state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrValues");
 
-    state.mkList(v, args[0]->attrs->size());
+    auto list = state.buildList(args[0]->attrs->size());
 
-    unsigned int n = 0;
+    for (const auto & [n, i] : enumerate(*args[0]->attrs))
-    for (auto & i : *args[0]->attrs)
+        list[n] = (Value *) &i;
-        v.listElems()[n++] = (Value *) &i;
 
-    std::sort(v.listElems(), v.listElems() + n,
+    std::sort(list.begin(), list.end(),
         [&](Value * v1, Value * v2) {
             std::string_view s1 = state.symbols[((Attr *) v1)->name],
                 s2 = state.symbols[((Attr *) v2)->name];
             return s1 < s2;
         });
 
-    for (unsigned int i = 0; i < n; ++i)
+    for (auto & v : list)
-        v.listElems()[i] = ((Attr *) v.listElems()[i])->value;
+        v = ((Attr *) v)->value;
+
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_attrValues({
@@ -2805,9 +2807,10 @@ static void prim_catAttrs(EvalState & state, const PosIdx pos, Value * * args, V
             res[found++] = i->value;
     }
 
-    state.mkList(v, found);
+    auto list = state.buildList(found);
     for (unsigned int n = 0; n < found; ++n)
-        v.listElems()[n] = res[n];
+        list[n] = res[n];
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_catAttrs({
@@ -2908,43 +2911,50 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
     // attribute with the merge function application. this way we need not
     // use (slightly slower) temporary storage the GC does not know about.
 
-    std::map<Symbol, std::pair<size_t, Value * *>> attrsSeen;
+    struct Item
+    {
+        size_t size = 0;
+        size_t pos = 0;
+        std::optional<ListBuilder> list;
+    };
+
+    std::map<Symbol, Item> attrsSeen;
 
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.zipAttrsWith");
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.zipAttrsWith");
-    const auto listSize = args[1]->listSize();
+    const auto listItems = args[1]->listItems();
-    const auto listElems = args[1]->listElems();
 
-    for (unsigned int n = 0; n < listSize; ++n) {
+    for (auto & vElem : listItems) {
-        Value * vElem = listElems[n];
         state.forceAttrs(*vElem, noPos, "while evaluating a value of the list passed as second argument to builtins.zipAttrsWith");
         for (auto & attr : *vElem->attrs)
-            attrsSeen[attr.name].first++;
+            attrsSeen.try_emplace(attr.name).first->second.size++;
+    }
+
+    for (auto & [sym, elem] : attrsSeen)
+        elem.list.emplace(state.buildList(elem.size));
+
+    for (auto & vElem : listItems) {
+        for (auto & attr : *vElem->attrs) {
+            auto & item = attrsSeen.at(attr.name);
+            (*item.list)[item.pos++] = attr.value;
+        }
     }
 
     auto attrs = state.buildBindings(attrsSeen.size());
+
     for (auto & [sym, elem] : attrsSeen) {
-        auto & list = attrs.alloc(sym);
-        state.mkList(list, elem.first);
-        elem.second = list.listElems();
-    }
-    v.mkAttrs(attrs.alreadySorted());
-
-    for (unsigned int n = 0; n < listSize; ++n) {
-        Value * vElem = listElems[n];
-        for (auto & attr : *vElem->attrs)
-            *attrsSeen[attr.name].second++ = attr.value;
-    }
-
-    for (auto & attr : *v.attrs) {
         auto name = state.allocValue();
-        name->mkString(state.symbols[attr.name]);
+        name->mkString(state.symbols[sym]);
         auto call1 = state.allocValue();
         call1->mkApp(args[0], name);
         auto call2 = state.allocValue();
-        call2->mkApp(call1, attr.value);
+        auto arg = state.allocValue();
-        attr.value = call2;
+        arg->mkList(*elem.list);
+        call2->mkApp(call1, arg);
+        attrs.insert(sym, call2);
     }
+
+    v.mkAttrs(attrs.alreadySorted());
 }
 
 static RegisterPrimOp primop_zipAttrsWith({
@@ -3055,9 +3065,10 @@ static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value
     if (args[0]->listSize() == 0)
         state.error<EvalError>("'tail' called on an empty list").atPos(pos).debugThrow();
 
