Improve handling of frame slots (#1634)

This makes various improvements to the handling of frame slot vars, and
includes some bug fixes introduced by
https://github.com/apple/pkl/pull/1622

* Refactor SymbolTable to track for-generator and parameter slots in
each scope
* Execute let expressions in their own root node in some places
* Unify how frame slots are managed; they are all represented as
`FrameSlotVariable`, created in `AstBuilder`, and passed into
`SymbolTable`.
* Fix how let expressions are executed in custom this scopes (introduce
a new root node when needed)

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -52,9 +52,10 @@ import org.pkl.core.ast.builder.MethodResolution.ImplicitThisMethod;
 import org.pkl.core.ast.builder.MethodResolution.LexicalMethod;
 import org.pkl.core.ast.builder.SymbolTable.AnnotationScope;
 import org.pkl.core.ast.builder.SymbolTable.ClassScope;
+import org.pkl.core.ast.builder.SymbolTable.LetExpressionScope;
 import org.pkl.core.ast.builder.SymbolTable.ModuleScope;
 import org.pkl.core.ast.builder.SymbolTable.ObjectScope;
-import org.pkl.core.ast.builder.VariableResolution.ForGeneratorOrLetVariable;
+import org.pkl.core.ast.builder.VariableResolution.ForGeneratorVariableOrLetBinding;
 import org.pkl.core.ast.builder.VariableResolution.ImplicitBaseProperty;
 import org.pkl.core.ast.builder.VariableResolution.ImplicitThisProperty;
 import org.pkl.core.ast.builder.VariableResolution.LexicalProperty;
@@ -121,6 +122,7 @@ import org.pkl.core.ast.expression.member.ReadLocalPropertyNode;
 import org.pkl.core.ast.expression.member.ReadPropertyNodeGen;
 import org.pkl.core.ast.expression.member.ReadSuperEntryNode;
 import org.pkl.core.ast.expression.member.ReadSuperPropertyNode;
+import org.pkl.core.ast.expression.primary.ExecuteCustomThisWithRootNode;
 import org.pkl.core.ast.expression.primary.GetEnclosingReceiverNode;
 import org.pkl.core.ast.expression.primary.GetMemberKeyNode;
 import org.pkl.core.ast.expression.primary.GetModuleNode;
@@ -179,6 +181,7 @@ import org.pkl.core.module.ResolvedModuleKey;
 import org.pkl.core.packages.PackageLoadError;
 import org.pkl.core.runtime.BaseModule;
 import org.pkl.core.runtime.FrameDescriptorBuilder;
+import org.pkl.core.runtime.FrameSlotVariable;
 import org.pkl.core.runtime.ModuleInfo;
 import org.pkl.core.runtime.ModuleResolver;
 import org.pkl.core.runtime.VmBytes;
@@ -417,14 +420,33 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
         scope -> {
           var exprs = type.getExprs();
           var constraints = new TypeConstraintNode[exprs.size()];
-          for (int i = 0; i < constraints.length; i++) {
+          for (var i = 0; i < constraints.length; i++) {
+            var currentFrameDescriptorSize = scope.frameDescriptorBuilder.getSize();
             var expr = visitExpr(exprs.get(i));
+            var writesFrameSlotVars =
+                scope.frameDescriptorBuilder.getSize() > currentFrameDescriptorSize;
+            // if a constraint expression writes to frame slots (only known case: is a `let` expr),
+            // create a new root node and execute the constraint within this root node
+            // e.g. String(let (x = this) x.length > 5)
+            if (writesFrameSlotVars) {
+              expr = getExprWithinCustomThis(scope, expr);
+            }
             constraints[i] = TypeConstraintNodeGen.create(expr.getSourceSection(), expr);
           }
           return new Constrained(createSourceSection(type), language, childNode, constraints);
         });
   }
 
+  private ExpressionNode getExprWithinCustomThis(SymbolTable.Scope scope, ExpressionNode expr) {
+    return new ExecuteCustomThisWithRootNode(
+        expr.getSourceSection(),
+        expr,
+        scope.frameDescriptorBuilder.build(),
+        scope.getQualifiedName(),
+        scope.forGeneratorSlots,
+        scope.parameterSlots);
+  }
+
   @Override
   public UnresolvedTypeNode visitUnionType(UnionType type) {
     var elementTypes = type.getTypes();
@@ -682,13 +704,13 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
           MemberLookupMode.IMPLICIT_LEXICAL,
           needsConst,
           p.levelsUp() == 0 ? new GetReceiverNode() : new GetEnclosingReceiverNode(p.levelsUp()));
-    } else if (resolution instanceof ForGeneratorOrLetVariable p) {
+    } else if (resolution instanceof ForGeneratorVariableOrLetBinding p) {
       // Parameters can possibly write to frame slots actually in a frame that is one level
       // higher than what we can tell at parse time. However, let exprs and for generator variables
       // always write to frame slots in the same frame.
       //
       // function foo(bar) = new Mixin {
-      //   [bar] = 1            <--- actually 1 level, not 0
+      //   res = bar            <--- actually 1 level, not 0
       //   for (elem in qux) {
       //     elem               <--- actually 0 level
       //   }
@@ -1096,26 +1118,30 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     var sourceSection = createSourceSection(letExpr);
     var parameter = letExpr.getParameter();
     UnresolvedTypeNode typeNode = null;
-    String binding = null;
+    @Nullable FrameSlotVariable binding = null;
-    var slot = -1;
     var frameDescriptorBuilder = symbolTable.getCurrentScope().frameDescriptorBuilder;
     if (parameter instanceof TypedIdentifier par) {
       typeNode = visitTypeAnnotation(par.getTypeAnnotation());
-      slot =
+      binding =
           frameDescriptorBuilder.addSlot(
-              FrameSlotKind.Illegal, toIdentifier(par.getIdentifier().getValue()), null);
+              FrameSlotKind.Illegal,
-      binding = par.getIdentifier().getValue();
+              toIdentifier(par.getIdentifier().getValue()),
+              LetExpressionScope.LET_BINDING_SLOT);
     }
     var bindingExpr = visitExpr(letExpr.getBindingExpr());
     var t = typeNode;
-    var s = slot;
+    var b = binding;
     return symbolTable.enterLetExpression(
         binding,
-        slot,
         scope -> {
           var bodyExpr = visitExpr(letExpr.getExpr());
           return LetExprNodeGen.create(
-              sourceSection, scope.getQualifiedName(), t, bodyExpr, s, bindingExpr);
+              sourceSection,
+              scope.getQualifiedName(),
+              t,
+              bodyExpr,
+              b == null ? -1 : b.slot(),
+              bindingExpr);
         });
   }
 
@@ -1123,9 +1149,10 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
   public ExpressionNode visitFunctionLiteralExpr(FunctionLiteralExpr expr) {
     var sourceSection = createSourceSection(expr);
     var params = expr.getParameterList();
-    var descriptorBuilder = createFrameDescriptorBuilder(params);
+    var descriptorBuilderAndBindings = createFrameDescriptorBuilderAndSlotVariables(params);
     var paramCount = params.getParameters().size();
-
+    var descriptorBuilder = descriptorBuilderAndBindings.first;
+    var bindings = descriptorBuilderAndBindings.second;
     if (paramCount > 5) {
       throw exceptionBuilder()
           .evalError("tooManyFunctionParameters")
@@ -1133,8 +1160,6 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
           .build();
     }
 
-    var bindings = getParameterNames(params);
-
     var isCustomThisScope = symbolTable.getCurrentScope().isCustomThisScope();
 
     return symbolTable.enterLambda(
@@ -1281,7 +1306,18 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
   public GeneratorMemberNode visitMemberPredicate(MemberPredicate ctx) {
     var keyNode =
         symbolTable.enterEagerGenerator(
-            (scp) -> symbolTable.enterCustomThisScope(scope -> visitExpr(ctx.getPred())));
+            (scp) ->
+                symbolTable.enterCustomThisScope(
+                    scope -> {
+                      var currentFrameDescriptorSize = scope.frameDescriptorBuilder.getSize();
+                      var expr = visitExpr(ctx.getPred());
+                      var writesFrameSlotVars =
+                          scope.frameDescriptorBuilder.getSize() > currentFrameDescriptorSize;
+                      if (writesFrameSlotVars) {
+                        return getExprWithinCustomThis(scope, expr);
+                      }
+                      return expr;
+                    }));
     var member =
         doVisitObjectEntryBody(createSourceSection(ctx), keyNode, ctx.getExpr(), ctx.getBodyList());
     var isFrameStored =
@@ -1339,82 +1375,81 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     return doVisitGeneratorMemberNodes(body.getMembers());
   }
 
