fix bug with for generator variables in mixin (#297)

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/AmendFunctionNode.java

@@ -61,6 +61,7 @@ public final class AmendFunctionNode extends PklNode {
     } else {
       parameterSlots = new int[0];
     }
+    var hasForGenVars = false;
     for (var i = 0; i < hostFrameDesecriptor.getNumberOfSlots(); i++) {
       var slotInfo = hostFrameDesecriptor.getSlotInfo(i);
       // Copy for-generator variables from the outer frame descriptor into inner lambda.
@@ -74,10 +75,22 @@ public final class AmendFunctionNode extends PklNode {
       // frame (e.g. with `new Mixin { ... }` syntax), so it injects for-generator vars into the
       // wrong frame.
       //
-      // As a remedy, we simply copy outer for-generator variables into this frame.
+      // As a remedy, we simply copy outer variables into this frame if there are any for generator
+      // variables.
+      //
+      // We need to preserve the frame slot index, so we insert dummy identifiers
+      // for other slots that aren't for generator variables.
       if (slotInfo != null && slotInfo.equals(SymbolTable.FOR_GENERATOR_VARIABLE)) {
+        if (!hasForGenVars) {
+          hasForGenVars = true;
+          for (var j = 0; j < i; j++) {
+            builder.addSlot(FrameSlotKind.Illegal, Identifier.DUMMY, null);
+          }
+        }
         builder.addSlot(
             hostFrameDesecriptor.getSlotKind(i), hostFrameDesecriptor.getSlotName(i), null);
+      } else if (hasForGenVars) {
+        builder.addSlot(FrameSlotKind.Illegal, Identifier.DUMMY, null);
       }
     }
     var objectToAmendSlot = builder.addSlot(FrameSlotKind.Object, new Object(), null);

pkl-core/src/main/java/org/pkl/core/runtime/Identifier.java

@@ -152,6 +152,9 @@ public final class Identifier implements Comparable<Identifier> {
   // common in lambdas etc
   public static final Identifier IT = get("it");
 
+  // dummy, unrepresentable identifier
+  public static final Identifier DUMMY = get("`#_");
+
   private final String name;
 
   private Identifier(String name) {

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorInMixins.pkl

@@ -38,3 +38,13 @@ function mapEnvLiteral(_env: Dynamic) = (it) -> (it) {
     }
   }
 }
+
+function addElements(keys: List<String>): Mixin<Mapping<String, String>> = new {
+  for (key in keys) {
+    [key] = key
+  }
+}
+
+res = new Mapping<String, String> {
+  ["base"] = "alreadyThere"
+} |> addElements(List("newElement"))

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorInMixins.pcf

@@ -36,3 +36,7 @@ foo {
     }
   }
 }
+res {
+  ["base"] = "alreadyThere"
+  ["newElement"] = "newElement"
+}