libexpr: remove matchAttrs boolean from ExprLambda

The boolean is only used to determine if the formals are set to a
non-null pointer in all our cases. We can get rid of that allocation and
instead just compare the pointer value with NULL. Saving up to
sizeof(bool) + platform specific alignment per ExprLambda instace.
Probably not a lot of memory but perhaps a few kilobyte with nixpkgs?

This also gets rid of a potential issue with dereferencing formals based on
the value of the boolean that didn't have to be aligned with the formals
pointer but was in all our cases.

src/libexpr/eval.cc

@@ -1316,13 +1316,13 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
 
     auto size =
         (lambda.arg.empty() ? 0 : 1) +
-        (lambda.matchAttrs ? lambda.formals->formals.size() : 0);
+        (lambda.hasFormals() ? lambda.formals->formals.size() : 0);
     Env & env2(allocEnv(size));
     env2.up = fun.lambda.env;
 
     size_t displ = 0;
 
-    if (!lambda.matchAttrs)
+    if (!lambda.hasFormals())
         env2.values[displ++] = &arg;
 
     else {
@@ -1402,7 +1402,7 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
         }
     }
 
-    if (!fun.isLambda() || !fun.lambda.fun->matchAttrs) {
+    if (!fun.isLambda() || !fun.lambda.fun->hasFormals()) {
         res = fun;
         return;
     }

src/libexpr/flake/flake.cc

@@ -231,7 +231,7 @@ static Flake getFlake(
     if (auto outputs = vInfo.attrs->get(sOutputs)) {
         expectType(state, nFunction, *outputs->value, *outputs->pos);
 
-        if (outputs->value->isLambda() && outputs->value->lambda.fun->matchAttrs) {
+        if (outputs->value->isLambda() && outputs->value->lambda.fun->hasFormals()) {
             for (auto & formal : outputs->value->lambda.fun->formals->formals) {
                 if (formal.name != state.sSelf)
                     flake.inputs.emplace(formal.name, FlakeInput {

src/libexpr/nixexpr.cc

@@ -124,7 +124,7 @@ void ExprList::show(std::ostream & str) const
 void ExprLambda::show(std::ostream & str) const
 {
     str << "(";
-    if (matchAttrs) {
+    if (hasFormals()) {
         str << "{ ";
         bool first = true;
         for (auto & i : formals->formals) {
@@ -348,7 +348,7 @@ void ExprLambda::bindVars(const StaticEnv & env)
 
     if (!arg.empty()) newEnv.vars[arg] = displ++;
 
-    if (matchAttrs) {
+    if (hasFormals()) {
         for (auto & i : formals->formals)
             newEnv.vars[i.name] = displ++;
 

src/libexpr/nixexpr.hh

@@ -233,11 +233,10 @@ struct ExprLambda : Expr
     Pos pos;
     Symbol name;
     Symbol arg;
-    bool matchAttrs;
     Formals * formals;
     Expr * body;
-    ExprLambda(const Pos & pos, const Symbol & arg, bool matchAttrs, Formals * formals, Expr * body)
+    ExprLambda(const Pos & pos, const Symbol & arg, Formals * formals, Expr * body)
-        : pos(pos), arg(arg), matchAttrs(matchAttrs), formals(formals), body(body)
+        : pos(pos), arg(arg), formals(formals), body(body)
     {
         if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
             throw ParseError({
@@ -247,6 +246,7 @@ struct ExprLambda : Expr
     };
     void setName(Symbol & name);
     string showNamePos() const;
+    inline bool hasFormals() const { return formals != nullptr; }
     COMMON_METHODS
 };
 

src/libexpr/parser.y

@@ -324,13 +324,13 @@ expr: expr_function;
 
 expr_function
   : ID ':' expr_function
-    { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), false, 0, $3); }
+    { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), 0, $3); }
   | '{' formals '}' ':' expr_function
-    { $$ = new ExprLambda(CUR_POS, data->symbols.create(""), true, $2, $5); }
+    { $$ = new ExprLambda(CUR_POS, data->symbols.create(""), $2, $5); }
   | '{' formals '}' '@' ID ':' expr_function
-    { $$ = new ExprLambda(CUR_POS, data->symbols.create($5), true, $2, $7); }
+    { $$ = new ExprLambda(CUR_POS, data->symbols.create($5), $2, $7); }
   | ID '@' '{' formals '}' ':' expr_function
-    { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), true, $4, $7); }
+    { $$ = new ExprLambda(CUR_POS, data->symbols.create($1), $4, $7); }
   | ASSERT expr ';' expr_function
     { $$ = new ExprAssert(CUR_POS, $2, $4); }
   | WITH expr ';' expr_function

src/libexpr/primops.cc

@@ -2365,7 +2365,7 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
             .errPos = pos
         });
 
-    if (!args[0]->lambda.fun->matchAttrs) {
+    if (!args[0]->lambda.fun->hasFormals()) {
         state.mkAttrs(v, 0);
         return;
     }

src/libexpr/value-to-xml.cc

@@ -135,7 +135,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
             if (location) posToXML(xmlAttrs, v.lambda.fun->pos);
             XMLOpenElement _(doc, "function", xmlAttrs);
 
-            if (v.lambda.fun->matchAttrs) {
+            if (v.lambda.fun->hasFormals()) {
                 XMLAttrs attrs;
                 if (!v.lambda.fun->arg.empty()) attrs["name"] = v.lambda.fun->arg;
                 if (v.lambda.fun->formals->ellipsis) attrs["ellipsis"] = "1";

src/nix/flake.cc

@@ -346,10 +346,10 @@ struct CmdFlakeCheck : FlakeCommand
         auto checkOverlay = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
                 state->forceValue(v, pos);
-                if (!v.isLambda() || v.lambda.fun->matchAttrs || std::string(v.lambda.fun->arg) != "final")
+                if (!v.isLambda() || v.lambda.fun->hasFormals() || std::string(v.lambda.fun->arg) != "final")
                     throw Error("overlay does not take an argument named 'final'");
                 auto body = dynamic_cast<ExprLambda *>(v.lambda.fun->body);
-                if (!body || body->matchAttrs || std::string(body->arg) != "prev")
+                if (!body || body->hasFormals() || std::string(body->arg) != "prev")
                     throw Error("overlay does not take an argument named 'prev'");
                 // FIXME: if we have a 'nixpkgs' input, use it to
                 // evaluate the overlay.
@@ -363,7 +363,7 @@ struct CmdFlakeCheck : FlakeCommand
             try {
                 state->forceValue(v, pos);
                 if (v.isLambda()) {
-                    if (!v.lambda.fun->matchAttrs || !v.lambda.fun->formals->ellipsis)
+                    if (!v.lambda.fun->hasFormals() || !v.lambda.fun->formals->ellipsis)
                         throw Error("module must match an open attribute set ('{ config, ... }')");
                 } else if (v.type() == nAttrs) {
                     for (auto & attr : *v.attrs)