+  private @Nullable FrameSlotVariable makeBinding(
+      org.pkl.parser.syntax.@Nullable Parameter parameter) {
+    if (!(parameter instanceof TypedIdentifier typedIdentifier)) {
+      return null;
+    }
+    var name = typedIdentifier.getIdentifier().getValue();
+    return symbolTable
+        .getCurrentScope()
+        .frameDescriptorBuilder
+        .addSlot(FrameSlotKind.Illegal, toIdentifier(name), null);
+  }
+
   @Override
   public GeneratorMemberNode visitForGenerator(ForGenerator ctx) {
+
     var keyParameter = ctx.getP2() == null ? null : ctx.getP1();
     var valueParameter = ctx.getP2() == null ? ctx.getP1() : ctx.getP2();
-    TypedIdentifier keyTypedIdentifier = null;
+    var keyBinding = makeBinding(keyParameter);
-    if (keyParameter instanceof TypedIdentifier ti) keyTypedIdentifier = ti;
+    var valueBinding = makeBinding(valueParameter);
-    TypedIdentifier valueTypedIdentifier = null;
-    if (valueParameter instanceof TypedIdentifier ti) valueTypedIdentifier = ti;
-
-    var params = new ArrayList<String>();
-    if (ctx.getP1() instanceof TypedIdentifier ti) {
-      params.add(ti.getIdentifier().getValue());
-    }
-    if (ctx.getP2() != null) {
-      if (ctx.getP2() instanceof TypedIdentifier ti) {
-        params.add(ti.getIdentifier().getValue());
-      }
-    }
-
     var keyIdentifier =
-        keyTypedIdentifier == null
+        keyParameter instanceof TypedIdentifier keyTypedIdentifier ? keyTypedIdentifier : null;
-            ? null
-            : toIdentifier(keyTypedIdentifier.getIdentifier().getValue());
     var valueIdentifier =
-        valueTypedIdentifier == null
+        valueParameter instanceof TypedIdentifier valueTypedIdentifier
-            ? null
+            ? valueTypedIdentifier
-            : toIdentifier(valueTypedIdentifier.getIdentifier().getValue());
+            : null;
-    if (valueIdentifier != null && valueIdentifier == keyIdentifier) {
+    if (keyIdentifier != null
+        && valueIdentifier != null
+        && keyIdentifier
+            .getIdentifier()
+            .getValue()
+            .equals(valueIdentifier.getIdentifier().getValue())) {
       throw exceptionBuilder()
-          .evalError("duplicateDefinition", valueIdentifier)
+          .evalError("duplicateDefinition", valueIdentifier.getIdentifier().getValue())
-          .withSourceSection(createSourceSection(valueTypedIdentifier.getIdentifier()))
+          .withSourceSection(createSourceSection(valueIdentifier))
           .build();
     }
-    var currentScope = symbolTable.getCurrentScope();
+
-    var generatorDescriptorBuilder = currentScope.newFrameDescriptorBuilder();
-    var keySlot = -1;
-    var valueSlot = -1;
-    if (keyIdentifier != null) {
-      keySlot = generatorDescriptorBuilder.addSlot(FrameSlotKind.Illegal, keyIdentifier, null);
-    }
-    if (valueIdentifier != null) {
-      valueSlot = generatorDescriptorBuilder.addSlot(FrameSlotKind.Illegal, valueIdentifier, null);
-    }
     var unresolvedKeyTypeNode =
-        keyTypedIdentifier == null
+        keyIdentifier == null ? null : visitTypeAnnotation(keyIdentifier.getTypeAnnotation());
-            ? null
-            : visitTypeAnnotation(keyTypedIdentifier.getTypeAnnotation());
     var unresolvedValueTypeNode =
-        valueTypedIdentifier == null
+        valueIdentifier == null ? null : visitTypeAnnotation(valueIdentifier.getTypeAnnotation());
-            ? null
-            : visitTypeAnnotation(valueTypedIdentifier.getTypeAnnotation());
     // if possible, initialize immediately to avoid later insert
     var keyTypeNode =
-        unresolvedKeyTypeNode == null && keySlot != -1
+        unresolvedKeyTypeNode == null && keyBinding != null
             ? new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
-                .initWriteSlotNode(keySlot)
+                .initWriteSlotNode(keyBinding.slot())
             : null;
     // if possible, initialize immediately to avoid later insert
     var valueTypeNode =
-        unresolvedValueTypeNode == null && valueSlot != -1
+        unresolvedValueTypeNode == null && valueBinding != null
             ? new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
-                .initWriteSlotNode(valueSlot)
+                .initWriteSlotNode(valueBinding.slot())
             : null;
     var iterableNode = symbolTable.enterEagerGenerator(scope -> visitExpr(ctx.getExpr()));
-    var memberNodes =
+    var outerScope = symbolTable.getCurrentScope();
-        symbolTable.enterForGenerator(
+    return symbolTable.enterForGenerator(
-            params, generatorDescriptorBuilder, scope -> doVisitForWhenBody(ctx.getBody()));
+        keyBinding,
-    return GeneratorForNodeGen.create(
+        valueBinding,
-        createSourceSection(ctx),
+        symbolTable.getCurrentScope().frameDescriptorBuilder,
-        generatorDescriptorBuilder.build(),
+        scope -> {
-        iterableNode,
+          var memberNodes = doVisitForWhenBody(ctx.getBody());
-        unresolvedKeyTypeNode,
+          return GeneratorForNodeGen.create(
-        unresolvedValueTypeNode,
+              createSourceSection(ctx),
-        memberNodes,
+              scope.frameDescriptorBuilder.build(),
-        keyTypeNode,
+              iterableNode,
-        valueTypeNode);
+              unresolvedKeyTypeNode,
+              unresolvedValueTypeNode,
+              memberNodes,
+              keyTypeNode,
+              valueTypeNode,
+              keyBinding == null ? -1 : keyBinding.slot(),
+              valueBinding == null ? -1 : valueBinding.slot(),
+              outerScope.getForGeneratorSlots(),
+              scope.getParameterSlots());
+        });
   }
 
   @Override
@@ -1937,10 +1972,11 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
 
     var bodyContext = entry.getExpr();
     var paramListCtx = entry.getParameterList();
-    var descriptorBuilder = createFrameDescriptorBuilder(paramListCtx);
+    var descriptorBuilderAndBindings = createFrameDescriptorBuilderAndSlotVariables(paramListCtx);
     var paramCount = paramListCtx.getParameters().size();
 
-    var bindings = getParameterNames(paramListCtx);
+    var descriptorBuilder = descriptorBuilderAndBindings.first;
+    var bindings = descriptorBuilderAndBindings.second;
 
     var annotations = doVisitAnnotations(entry.getAnnotations(), methodName);
 
@@ -2221,8 +2257,14 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
   }
 
   private ExpressionNode doVisitObjectBody(ObjectBody body, ExpressionNode parentNode) {
+    var parametersDescriptorAndBindings = createFrameDescriptorBuilderAndSlotVariables(body);
+    var bindings =
+        parametersDescriptorAndBindings == null
+            ? new FrameSlotVariable[0]
+            : parametersDescriptorAndBindings.second;
+
     return symbolTable.enterObjectScope(
-        body,
+        bindings,
         (scope) -> {
           addObjectNamesToScope(scope, body);
           var objectMembers = body.getMembers();
@@ -2232,7 +2274,10 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
           }
           var sourceSection = createSourceSection(body.parent());
 
