libexpr: remove maxPrimOpArity

this was only used to enable an optimization we no longer need such
strong guarantees for: SmallVector can be used with a dynamic size.

Change-Id: I4513fb7e665827fe363ce6308448656e6c5badb7

lix/libexpr/eval.cc

@@ -458,14 +458,6 @@ void EvalState::addConstant(const std::string & name, Value * v, Constant info)
 }
 
 
-void PrimOp::check()
-{
-    if (arity > maxPrimOpArity) {
-        throw Error("primop arity must not exceed %1%", maxPrimOpArity);
-    }
-}
-
-
 std::ostream & operator<<(std::ostream & output, PrimOp & primOp)
 {
     output << "primop " << primOp.name;
@@ -1636,7 +1628,8 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 /* We have all the arguments, so call the primop with
                    the previous and new arguments. */
 
-                Value * vArgs[maxPrimOpArity];
+                // max arity as of writing is 3. even 4 seems excessive though.
+                SmallVector<Value *, 4> vArgs(arity);
                 auto n = argsDone;
                 for (Value * arg = &vCur; arg->isPrimOpApp(); arg = arg->primOpApp.left)
                     vArgs[--n] = arg->primOpApp.right;
@@ -1653,7 +1646,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                     // 1. Unify this and above code. Heavily redundant.
                     // 2. Create a fake env (arg1, arg2, etc.) and a fake expr (arg1: arg2: etc: builtins.name arg1 arg2 etc)
                     //    so the debugger allows to inspect the wrong parameters passed to the builtin.
-                    fn->fun(*this, vCur.determinePos(noPos), vArgs, vCur);
+                    fn->fun(*this, vCur.determinePos(noPos), vArgs.data(), vCur);
                 } catch (Error & e) {
                     e.addTrace(positions[pos], "while calling the '%1%' builtin", fn->name);
                     throw;

lix/libexpr/eval.hh

@@ -22,13 +22,6 @@
 
 namespace nix {
 
-/**
- * We put a limit on primop arity because it lets us use a fixed size array on
- * the stack. 8 is already an impractical number of arguments. Use an attrset
- * argument for such overly complicated functions.
- */
-constexpr size_t maxPrimOpArity = 8;
-
 class Store;
 class EvalState;
 class StorePath;
@@ -82,12 +75,6 @@ struct PrimOp
      * Optional experimental for this to be gated on.
      */
     std::optional<ExperimentalFeature> experimentalFeature;
-
-    /**
-     * Validity check to be performed by functions that introduce primops,
-     * such as RegisterPrimOp() and Value::mkPrimOp().
-     */
-    void check();
 };
 
 std::ostream & operator<<(std::ostream & output, PrimOp & primOp);

lix/libexpr/value.cc

@@ -27,7 +27,6 @@ Value::Value(primop_t, PrimOp & primop)
     , primOp(&primop)
     , _primop_pad(0)
 {
-    primop.check();
 }
 
 
@@ -76,7 +75,6 @@ PrimOp * Value::primOpAppPrimOp() const
 
 void Value::mkPrimOp(PrimOp * p)
 {
-    p->check();
     clearValue();
     internalType = tPrimOp;
     primOp = p;