-    state.mkList(v, args[0]->listSize() - 1);
+    auto list = state.buildList(args[0]->listSize() - 1);
-    for (unsigned int n = 0; n < v.listSize(); ++n)
+    for (const auto & [n, v] : enumerate(list))
-        v.listElems()[n] = args[0]->listElems()[n + 1];
+        v = args[0]->listElems()[n + 1];
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_tail({
@@ -3088,10 +3099,11 @@ static void prim_map(EvalState & state, const PosIdx pos, Value * * args, Value
 
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.map");
 
-    state.mkList(v, args[1]->listSize());
+    auto list = state.buildList(args[1]->listSize());
-    for (unsigned int n = 0; n < v.listSize(); ++n)
+    for (const auto & [n, v] : enumerate(list))
-        (v.listElems()[n] = state.allocValue())->mkApp(
+        (v = state.allocValue())->mkApp(
             args[0], args[1]->listElems()[n]);
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_map({
@@ -3140,8 +3152,9 @@ static void prim_filter(EvalState & state, const PosIdx pos, Value * * args, Val
     if (same)
         v = *args[1];
     else {
-        state.mkList(v, k);
+        auto list = state.buildList(k);
-        for (unsigned int n = 0; n < k; ++n) v.listElems()[n] = vs[n];
+        for (const auto & [n, v] : enumerate(list)) v = vs[n];
+        v.mkList(list);
     }
 }
 
@@ -3316,12 +3329,13 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     // as evaluating map without accessing any values makes little sense.
     state.forceFunction(*args[0], noPos, "while evaluating the first argument passed to builtins.genList");
 
-    state.mkList(v, len);
+    auto list = state.buildList(len);
-    for (unsigned int n = 0; n < (unsigned int) len; ++n) {
+    for (const auto & [n, v] : enumerate(list)) {
         auto arg = state.allocValue();
         arg->mkInt(n);
-        (v.listElems()[n] = state.allocValue())->mkApp(args[0], arg);
+        (v = state.allocValue())->mkApp(args[0], arg);
     }
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_genList({
@@ -3355,11 +3369,9 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
 
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.sort");
 
-    state.mkList(v, len);
+    auto list = state.buildList(len);
-    for (unsigned int n = 0; n < len; ++n) {
+    for (const auto & [n, v] : enumerate(list))
-        state.forceValue(*args[1]->listElems()[n], pos);
+        state.forceValue(*(v = args[1]->listElems()[n]), pos);
-        v.listElems()[n] = args[1]->listElems()[n];
-    }
 
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
@@ -3378,7 +3390,9 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
     /* FIXME: std::sort can segfault if the comparator is not a strict
        weak ordering. What to do? std::stable_sort() seems more
        resilient, but no guarantees... */
-    std::stable_sort(v.listElems(), v.listElems() + len, comparator);
+    std::stable_sort(list.begin(), list.end(), comparator);
+
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_sort({
@@ -3424,17 +3438,17 @@ static void prim_partition(EvalState & state, const PosIdx pos, Value * * args,
 
     auto attrs = state.buildBindings(2);
 
-    auto & vRight = attrs.alloc(state.sRight);
     auto rsize = right.size();
-    state.mkList(vRight, rsize);
+    auto rlist = state.buildList(rsize);
     if (rsize)
-        memcpy(vRight.listElems(), right.data(), sizeof(Value *) * rsize);
+        memcpy(rlist.elems, right.data(), sizeof(Value *) * rsize);
+    attrs.alloc(state.sRight).mkList(rlist);
 