-          var parametersDescriptorBuilder = createFrameDescriptorBuilder(body);
+          var parametersDescriptor =
+              parametersDescriptorAndBindings == null
+                  ? null
+                  : parametersDescriptorAndBindings.first.build();
           var parameterTypes = doVisitParameterTypes(body);
 
           var members = EconomicMaps.<Object, ObjectMember>create();
@@ -2278,8 +2323,6 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
           }
 
           var currentScope = symbolTable.getCurrentScope();
-          var parametersDescriptor =
-              parametersDescriptorBuilder == null ? null : parametersDescriptorBuilder.build();
           if (!elements.isEmpty()) {
             if (isConstantKeyNodes) { // true if zero key nodes
               addConstantEntries(members, keyNodes, values);
@@ -2516,8 +2559,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
   }
 
   private ObjectMember doVisitObjectElement(ObjectElement element) {
-    var isForGeneratorScope = symbolTable.getCurrentScope().isForGeneratorScope();
+    var outerScope = symbolTable.getCurrentScope();
-    return symbolTable.enterEntry(
+    var isForGeneratorScope = outerScope.isForGeneratorScope();
+    return symbolTable.enterEntryOrElement(
         null,
         scope -> {
           var elementNode = visitExpr(element.getExpr());
@@ -2535,7 +2579,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
             member.initConstantValue(constantNode);
           } else {
             if (isForGeneratorScope) {
-              elementNode = new RestoreForBindingsNode(elementNode);
+              elementNode =
+                  new RestoreForBindingsNode(
+                      elementNode, scope.getParameterSlots(), scope.getForGeneratorSlots());
             }
             member.initMemberNode(
                 ElementOrEntryNodeGen.create(
@@ -2586,9 +2632,10 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
 
     var methodName = org.pkl.core.runtime.Identifier.method(identifier.getValue(), true);
 
-    var frameDescriptorBuilder = createFrameDescriptorBuilder(paramList);
+    var frameDescriptorBuilderAndBindings = createFrameDescriptorBuilderAndSlotVariables(paramList);
 
-    var bindings = getParameterNames(paramList);
+    var frameDescriptorBuilder = frameDescriptorBuilderAndBindings.first;
+    var bindings = frameDescriptorBuilderAndBindings.second;
 
     return symbolTable.enterMethod(
         methodName,
@@ -2629,7 +2676,8 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
 
   private GeneratorObjectLiteralNode doVisitGeneratorObjectBody(
       ObjectBody body, ExpressionNode parentNode) {
-    var parametersDescriptor = createFrameDescriptorBuilder(body);
+    var parametersDescriptorBuilderAndFrameSlotVariables =
+        createFrameDescriptorBuilderAndSlotVariables(body);
     var parameterTypes = doVisitParameterTypes(body);
     var memberNodes = doVisitGeneratorMemberNodes(body.getMembers());
     var currentScope = symbolTable.getCurrentScope();
@@ -2639,7 +2687,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
         language,
         currentScope.getQualifiedName(),
         currentScope.isCustomThisScope(),
-        parametersDescriptor == null ? null : parametersDescriptor.build(),
+        parametersDescriptorBuilderAndFrameSlotVariables == null
+            ? null
+            : parametersDescriptorBuilderAndFrameSlotVariables.first.build(),
         parameterTypes,
         memberNodes,
         parentNode);
@@ -2838,8 +2888,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
       ExpressionNode keyNode,
       @Nullable Expr valueCtx,
       @Nullable List<? extends ObjectBody> objectBodyCtxs) {
-    var isForGeneratorScope = symbolTable.getCurrentScope().isForGeneratorScope();
+    var outerScope = symbolTable.getCurrentScope();
-    return symbolTable.enterEntry(
+    var isForGeneratorScope = outerScope.isForGeneratorScope();
+    return symbolTable.enterEntryOrElement(
         keyNode,
         scope -> {
           var modifier = VmModifier.ENTRY;
@@ -2856,7 +2907,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
               member.initConstantValue(constantNode);
             } else {
               if (isForGeneratorScope) {
-                valueNode = new RestoreForBindingsNode(valueNode);
+                valueNode =
+                    new RestoreForBindingsNode(
+                        valueNode, scope.getParameterSlots(), scope.getForGeneratorSlots());
               }
               member.initMemberNode(
                   ElementOrEntryNodeGen.create(
@@ -2869,7 +2922,9 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
                     objectBodyCtxs,
                     new ReadSuperEntryNode(unavailableSourceSection(), new GetMemberKeyNode()));
             if (isForGeneratorScope) {
-              objectBody = new RestoreForBindingsNode(objectBody);
+              objectBody =
+                  new RestoreForBindingsNode(
+                      objectBody, scope.getParameterSlots(), scope.getForGeneratorSlots());
             }
             member.initMemberNode(
                 ElementOrEntryNodeGen.create(
@@ -2903,39 +2958,34 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     return needsConst;
   }
 
-  private static List<String> getParameterNames(ParameterList parameterList) {
+  private FrameSlotVariable[] getSlotVariables(
-    var result = new ArrayList<String>(parameterList.getParameters().size());
+      List<org.pkl.parser.syntax.Parameter> params, FrameDescriptorBuilder frameDescriptorBuilder) {
-    for (var param : parameterList.getParameters()) {
+    var slotVariables = new FrameSlotVariable[params.size()];
-      var name = param instanceof TypedIdentifier id ? id.getIdentifier().getValue() : "_";
+    for (var i = 0; i < params.size(); i++) {
-      result.add(name);
+      var param = params.get(i);
-    }
+      org.pkl.core.runtime.Identifier identifier;
-    return result;
-  }
-
-  private FrameDescriptorBuilder createFrameDescriptorBuilder(ParameterList params) {
-    var builder = new FrameDescriptorBuilder(params.getParameters().size());
-    for (var param : params.getParameters()) {
-      org.pkl.core.runtime.Identifier identifier = null;
       if (param instanceof TypedIdentifier typedIdentifier) {
         identifier = toIdentifier(typedIdentifier.getIdentifier().getValue());
+      } else {
+        identifier = org.pkl.core.runtime.Identifier.ILLEGAL;
       }
-      builder.addSlot(FrameSlotKind.Illegal, identifier, null);
+      slotVariables[i] = frameDescriptorBuilder.addSlot(FrameSlotKind.Illegal, identifier, null);
     }
-    return builder;
+    return slotVariables;
   }
 
-  private FrameDescriptor.@Nullable Builder createFrameDescriptorBuilder(ObjectBody body) {
+  private Pair<FrameDescriptorBuilder, FrameSlotVariable[]>
+      createFrameDescriptorBuilderAndSlotVariables(ParameterList params) {
+    var builder = new FrameDescriptorBuilder(params.getParameters().size());
+    return Pair.of(builder, getSlotVariables(params.getParameters(), builder));
+  }
+
+  private @Nullable Pair<@Nullable FrameDescriptorBuilder, FrameSlotVariable[]>
+      createFrameDescriptorBuilderAndSlotVariables(ObjectBody body) {
     if (body.getParameters().isEmpty()) return null;
 
-    var builder = FrameDescriptor.newBuilder(body.getParameters().size());
+    var builder = new FrameDescriptorBuilder(body.getParameters().size());
-    for (var param : body.getParameters()) {
+    return Pair.of(builder, getSlotVariables(body.getParameters(), builder));
-      org.pkl.core.runtime.Identifier identifier = null;
-      if (param instanceof TypedIdentifier typedIdentifier) {
-        identifier = toIdentifier(typedIdentifier.getIdentifier().getValue());
-      }
-      builder.addSlot(FrameSlotKind.Illegal, identifier, null);
-    }
-    return builder;
   }
 
   private void checkNotInsideForGenerator(Node ctx, String errorMessageKey) {

pkl-core/src/main/java/org/pkl/core/ast/builder/SymbolTable.java

@@ -15,6 +15,8 @@
  */
 package org.pkl.core.ast.builder;
 
