perf: optimize std.makeArray for constant-body functions

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -857,6 +857,7 @@ class Evaluator(
       scope: ValScope): Val.Func =
     new Val.Func(outerPos, scope, params) {
       override def functionName: String = name
+      override val bodyExpr: Expr = rhs
       def evalRhs(vs: ValScope, es: EvalScope, fs: FileScope, pos: Position): Val =
         visitExprWithTailCallSupport(rhs)(vs)
       override def evalDefault(expr: Expr, vs: ValScope, es: EvalScope): Val = {

sjsonnet/src/sjsonnet/Val.scala

@@ -770,6 +770,12 @@ object Val {
 
     def evalRhs(scope: ValScope, ev: EvalScope, fs: FileScope, pos: Position): Val
 
+    /**
+     * Override to expose the function's body AST for pattern detection (e.g. constant-body
+     * makeArray optimization). Returns null by default.
+     */
+    def bodyExpr: Expr = null
+
     // Convenience wrapper: evaluates the function body and resolves any TailCall sentinel.
     // Use this instead of raw `evalRhs` at call sites that bypass `apply*` and consume
     // the result directly (e.g. stdlib scope-reuse fast paths).

sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala

@@ -469,11 +469,19 @@ object ArrayModule extends AbstractFunctionModule {
         pos, {
           val sz = size.cast[Val.Num].asPositiveInt
           val a = new Array[Eval](sz)
-          val noOff = pos.noOffset
-          var i = 0
-          while (i < sz) {
-            a(i) = new LazyApply1(func, Val.Num(pos, i), noOff, ev)
-            i += 1
+          val body = func.bodyExpr
+          if (func.params.names.length == 1 && body != null && body.isInstanceOf[Val.Literal]) {
+            // Function body is a constant (e.g. `function(_) 'x'`).
+            // Skip lazy thunk + Val.Num(index) allocation per element.
+            val constVal = body.asInstanceOf[Val]
+            java.util.Arrays.fill(a.asInstanceOf[Array[AnyRef]], constVal)
+          } else {
+            val noOff = pos.noOffset
+            var i = 0
+            while (i < sz) {
+              a(i) = new LazyApply1(func, Val.Num(pos, i), noOff, ev)
+              i += 1
+            }
           }
           a
         }