-    auto & vWrong = attrs.alloc(state.sWrong);
     auto wsize = wrong.size();
-    state.mkList(vWrong, wsize);
+    auto wlist = state.buildList(wsize);
     if (wsize)
-        memcpy(vWrong.listElems(), wrong.data(), sizeof(Value *) * wsize);
+        memcpy(wlist.elems, wrong.data(), sizeof(Value *) * wsize);
+    attrs.alloc(state.sWrong).mkList(wlist);
 
     v.mkAttrs(attrs);
 }
@@ -3481,10 +3495,10 @@ static void prim_groupBy(EvalState & state, const PosIdx pos, Value * * args, Va
     auto attrs2 = state.buildBindings(attrs.size());
 
     for (auto & i : attrs) {
-        auto & list = attrs2.alloc(i.first);
         auto size = i.second.size();
-        state.mkList(list, size);
+        auto list = state.buildList(size);
-        memcpy(list.listElems(), i.second.data(), sizeof(Value *) * size);
+        memcpy(list.elems, i.second.data(), sizeof(Value *) * size);
+        attrs2.alloc(i.first).mkList(list);
     }
 
     v.mkAttrs(attrs2.alreadySorted());
@@ -3531,14 +3545,15 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
         len += lists[n].listSize();
     }
 
-    state.mkList(v, len);
+    auto list = state.buildList(len);
-    auto out = v.listElems();
+    auto out = list.elems;
     for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
         auto l = lists[n].listSize();
         if (l)
             memcpy(out + pos, lists[n].listElems(), l * sizeof(Value *));
         pos += l;
     }
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_concatMap({
@@ -3986,14 +4001,13 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
         }
 
         // the first match is the whole string
-        const size_t len = match.size() - 1;
+        auto list = state.buildList(match.size() - 1);
-        state.mkList(v, len);
+        for (const auto & [i, v2] : enumerate(list))
-        for (size_t i = 0; i < len; ++i) {
             if (!match[i + 1].matched)
-                (v.listElems()[i] = state.allocValue())->mkNull();
+                v2 = &state.vNull;
             else
-                (v.listElems()[i] = state.allocValue())->mkString(match[i + 1].str());
+                (v2 = state.allocValue())->mkString(match[i + 1].str());
-        }
+        v.mkList(list);
 
     } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
@@ -4062,11 +4076,12 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 
         // Any matches results are surrounded by non-matching results.
         const size_t len = std::distance(begin, end);
-        state.mkList(v, 2 * len + 1);
+        auto list = state.buildList(2 * len + 1);
         size_t idx = 0;
 
         if (len == 0) {
-            v.listElems()[idx++] = args[1];
+            list[0] = args[1];
+            v.mkList(list);
             return;
         }
 
@@ -4075,28 +4090,31 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
             auto match = *i;
 
             // Add a string for non-matched characters.
-            (v.listElems()[idx++] = state.allocValue())->mkString(match.prefix().str());
+            (list[idx++] = state.allocValue())->mkString(match.prefix().str());
 
             // Add a list for matched substrings.
             const size_t slen = match.size() - 1;
-            auto elem = v.listElems()[idx++] = state.allocValue();
 
             // Start at 1, beacause the first match is the whole string.
-            state.mkList(*elem, slen);
+            auto list2 = state.buildList(slen);
-            for (size_t si = 0; si < slen; ++si) {
+            for (const auto & [si, v2] : enumerate(list2)) {
                 if (!match[si + 1].matched)
-                    (elem->listElems()[si] = state.allocValue())->mkNull();
+                    v2 = &state.vNull;
                 else
-                    (elem->listElems()[si] = state.allocValue())->mkString(match[si + 1].str());
+                    (v2 = state.allocValue())->mkString(match[si + 1].str());
             }
 
+            (list[idx++] = state.allocValue())->mkList(list2);
+
             // Add a string for non-matched suffix characters.
             if (idx == 2 * len)
-                (v.listElems()[idx++] = state.allocValue())->mkString(match.suffix().str());
+                (list[idx++] = state.allocValue())->mkString(match.suffix().str());
         }
 
         assert(idx == 2 * len + 1);
 