+import static org.pkl.core.util.ArrayUtils.EMPTY_INT_ARRAY;
+
 import com.oracle.truffle.api.frame.FrameDescriptor;
 import java.util.*;
 import java.util.function.Function;
@@ -26,19 +28,21 @@ import org.pkl.core.ast.VmModifier;
 import org.pkl.core.ast.builder.MethodResolution.ImplicitBaseMethod;
 import org.pkl.core.ast.builder.MethodResolution.ImplicitThisMethod;
 import org.pkl.core.ast.builder.MethodResolution.LexicalMethod;
-import org.pkl.core.ast.builder.VariableResolution.ForGeneratorOrLetVariable;
+import org.pkl.core.ast.builder.VariableResolution.ForGeneratorVariableOrLetBinding;
 import org.pkl.core.ast.builder.VariableResolution.ImplicitBaseProperty;
 import org.pkl.core.ast.builder.VariableResolution.LexicalProperty;
+import org.pkl.core.ast.builder.VariableResolution.Parameter;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.BaseModuleMembers;
 import org.pkl.core.runtime.FrameDescriptorBuilder;
+import org.pkl.core.runtime.FrameSlotVariable;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.ModuleInfo;
 import org.pkl.core.runtime.VmDataSize;
 import org.pkl.core.runtime.VmDuration;
+import org.pkl.core.util.ArrayUtils;
 import org.pkl.core.util.LateInit;
 import org.pkl.parser.Lexer;
-import org.pkl.parser.syntax.ObjectBody;
 
 public final class SymbolTable {
 
@@ -94,7 +98,7 @@ public final class SymbolTable {
   public <T> T enterMethod(
       Identifier name,
       ConstLevel constLevel,
-      List<String> bindings,
+      FrameSlotVariable[] bindings,
       FrameDescriptorBuilder frameDescriptorBuilder,
       List<TypeParameter> typeParameters,
       Function<MethodScope, T> nodeFactory) {
@@ -115,17 +119,22 @@ public final class SymbolTable {
   }
 
   public <T> T enterForGenerator(
-      List<String> params,
+      @Nullable FrameSlotVariable keyBinding,
+      @Nullable FrameSlotVariable valueBinding,
       FrameDescriptorBuilder frameDescriptorBuilder,
       Function<ForGeneratorScope, T> nodeFactory) {
     return doEnter(
         new ForGeneratorScope(
-            currentScope, currentScope.qualifiedName, params, frameDescriptorBuilder),
+            currentScope,
+            currentScope.qualifiedName,
+            keyBinding,
+            valueBinding,
+            frameDescriptorBuilder),
         nodeFactory);
   }
 
   public <T> T enterLambda(
-      List<String> bindings,
+      FrameSlotVariable[] bindings,
       FrameDescriptorBuilder frameDescriptorBuilder,
       Function<LambdaScope, T> nodeFactory) {
 
@@ -144,7 +153,7 @@ public final class SymbolTable {
   }
 
   public <T> T enterLetExpression(
-      @Nullable String binding, int slot, Function<LetExpressionScope, T> nodeFactory) {
+      @Nullable FrameSlotVariable binding, Function<LetExpressionScope, T> nodeFactory) {
 
     // flatten names of let exprs inside other let exprs for presentation purposes
     var parentScope = currentScope;
@@ -154,7 +163,7 @@ public final class SymbolTable {
 
     assert parentScope != null;
     var qualifiedName = parentScope.qualifiedName + "." + "<let expr>";
-    return doEnter(new LetExpressionScope(currentScope, binding, slot, qualifiedName), nodeFactory);
+    return doEnter(new LetExpressionScope(currentScope, binding, qualifiedName), nodeFactory);
   }
 
   public <T> T enterProperty(
@@ -165,16 +174,16 @@ public final class SymbolTable {
         nodeFactory);
   }
 
-  public <T> T enterEntry(
+  public <T> T enterEntryOrElement(
       @Nullable ExpressionNode keyNode, // null for listing elements
-      Function<EntryScope, T> nodeFactory) {
+      Function<EntryOrElementScope, T> nodeFactory) {
 
     var qualifiedName = currentScope.getQualifiedName() + currentScope.getNextEntryName(keyNode);
     var builder =
         currentScope instanceof ForGeneratorScope forScope
             ? forScope.frameDescriptorBuilder
             : new FrameDescriptorBuilder();
-    return doEnter(new EntryScope(currentScope, qualifiedName, builder), nodeFactory);
+    return doEnter(new EntryOrElementScope(currentScope, qualifiedName, builder), nodeFactory);
   }
 
   public <T> T enterCustomThisScope(Function<CustomThisScope, T> nodeFactory) {
@@ -193,9 +202,10 @@ public final class SymbolTable {
         nodeFactory);
   }
 
-  public <T> T enterObjectScope(ObjectBody body, Function<ObjectScope, T> nodeFactory) {
+  public <T> T enterObjectScope(
+      FrameSlotVariable[] bindings, Function<ObjectScope, T> nodeFactory) {
     return doEnter(
-        new ObjectScope(currentScope, body, currentScope.frameDescriptorBuilder), nodeFactory);
+        new ObjectScope(currentScope, bindings, currentScope.frameDescriptorBuilder), nodeFactory);
   }
 
   private <T, S extends Scope> T doEnter(S scope, Function<S, T> nodeFactory) {
@@ -224,17 +234,35 @@ public final class SymbolTable {
     protected final FrameDescriptorBuilder frameDescriptorBuilder;
     private final ConstLevel constLevel;
     protected boolean isBaseModule;
+    // all for-generator slots in this scope (excludes args and let bindings).
+    protected final int[] forGeneratorSlots;
+    // all parameter slots in this scope; includes let bindings, function params, method params,
+    // but excludes object body params (they are written one level higher)
+    protected final int[] parameterSlots;
     // The properties defined on this (lexical) scope
     protected final Map<String, Member> properties = new HashMap<>();
     // The methods defined on this (lexical) scope
     protected final Map<String, Member> methods = new HashMap<>();
 
+    static int[] getSlots(FrameSlotVariable[] bindings) {
+      if (bindings.length == 0) {
+        return EMPTY_INT_ARRAY;
+      }
+      var ret = new int[bindings.length];
+      for (var i = 0; i < ret.length; i++) {
+        ret[i] = bindings[i].slot();
+      }
+      return ret;
+    }
+
     private Scope(
         @Nullable Scope parent,
         @Nullable Identifier name,
         String qualifiedName,
         ConstLevel constLevel,
-        FrameDescriptorBuilder frameDescriptorBuilder) {
+        FrameDescriptorBuilder frameDescriptorBuilder,
+        int[] forGeneratorSlots,
+        int[] parameterSlots) {
       this.parent = parent;
       this.name = name;
       this.qualifiedName = qualifiedName;
@@ -247,6 +275,8 @@ public final class SymbolTable {
           parent != null && parent.constLevel.biggerOrEquals(constLevel)
               ? parent.constLevel
               : constLevel;
+      this.forGeneratorSlots = forGeneratorSlots;
+      this.parameterSlots = parameterSlots;
     }
 
     public final @Nullable Scope getParent() {
@@ -266,16 +296,26 @@ public final class SymbolTable {
       return qualifiedName;
     }
 
-    public FrameDescriptor buildFrameDescriptor() {
+    /**
-      return frameDescriptorBuilder.build();
+     * Returns the frame slots inhabited by for-generator variables in this scope.
+     *
+     * <p>Includes outer for-generator variables.
+     */
+    public int[] getForGeneratorSlots() {
+      return forGeneratorSlots;
     }
 
     /**
-     * Returns a new descriptor builder that contains the same slots as the current scope's frame
+     * Returns the parameter slots in this scope.
-     * descriptor.
+     *
+     * <p>Includes let bindings, object body params, method params, lambda params
      */
-    public FrameDescriptorBuilder newFrameDescriptorBuilder() {
+    public int[] getParameterSlots() {
-      return new FrameDescriptorBuilder(buildFrameDescriptor());
+      return parameterSlots;
+    }
+
+    public FrameDescriptor buildFrameDescriptor() {
+      return frameDescriptorBuilder.build();
     }
 
     public @Nullable TypeParameter getTypeParameter(String name) {
@@ -389,6 +429,13 @@ public final class SymbolTable {
     }
 
     public final boolean isCustomThisScope() {
+      if (this instanceof LetExpressionScope) {
+        var myParent = parent;
+        while (myParent instanceof LetExpressionScope) {
+          myParent = myParent.getParent();
+        }
+        return myParent instanceof CustomThisScope;
+      }
       return this instanceof CustomThisScope;
     }
 
@@ -505,21 +552,45 @@ public final class SymbolTable {
   }
 
   public static class ObjectScope extends Scope implements LexicalScope {
-    private final Map<String, Integer> params;
+    private final FrameSlotVariable[] bindings;
 
+    /**
+     * NOTE: object body params desugar to wrapping this object with a lambda call.
+     *
+     * <p>So, the object itself does not contribute to parameter slots in the object's frame
+     * descriptor.
+     *
+     * <p>This code:
+     *
+     * <pre>{@code
+     * foo { param ->
+     *   res = param
+     * }
+     * }</pre>
+     *
+     * Is sugar for:
+     *
+     * <pre>{@code
+     * foo = (param) -> (super.foo.apply(param)) {
+     *   res = param
+     * }
+     * }</pre>
+     */
     private ObjectScope(
-        Scope parent, ObjectBody body, FrameDescriptorBuilder frameDescriptorBuilder) {
+        Scope parent, FrameSlotVariable[] bindings, FrameDescriptorBuilder frameDescriptorBuilder) {
       super(
           parent,
           parent.getNameOrNull(),
           parent.getQualifiedName(),
           ConstLevel.NONE,
-          frameDescriptorBuilder);
+          frameDescriptorBuilder,
-      params = collectParams(body);
+          parent.forGeneratorSlots,
+          EMPTY_INT_ARRAY);
+      this.bindings = bindings;
     }
 
     public boolean hasParams() {
-      return !params.isEmpty();
+      return bindings.length > 0;
     }
 
     @Override
@@ -532,10 +603,11 @@ public final class SymbolTable {
       if (prop != null) {
         return new VariableResolution.LexicalProperty(false, prop.modifiers, levelsUp);
       }
-      var paramIndex = params.get(name);
+      for (var binding : bindings) {
-      if (paramIndex != null) {
+        if (binding.name().equals(name)) {
-        // params are on a higher level than the properties
+          // params are on a higher level than the properties
-        return new VariableResolution.Parameter(paramIndex, levelsUp + 1);
+          return new VariableResolution.Parameter(binding.slot(), levelsUp + 1);
+        }
       }
       return null;
     }
@@ -548,19 +620,6 @@ public final class SymbolTable {
       }
       return null;
     }
-
-    private static Map<String, Integer> collectParams(ObjectBody body) {
-      var params = new HashMap<String, Integer>();
-      for (var i = 0; i < body.getParameters().size(); i++) {
-        var param = body.getParameters().get(i);
-        if (param instanceof org.pkl.parser.syntax.Parameter.TypedIdentifier ti) {
-          params.put(ti.getIdentifier().getValue(), i);
-        } else {
-          params.put("_", i);
-        }
-      }
-      return params;
-    }
   }
 
   public abstract static class TypeParameterizableScope extends Scope {
@@ -572,8 +631,17 @@ public final class SymbolTable {
         String qualifiedName,
         ConstLevel constLevel,
         FrameDescriptorBuilder frameDescriptorBuilder,
-        List<TypeParameter> typeParameters) {
+        List<TypeParameter> typeParameters,
-      super(parent, name, qualifiedName, constLevel, frameDescriptorBuilder);
+        int[] forGeneratorSlots,
+        int[] parameterSlots) {
+      super(
+          parent,
+          name,
+          qualifiedName,
+          constLevel,
+          frameDescriptorBuilder,
+          forGeneratorSlots,
+          parameterSlots);
       this.typeParameters = typeParameters;
     }
 
@@ -593,7 +661,14 @@ public final class SymbolTable {
     private final boolean isAmend;
 
     public ModuleScope(ModuleInfo moduleInfo, boolean isBaseModule) {
-      super(null, null, moduleInfo.getModuleName(), ConstLevel.NONE, new FrameDescriptorBuilder());
+      super(
+          null,
+          null,
+          moduleInfo.getModuleName(),
+          ConstLevel.NONE,
+          new FrameDescriptorBuilder(),
+          EMPTY_INT_ARRAY,
+          EMPTY_INT_ARRAY);
       this.isBaseModule = isBaseModule;
       this.moduleInfo = moduleInfo;
       this.isAmend = moduleInfo.isAmend();
@@ -622,17 +697,25 @@ public final class SymbolTable {
   }
 
   public static final class MethodScope extends TypeParameterizableScope implements LexicalScope {
-    private final List<String> bindings;
+    private final FrameSlotVariable[] bindings;
 
-    public MethodScope(
+    MethodScope(
         Scope parent,
         Identifier name,
         String qualifiedName,
         ConstLevel constLevel,
-        List<String> bindings,
+        FrameSlotVariable[] bindings,
         FrameDescriptorBuilder frameDescriptorBuilder,
         List<TypeParameter> typeParameters) {
-      super(parent, name, qualifiedName, constLevel, frameDescriptorBuilder, typeParameters);
+      super(
+          parent,
+          name,
+          qualifiedName,
+          constLevel,
+          frameDescriptorBuilder,
+          typeParameters,
+          EMPTY_INT_ARRAY,
+          getSlots(bindings));
       this.bindings = bindings;
     }
 
@@ -648,14 +731,21 @@ public final class SymbolTable {
   }
 
   public static final class LambdaScope extends Scope implements LexicalScope {
-    private final List<String> bindings;
+    private final FrameSlotVariable[] bindings;
 
     public LambdaScope(
         Scope parent,
-        List<String> bindings,
+        FrameSlotVariable[] bindings,
         String qualifiedName,
         FrameDescriptorBuilder frameDescriptorBuilder) {
-      super(parent, null, qualifiedName, parent.getConstLevel(), frameDescriptorBuilder);
+      super(
+          parent,
+          null,
+          qualifiedName,
+          parent.getConstLevel(),
+          frameDescriptorBuilder,
+          EMPTY_INT_ARRAY,
+          getSlots(bindings));
       this.bindings = bindings;
     }
 
@@ -671,8 +761,8 @@ public final class SymbolTable {
   }
 
   public static final class LetExpressionScope extends Scope implements LexicalScope {
-    private final @Nullable String binding;
+    public static final Object LET_BINDING_SLOT = new Object();
-    private final int slot;
+    private final @Nullable FrameSlotVariable binding;
 
     private static @Nullable Identifier getParentName(Scope parent) {
       while (parent != null && parent.name == null) {
@@ -681,25 +771,34 @@ public final class SymbolTable {
       return parent == null ? null : parent.name;
     }
 
+    private static int[] getMyParameterSlots(Scope parent, @Nullable FrameSlotVariable binding) {
+      var parentSlots = parent.parameterSlots;
+      if (binding == null) {
+        return parentSlots;
+      }
+      return ArrayUtils.append(parentSlots, binding.slot());
+    }
+
     public LetExpressionScope(
-        Scope parent, @Nullable String binding, int slot, String qualifiedName) {
+        Scope parent, @Nullable FrameSlotVariable binding, String qualifiedName) {
       super(
           parent,
           getParentName(parent),
           qualifiedName,
           parent.getConstLevel(),
-          parent.frameDescriptorBuilder);
+          parent.frameDescriptorBuilder,
+          parent.forGeneratorSlots,
+          getMyParameterSlots(parent, binding));
       this.binding = binding;
-      this.slot = slot;
     }
 
     @Override
     public @Nullable VariableResolution doResolveProperty(String name, int levelsUp) {
-      if (name.equals("_")) {
+      if (name.equals("_") || binding == null) {
         return null;
       }
-      if (name.equals(binding)) {
+      if (name.equals(binding.name())) {
-        return new ForGeneratorOrLetVariable(slot, levelsUp);
+        return new ForGeneratorVariableOrLetBinding(binding.slot(), levelsUp);
       }
       return null;
     }
@@ -718,21 +817,87 @@ public final class SymbolTable {
       return grandParent.frameDescriptorBuilder;
     }
 