+        v.mkList(list);
+
     } catch (std::regex_error & e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
@@ -4316,9 +4334,10 @@ static void prim_splitVersion(EvalState & state, const PosIdx pos, Value * * arg
             break;
         components.emplace_back(component);
     }
-    state.mkList(v, components.size());
+    auto list = state.buildList(components.size());
     for (const auto & [n, component] : enumerate(components))
-        (v.listElems()[n] = state.allocValue())->mkString(std::move(component));
+        (list[n] = state.allocValue())->mkString(std::move(component));
+    v.mkList(list);
 }
 
 static RegisterPrimOp primop_splitVersion({
@@ -4411,8 +4430,7 @@ void EvalState::createBaseEnv()
         )",
     });
 
-    v.mkNull();
+    addConstant("null", &vNull, {
-    addConstant("null", v, {
         .type = nNull,
         .doc = R"(
           Primitive value.
@@ -4559,14 +4577,14 @@ void EvalState::createBaseEnv()
     });
 
     /* Add a value containing the current Nix expression search path. */
-    mkList(v, searchPath.elements.size());
+    auto list = buildList(searchPath.elements.size());
-    int n = 0;
+    for (const auto & [n, i] : enumerate(searchPath.elements)) {
-    for (auto & i : searchPath.elements) {
         auto attrs = buildBindings(2);
         attrs.alloc("path").mkString(i.path.s);
         attrs.alloc("prefix").mkString(i.prefix.s);
-        (v.listElems()[n++] = allocValue())->mkAttrs(attrs);
+        (list[n] = allocValue())->mkAttrs(attrs);
     }
+    v.mkList(list);
     addConstant("__nixPath", v, {
         .type = nList,
         .doc = R"(

src/libexpr/primops/context.cc

@@ -207,10 +207,10 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
         if (info.second.allOutputs)
             infoAttrs.alloc(sAllOutputs).mkBool(true);
         if (!info.second.outputs.empty()) {
-            auto & outputsVal = infoAttrs.alloc(state.sOutputs);
+            auto list = state.buildList(info.second.outputs.size());
-            state.mkList(outputsVal, info.second.outputs.size());
             for (const auto & [i, output] : enumerate(info.second.outputs))
-                (outputsVal.listElems()[i] = state.allocValue())->mkString(output);
+                (list[i] = state.allocValue())->mkString(output);
+            infoAttrs.alloc(state.sOutputs).mkList(list);
         }
         attrs.alloc(state.store->printStorePath(info.first)).mkAttrs(infoAttrs);
     }

src/libexpr/primops/fromTOML.cc

@@ -38,10 +38,10 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
                 {
                     auto array = toml::get<std::vector<toml::value>>(t);
 
-                    size_t size = array.size();
+                    auto list = state.buildList(array.size());
-                    state.mkList(v, size);
+                    for (const auto & [n, v] : enumerate(list))
-                    for (size_t i = 0; i < size; ++i)
+                        visit(*(v = state.allocValue()), array[n]);
-                        visit(*(v.listElems()[i] = state.allocValue()), array[i]);
+                    v.mkList(list);
                 }
                 break;;
             case toml::value_t::boolean:

src/libexpr/value.hh

@@ -18,6 +18,7 @@
 
 namespace nix {
 
+struct Value;
 class BindingsBuilder;
 
 
@@ -134,6 +135,34 @@ class ExternalValueBase
 std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
 
 
+class ListBuilder
+{
+    const size_t size;
+    Value * inlineElems[2] = {nullptr, nullptr};
+public:
+    Value * * elems;
+    ListBuilder(EvalState & state, size_t size);
+
+    ListBuilder(ListBuilder && x)
+        : size(x.size)
+        , inlineElems{x.inlineElems[0], x.inlineElems[1]}
+        , elems(size <= 2 ? inlineElems : x.elems)
+    { }
+
+    Value * & operator [](size_t n)
+    {
+        return elems[n];
+    }
+
+    typedef Value * * iterator;
+
+    iterator begin() { return &elems[0]; }
+    iterator end() { return &elems[size]; }
+
+    friend class Value;
+};
+
+
 struct Value
 {
 private:
@@ -217,7 +246,7 @@ public:
         Bindings * attrs;
         struct {
             size_t size;
-            Value * * elems;
+            Value * const * elems;
         } bigList;
         Value * smallList[2];
         ClosureThunk thunk;
@@ -323,16 +352,20 @@ public:
 