+    private static int[] getForGeneratorSlots(Scope parent) {
+      var grandParent = parent.parent;
+      assert grandParent != null;
+      return grandParent.forGeneratorSlots;
+    }
+
+    private static int[] getParameterSlots(Scope parent) {
+      var grandParent = parent.parent;
+      assert grandParent != null;
+      return grandParent.parameterSlots;
+    }
+
     private EagerGeneratorScope(Scope parent, String qualifiedName) {
-      super(parent, null, qualifiedName, ConstLevel.NONE, getFrameDescriptorBuilder(parent));
+      super(
+          parent,
+          null,
+          qualifiedName,
+          ConstLevel.NONE,
+          getFrameDescriptorBuilder(parent),
+          getForGeneratorSlots(parent),
+          getParameterSlots(parent));
     }
   }
 
   public static final class ForGeneratorScope extends Scope implements LexicalScope {
-    final List<String> params;
+    private final @Nullable FrameSlotVariable keyBinding;
+    private final @Nullable FrameSlotVariable valueBinding;
+
+    private static int[] getMyForGeneratorSlots(
+        Scope parentScope,
+        @Nullable FrameSlotVariable keyBinding,
+        @Nullable FrameSlotVariable valueBinding) {
+      var slots = parentScope.forGeneratorSlots;
+      if (keyBinding != null && valueBinding != null) {
+        return ArrayUtils.append(slots, keyBinding.slot(), valueBinding.slot());
+      }
+      if (keyBinding != null) {
+        return ArrayUtils.append(slots, keyBinding.slot());
+      }
+      if (valueBinding != null) {
+        return ArrayUtils.append(slots, valueBinding.slot());
+      }
+      return slots;
+    }
+
+    // for-generators execute in the frame above their enclosing object.
+    // so, the parameters of the scope outside that object is visible.
+    //
+    // e.g. this for-generator reads param `it` as levels up == 0
+    // ```
+    // (it) -> new Listing {
+    //   for (elem in it) {
+    //     doSomething(elem)
+    //   }
+    // }
+    // ```
+    private static int[] getMyParameterSlots(Scope parent) {
+      if (parent instanceof ObjectScope) {
+        var grandParent = parent.parent;
+        assert grandParent != null;
+        return grandParent.parameterSlots;
+      }
+      return parent.parameterSlots;
+    }
 
     public ForGeneratorScope(
         Scope parent,
         String qualifiedName,
-        List<String> params,
+        @Nullable FrameSlotVariable keyBinding,
+        @Nullable FrameSlotVariable valueBinding,
         FrameDescriptorBuilder frameDescriptorBuilder) {
-      super(parent, null, qualifiedName, ConstLevel.NONE, frameDescriptorBuilder);
+      super(
-      this.params = params;
+          parent,
+          null,
+          qualifiedName,
+          ConstLevel.NONE,
+          frameDescriptorBuilder,
+          getMyForGeneratorSlots(parent, keyBinding, valueBinding),
+          getMyParameterSlots(parent));
+      this.keyBinding = keyBinding;
+      this.valueBinding = valueBinding;
     }
 
     @Override
@@ -744,12 +909,11 @@ public final class SymbolTable {
 
     @Override
     public @Nullable VariableResolution doResolveProperty(String name, int levelsUp) {
-      if (!params.contains(name)) {
+      if (keyBinding != null && keyBinding.name().equals(name)) {
-        return null;
+        return new ForGeneratorVariableOrLetBinding(keyBinding.slot(), levelsUp);
       }
-      var index = frameDescriptorBuilder.findSlot(Identifier.get(name));
+      if (valueBinding != null && valueBinding.name().equals(name)) {
-      if (index >= 0) {
+        return new ForGeneratorVariableOrLetBinding(valueBinding.slot(), levelsUp);
-        return new ForGeneratorOrLetVariable(index, levelsUp);
       }
       return null;
     }
@@ -767,14 +931,30 @@ public final class SymbolTable {
         String qualifiedName,
         ConstLevel constLevel,
         FrameDescriptorBuilder frameDescriptorBuilder) {
-      super(parent, name, qualifiedName, constLevel, frameDescriptorBuilder);
+      super(
+          parent,
+          name,
+          qualifiedName,
+          constLevel,
+          frameDescriptorBuilder,
+          // object members inherit the for-generator slots of the parent for-generator, if it
+          // exists.
+          parent instanceof ForGeneratorScope ? parent.forGeneratorSlots : EMPTY_INT_ARRAY,
+          parent.parameterSlots);
     }
   }
 
-  public static final class EntryScope extends Scope {
+  public static final class EntryOrElementScope extends Scope {
-    public EntryScope(
+    public EntryOrElementScope(
         Scope parent, String qualifiedName, FrameDescriptorBuilder frameDescriptorBuilder) {
-      super(parent, null, qualifiedName, ConstLevel.NONE, frameDescriptorBuilder);
+      super(
+          parent,
+          null,
+          qualifiedName,
+          ConstLevel.NONE,
+          frameDescriptorBuilder,
+          parent instanceof ForGeneratorScope ? parent.forGeneratorSlots : EMPTY_INT_ARRAY,
+          parent.parameterSlots);
     }
   }
 
@@ -788,7 +968,15 @@ public final class SymbolTable {
         int modifiers,
         FrameDescriptorBuilder frameDescriptorBuilder,
         List<TypeParameter> typeParameters) {
-      super(parent, name, qualifiedName, ConstLevel.MODULE, frameDescriptorBuilder, typeParameters);
+      super(
+          parent,
+          name,
+          qualifiedName,
+          ConstLevel.MODULE,
+          frameDescriptorBuilder,
+          typeParameters,
+          EMPTY_INT_ARRAY,
+          EMPTY_INT_ARRAY);
       isClosed = VmModifier.isClosed(modifiers);
     }
 
@@ -816,7 +1004,15 @@ public final class SymbolTable {
         String qualifiedName,
         FrameDescriptorBuilder frameDescriptorBuilder,
         List<TypeParameter> typeParameters) {
-      super(parent, name, qualifiedName, ConstLevel.MODULE, frameDescriptorBuilder, typeParameters);
+      super(
+          parent,
+          name,
+          qualifiedName,
+          ConstLevel.MODULE,
+          frameDescriptorBuilder,
+          typeParameters,
+          parent.forGeneratorSlots,
+          parent.parameterSlots);
     }
   }
 
@@ -841,7 +1037,9 @@ public final class SymbolTable {
           parent.getNameOrNull(),
           parent.getQualifiedName(),
           ConstLevel.NONE,
-          frameDescriptorBuilder);
+          frameDescriptorBuilder,
+          parent.forGeneratorSlots,
+          parent.parameterSlots);
     }
   }
 
@@ -849,18 +1047,22 @@ public final class SymbolTable {
     public AnnotationScope(
         Scope parent, String qualifiedName, FrameDescriptorBuilder frameDescriptorBuilder) {
       super(
-          parent, parent.getNameOrNull(), qualifiedName, ConstLevel.MODULE, frameDescriptorBuilder);
+          parent,
+          parent.getNameOrNull(),
+          qualifiedName,
+          ConstLevel.MODULE,
+          frameDescriptorBuilder,
+          EMPTY_INT_ARRAY,
+          EMPTY_INT_ARRAY);
     }
   }
 
   private static @Nullable VariableResolution resolveParameter(
-      String name, List<String> bindings, int levelsUp) {
+      String name, FrameSlotVariable[] bindings, int levelsUp) {
-    if (name.equals("_")) {
+    for (var binding : bindings) {
-      return null;
+      if (name.equals(binding.name())) {
-    }
+        return new Parameter(binding.slot(), levelsUp);
-    var index = bindings.indexOf(name);
+      }
-    if (index != -1) {
-      return new VariableResolution.Parameter(index, levelsUp);
     }
     return null;
   }

pkl-core/src/main/java/org/pkl/core/ast/builder/VariableResolution.java

@@ -34,7 +34,7 @@ public sealed interface VariableResolution {
   // method, lambda, object body param
   record Parameter(int slot, int levelsUp) implements VariableResolution {}
 
-  record ForGeneratorOrLetVariable(int slot, int levelsUp) implements VariableResolution {}
+  record ForGeneratorVariableOrLetBinding(int slot, int levelsUp) implements VariableResolution {}
 
   // Implicit base module lookup
   record ImplicitBaseProperty() implements VariableResolution {}

pkl-core/src/main/java/org/pkl/core/ast/expression/binary/LetExprNode.java

@@ -58,6 +58,7 @@ public abstract class LetExprNode extends ExpressionNode {
         typeNode = new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection());
       }
       typeNode.initWriteSlotNode(slot);
+      frame.getFrameDescriptor().setSlotKind(slot, typeNode.getFrameSlotKind());
       insert(typeNode);
     }
     assert typeNode != null;

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorForNode.java

@@ -28,6 +28,7 @@ import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.type.TypeNode;
 import org.pkl.core.ast.type.UnresolvedTypeNode;
 import org.pkl.core.runtime.*;
+import org.pkl.core.util.ArrayUtils;
 
 public abstract class GeneratorForNode extends GeneratorMemberNode {
   private final FrameDescriptor generatorDescriptor;
@@ -37,6 +38,9 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
   @Children private final GeneratorMemberNode[] childNodes;
   @Child private @Nullable TypeNode keyTypeNode;
   @Child private @Nullable TypeNode valueTypeNode;
+  private final int keySlot;
+  private final int valueSlot;
+  private final int[] slotsToCopy;
 
   public GeneratorForNode(
       SourceSection sourceSection,
@@ -52,7 +56,11 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
       @Nullable TypeNode keyTypeNode,
       // If this node can be constructed at parse time,
       // it should be passed instead of `unresolvedValueTypeNode`.
-      @Nullable TypeNode valueTypeNode) {
+      @Nullable TypeNode valueTypeNode,
+      int keySlot,
+      int valueSlot,
+      int[] outerForGeneratorSlots,
+      int[] parameterSlots) {
     super(sourceSection, false);
     this.generatorDescriptor = generatorDescriptor;
     this.iterableNode = iterableNode;
@@ -61,6 +69,9 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
     this.childNodes = childNodes;
     this.keyTypeNode = keyTypeNode;
     this.valueTypeNode = valueTypeNode;
+    this.keySlot = keySlot;
+    this.valueSlot = valueSlot;
+    this.slotsToCopy = ArrayUtils.concat(parameterSlots, outerForGeneratorSlots);
   }
 
   protected abstract void executeWithIterable(
@@ -154,12 +165,10 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
       VirtualFrame frame, Object parent, ObjectData data, Object key, Object value) {
 
     // GraalJS uses the same implementation technique here:
-    // https://github.com/oracle/graaljs/blob/44a11ce6e87/graal-js/src/com.oracle.truffle.js/
+    // https://github.com/oracle/graaljs/blob/44a11ce6e87/graal-js/src/com.oracle.truffle.js/src/com/oracle/truffle/js/nodes/function/IterationScopeNode.java#L86-L88
-    // src/com/oracle/truffle/js/nodes/function/IterationScopeNode.java#L86-L88
     var newFrame =
         Truffle.getRuntime().createVirtualFrame(frame.getArguments(), generatorDescriptor);
-    // the locals in `frame` (if any) are function arguments and/or outer for-generator bindings
+    VmUtils.copyLocals(frame, newFrame, slotsToCopy);
-    VmUtils.copyLocals(frame, 0, newFrame, 0, frame.getFrameDescriptor().getNumberOfSlots());
     if (keyTypeNode != null) {
       keyTypeNode.executeAndSet(newFrame, key);
     }
@@ -175,14 +184,12 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
   private void initialize(VirtualFrame frame) {
     if (unresolvedKeyTypeNode != null) {
       CompilerDirectives.transferToInterpreterAndInvalidate();
-      var keySlot = frame.getFrameDescriptor().getNumberOfSlots();
       keyTypeNode = insert(unresolvedKeyTypeNode.execute(frame)).initWriteSlotNode(keySlot);
       generatorDescriptor.setSlotKind(keySlot, keyTypeNode.getFrameSlotKind());
       unresolvedKeyTypeNode = null;
     }
     if (unresolvedValueTypeNode != null) {
       CompilerDirectives.transferToInterpreterAndInvalidate();
-      var valueSlot = frame.getFrameDescriptor().getNumberOfSlots() + (keyTypeNode != null ? 1 : 0);
       valueTypeNode = insert(unresolvedValueTypeNode.execute(frame)).initWriteSlotNode(valueSlot);
       generatorDescriptor.setSlotKind(valueSlot, valueTypeNode.getFrameSlotKind());
       unresolvedValueTypeNode = null;

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/RestoreForBindingsNode.java

@@ -18,27 +18,26 @@ package org.pkl.core.ast.expression.generator;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.runtime.VmUtils;
+import org.pkl.core.util.ArrayUtils;
 
 /**
  * Restores for-generator variable bindings when a member generated by a for-generator is executed.
  */
 public final class RestoreForBindingsNode extends ExpressionNode {
   private @Child ExpressionNode child;
+  private final int[] slotsToCopy;
 
-  public RestoreForBindingsNode(ExpressionNode child) {
+  public RestoreForBindingsNode(
+      ExpressionNode child, int[] parameterSlots, int[] forGeneratorSlots) {
     super(child.getSourceSection());
     this.child = child;
+    this.slotsToCopy = ArrayUtils.concat(parameterSlots, forGeneratorSlots);
   }
 
   @Override
   public Object executeGeneric(VirtualFrame frame) {
     var generatorFrame = ObjectData.getGeneratorFrame(frame);
-    // copying all slots includes function arguments, but the capture generator frame
+    VmUtils.copyLocals(generatorFrame, frame, slotsToCopy);
-    // and the host frame are guaranteed to have the same arguments and number of slots
-    // (guaranteed by AstBuilder).
-    assert frame.getFrameDescriptor().getNumberOfSlots()
-        == generatorFrame.getFrameDescriptor().getNumberOfSlots();
-    VmUtils.copyLocals(generatorFrame, 0, frame, 0, frame.getFrameDescriptor().getNumberOfSlots());
     return child.executeGeneric(frame);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/ExecuteCustomThisWithRootNode.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.ast.expression.primary;
+
+import com.oracle.truffle.api.frame.FrameDescriptor;
+import com.oracle.truffle.api.frame.VirtualFrame;
+import com.oracle.truffle.api.nodes.DirectCallNode;
+import com.oracle.truffle.api.source.SourceSection;
+import org.pkl.core.ast.ExpressionNode;
+import org.pkl.core.ast.SimpleRootNode;
+import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
+import org.pkl.core.runtime.VmLanguage;
+import org.pkl.core.runtime.VmUtils;
+
+/**
+ * A node that executes {@code expressionNode} within its own root node, with the custom this value
+ * set.
+ *
+ * <p>This is required, for example, when let expressions are used inside type constraints, because:
+ *
+ * <ul>
+ *   <li>let expressions write to frame slots.
+ *   <li>type nodes don't add slots to the enclosing frame.
+ * </ul>
+ */
+public final class ExecuteCustomThisWithRootNode extends ExpressionNode {
+  private @Child ExpressionNode customThisNode =
+      new CustomThisNode(VmUtils.unavailableSourceSection());
+  private @Child DirectCallNode callNode;
+
+  public ExecuteCustomThisWithRootNode(
+      SourceSection sourceSection,
+      ExpressionNode expressionNode,
+      FrameDescriptor frameDescriptor,
+      String qualifiedName,
+      int[] forGeneratorSlots,
+      int[] parameterSlots) {
+    super(sourceSection);
+    frameDescriptor.findOrAddAuxiliarySlot(CustomThisScope.FRAME_SLOT_ID);
+    var rootNode =
+        new SimpleRootNode(
+            VmLanguage.get(this),
+            frameDescriptor,
+            sourceSection,
+            qualifiedName,
+            new WithCustomThisExpression(expressionNode, forGeneratorSlots, parameterSlots));
+    this.callNode = DirectCallNode.create(rootNode.getCallTarget());
+  }
+
+  @Override
+  public Object executeGeneric(VirtualFrame frame) {
+    var customThis = customThisNode.executeGeneric(frame);
+    return callNode.call(VmUtils.getReceiver(frame), VmUtils.getOwner(frame), customThis, frame);
+  }
+}