     Value & mkAttrs(BindingsBuilder & bindings);
 
-    inline void mkList(size_t size)
+    void mkList(const ListBuilder & builder)
     {
         clearValue();
-        if (size == 1)
+        if (builder.size == 1) {
+            smallList[0] = builder.inlineElems[0];
             internalType = tList1;
-        else if (size == 2)
+        } else if (builder.size == 2) {
+            smallList[0] = builder.inlineElems[0];
+            smallList[1] = builder.inlineElems[1];
             internalType = tList2;
-        else {
+        } else {
+            bigList.size = builder.size;
+            bigList.elems = builder.elems;
             internalType = tListN;
-            bigList.size = size;
         }
     }
 
@@ -392,7 +425,7 @@ public:
         return internalType == tList1 || internalType == tList2 || internalType == tListN;
     }
 
-    Value * * listElems()
+    Value * const * listElems()
     {
         return internalType == tList1 || internalType == tList2 ? smallList : bigList.elems;
     }

src/nix-env/nix-env.cc

@@ -172,7 +172,7 @@ static void loadSourceExpr(EvalState & state, const SourcePath & path, Value & v
        directory). */
     else if (st.type == InputAccessor::tDirectory) {
         auto attrs = state.buildBindings(maxAttrs);
-        state.mkList(attrs.alloc("_combineChannels"), 0);
+        attrs.insert(state.symbols.create("_combineChannels"), &state.vEmptyList);
         StringSet seen;
         getAllExprs(state, path, seen, attrs);
         v.mkAttrs(attrs);

src/nix-env/user-env.cc

@@ -49,10 +49,8 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
 
     /* Construct the whole top level derivation. */
     StorePathSet references;
-    Value manifest;
+    auto list = state.buildList(elems.size());
-    state.mkList(manifest, elems.size());
+    for (const auto & [n, i] : enumerate(elems)) {
-    size_t n = 0;
-    for (auto & i : elems) {
         /* Create a pseudo-derivation containing the name, system,
            output paths, and optionally the derivation path, as well
            as the meta attributes. */
@@ -72,10 +70,9 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
             attrs.alloc(state.sDrvPath).mkString(state.store->printStorePath(*drvPath));
 
         // Copy each output meant for installation.
-        auto & vOutputs = attrs.alloc(state.sOutputs);
+        auto outputsList = state.buildList(outputs.size());
-        state.mkList(vOutputs, outputs.size());
         for (const auto & [m, j] : enumerate(outputs)) {
-            (vOutputs.listElems()[m] = state.allocValue())->mkString(j.first);
+            (outputsList[m] = state.allocValue())->mkString(j.first);
             auto outputAttrs = state.buildBindings(2);
             outputAttrs.alloc(state.sOutPath).mkString(state.store->printStorePath(*j.second));
             attrs.alloc(j.first).mkAttrs(outputAttrs);
@@ -87,6 +84,7 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
 
             references.insert(*j.second);
         }
+        attrs.alloc(state.sOutputs).mkList(outputsList);
 
         // Copy the meta attributes.
         auto meta = state.buildBindings(metaNames.size());
@@ -98,11 +96,14 @@ bool createUserEnv(EvalState & state, PackageInfos & elems,
 
         attrs.alloc(state.sMeta).mkAttrs(meta);
 
-        (manifest.listElems()[n++] = state.allocValue())->mkAttrs(attrs);
+        (list[n] = state.allocValue())->mkAttrs(attrs);
 
         if (drvPath) references.insert(*drvPath);
     }
 
+    Value manifest;
+    manifest.mkList(list);
+
     /* Also write a copy of the list of user environment elements to
        the store; we need it for future modifications of the
        environment. */

tests/unit/libexpr/value/print.cc

@@ -79,11 +79,11 @@ TEST_F(ValuePrintingTests, tList)
     Value vTwo;
     vTwo.mkInt(2);
 
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
     Value vList;
-    state.mkList(vList, 5);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vOne;
-    vList.bigList.elems[1] = &vTwo;
-    vList.bigList.size = 3;
 
     test(vList, "[ 1 2 «nullptr» ]");
 }
@@ -249,12 +249,12 @@ TEST_F(ValuePrintingTests, depthList)
     Value vNested;
     vNested.mkAttrs(builder2.finish());
 
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    list.elems[2] = &vNested;
     Value vList;
-    state.mkList(vList, 5);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vOne;
-    vList.bigList.elems[1] = &vTwo;
-    vList.bigList.elems[2] = &vNested;
-    vList.bigList.size = 3;
 
     test(vList, "[ 1 2 { ... } ]", PrintOptions { .maxDepth = 1 });
     test(vList, "[ 1 2 { nested = { ... }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 2 });
@@ -539,11 +539,11 @@ TEST_F(ValuePrintingTests, ansiColorsList)
     Value vTwo;
     vTwo.mkInt(2);
 
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
     Value vList;
-    state.mkList(vList, 5);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vOne;
-    vList.bigList.elems[1] = &vTwo;
-    vList.bigList.size = 3;
 
     test(vList,
          "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_CYAN "2" ANSI_NORMAL " " ANSI_MAGENTA "«nullptr»" ANSI_NORMAL " ]",
@@ -670,11 +670,11 @@ TEST_F(ValuePrintingTests, ansiColorsListRepeated)
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
 
+    auto list = state.buildList(2);
+    list.elems[0] = &vEmpty;
+    list.elems[1] = &vEmpty;
     Value vList;
-    state.mkList(vList, 3);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vEmpty;
-    vList.bigList.elems[1] = &vEmpty;
-    vList.bigList.size = 2;
 
     test(vList,
          "[ { } " ANSI_MAGENTA "«repeated»" ANSI_NORMAL " ]",
@@ -690,11 +690,11 @@ TEST_F(ValuePrintingTests, listRepeated)
     Value vEmpty;
     vEmpty.mkAttrs(emptyBuilder.finish());
 
+    auto list = state.buildList(2);
+    list.elems[0] = &vEmpty;
+    list.elems[1] = &vEmpty;
     Value vList;
-    state.mkList(vList, 3);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vEmpty;
-    vList.bigList.elems[1] = &vEmpty;
-    vList.bigList.size = 2;
 
     test(vList, "[ { } «repeated» ]", PrintOptions { });
     test(vList,
@@ -750,11 +750,12 @@ TEST_F(ValuePrintingTests, ansiColorsListElided)
     Value vTwo;
     vTwo.mkInt(2);
 
+    {
+    auto list = state.buildList(2);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
     Value vList;
-    state.mkList(vList, 4);
+    vList.mkList(list);
-    vList.bigList.elems[0] = &vOne;
-    vList.bigList.elems[1] = &vTwo;
-    vList.bigList.size = 2;
 
     test(vList,
          "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT "«1 item elided»" ANSI_NORMAL " ]",
@@ -762,12 +763,18 @@ TEST_F(ValuePrintingTests, ansiColorsListElided)
              .ansiColors = true,
              .maxListItems = 1
          });
+    }
 
     Value vThree;
     vThree.mkInt(3);
 
-    vList.bigList.elems[2] = &vThree;
+    {
-    vList.bigList.size = 3;
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    list.elems[2] = &vThree;
+    Value vList;
+    vList.mkList(list);
 
     test(vList,
          "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT "«2 items elided»" ANSI_NORMAL " ]",
@@ -776,5 +783,6 @@ TEST_F(ValuePrintingTests, ansiColorsListElided)
              .maxListItems = 1
          });
     }
+}
 
 } // namespace nix