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/GetModuleNode.java

@@ -32,8 +32,8 @@ public final class GetModuleNode extends ExpressionNode {
   public Object executeGeneric(VirtualFrame frame) {
     CompilerDirectives.transferToInterpreter();
 
-    var levelsUp = 0;
+    var levelsUp = -1;
-    for (var current = VmUtils.getOwner(frame).getEnclosingOwner();
+    for (var current = VmUtils.getOwner(frame);
         current != null;
         current = current.getEnclosingOwner()) {
       if (!current.isParseTimeInvisibleScope()) {

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/WithCustomThisExpression.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.ast.expression.primary;
+
+import com.oracle.truffle.api.CompilerDirectives;
+import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
+import com.oracle.truffle.api.frame.VirtualFrame;
+import org.pkl.core.ast.ExpressionNode;
+import org.pkl.core.runtime.VmUtils;
+import org.pkl.core.util.ArrayUtils;
+
+/** Entrypoint node of {@link ExecuteCustomThisWithRootNode}. */
+public final class WithCustomThisExpression extends ExpressionNode {
+
+  private @Child ExpressionNode expressionNode;
+  private final int[] slotsToCopy;
+  @CompilationFinal private int customThisSlot = -1;
+
+  public WithCustomThisExpression(
+      ExpressionNode expressionNode, int[] forGeneratorSlots, int[] parameterSlots) {
+    super(expressionNode.getSourceSection());
+    this.expressionNode = expressionNode;
+    this.slotsToCopy = ArrayUtils.concat(parameterSlots, forGeneratorSlots);
+  }
+
+  public int getCustomThisSlot(VirtualFrame frame) {
+    if (customThisSlot == -1) {
+      CompilerDirectives.transferToInterpreterAndInvalidate();
+      customThisSlot = VmUtils.findCustomThisSlot(frame);
+    }
+    return customThisSlot;
+  }
+
+  @Override
+  public Object executeGeneric(VirtualFrame frame) {
+    var customThisSlot = getCustomThisSlot(frame);
+    // arguments passed in by `ExecuteCustomThisWithRootNode`
+    frame.setAuxiliarySlot(customThisSlot, frame.getArguments()[2]);
+    var originalFrame = (VirtualFrame) frame.getArguments()[3];
+    VmUtils.copyLocals(originalFrame, frame, slotsToCopy);
+    return expressionNode.executeGeneric(frame);
+  }
+}

pkl-core/src/main/java/org/pkl/core/ast/member/FunctionNode.java

@@ -148,8 +148,7 @@ public final class FunctionNode extends RegularMemberNode {
     var parameters = CollectionUtils.<String, PType>newLinkedHashMap(paramCount);
     for (var i = 0; i < paramCount; i++) {
       var slotName = getFrameDescriptor().getSlotName(i);
-      // Ignored parameters (`_`) have no name
+      var paramName = slotName == Identifier.ILLEGAL ? "_#" + i : slotName.toString();
-      var paramName = slotName == null ? "_#" + i : slotName.toString();
       parameters.put(paramName, TypeNode.export(parameterTypeNodes[i]));
     }
 

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

@@ -20,10 +20,7 @@ import com.oracle.truffle.api.frame.FrameSlotKind;
 import java.util.Arrays;
 import org.jspecify.annotations.Nullable;
 
-/**
+/** A wrapper for Truffle's {@link FrameDescriptor.Builder}, but also gives us the current size. */
- * A wrapper for Truffle's {@link FrameDescriptor.Builder}, but also lets us find the slot of a
- * given {@link Identifier}.
- */
 public class FrameDescriptorBuilder {
 
   private @Nullable Identifier[] names;
@@ -42,16 +39,6 @@ public class FrameDescriptorBuilder {
     this.names = new Identifier[capacity];
   }
 
-  public FrameDescriptorBuilder(FrameDescriptor descriptor) {
-    this(descriptor.getNumberOfSlots());
-    for (var i = 0; i < descriptor.getNumberOfSlots(); i++) {
-      addSlot(
-          descriptor.getSlotKind(i),
-          (Identifier) descriptor.getSlotName(i),
-          descriptor.getSlotInfo(i));
-    }
-  }
-
   private void ensureCapacity(int count) {
     if (names.length < size + count) {
       var newLength = Math.max(size + count, size * 2);
@@ -59,26 +46,19 @@ public class FrameDescriptorBuilder {
     }
   }
 
-  public int addSlot(FrameSlotKind kind, @Nullable Identifier name, @Nullable Object info) {
+  public FrameSlotVariable addSlot(FrameSlotKind kind, Identifier name, @Nullable Object info) {
     ensureCapacity(1);
     names[size] = name;
     size++;
-    return underlying.addSlot(kind, name, info);
+    var slot = underlying.addSlot(kind, name, info);
-  }
+    return new FrameSlotVariable(name.toString(), slot);
-
-  public int findSlot(Identifier identifier) {
-    // go backwards to account for shadowed variables
-    // (this happens in the case of nested for generators).
-    for (var i = size - 1; i >= 0; i--) {
-      var name = names[i];
-      if (name != null && name.equals(identifier)) {
-        return i;
-      }
-    }
-    return -1;
   }
 
   public FrameDescriptor build() {
     return underlying.build();
   }
+
+  public int getSize() {
+    return size;
+  }
 }

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

@@ -0,0 +1,18 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.runtime;
+
+public record FrameSlotVariable(String name, int slot) {}

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

@@ -163,6 +163,8 @@ public final class Identifier implements Comparable<Identifier> {
   public static final Identifier GLOB = get("glob");
   public static final Identifier COMPLETION_CANDIDATES = get("completionCandidates");
 
+  public static final Identifier ILLEGAL = get("`");
+
   // common in lambdas etc
   public static final Identifier IT = get("it");
 

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

@@ -401,50 +401,42 @@ public final class VmUtils {
   }
 
   /**
-   * Copies {@code numberOfLocalsToCopy} locals from {@code sourceFrame}, starting at {@code
+   * Copies the slots specified by {@code slotsToCopy} locals from {@code sourceFrame} into {@code
-   * firstSourceSlot}, to {@code firstSourceSlot}, starting at {@code firstTargetSlot}.
+   * targetFrame}.
    */
   public static void copyLocals(
-      VirtualFrame sourceFrame,
+      VirtualFrame sourceFrame, VirtualFrame targetFrame, int[] slotsToCopy) {
-      int firstSourceSlot,
+    if (slotsToCopy.length == 0) return;
-      VirtualFrame targetFrame,
-      int firstTargetSlot,
-      int numberOfLocalsToCopy) {
     var sourceDescriptor = sourceFrame.getFrameDescriptor();
     var targetDescriptor = targetFrame.getFrameDescriptor();
-    assert sourceDescriptor.getNumberOfSlots() <= targetDescriptor.getNumberOfSlots();
+    for (var slot : slotsToCopy) {
-    // Alternatively, locals could be copied with `numberOfLocalsToCopy`
-    // `ReadFrameSlotNode/WriteFrameSlotNode`'s.
-    for (int i = 0; i < numberOfLocalsToCopy; i++) {
-      var sourceSlot = firstSourceSlot + i;
-      var targetSlot = firstTargetSlot + i;
       // If, for a particular call site of this method,
       // slot kinds of `sourceDescriptor` will reach a steady state,
       // then slot kinds of `targetDescriptor` will too.
-      var slotKind = sourceDescriptor.getSlotKind(sourceSlot);
+      var slotKind = sourceDescriptor.getSlotKind(slot);
       switch (slotKind) {
         case Boolean -> {
-          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Boolean);
+          targetDescriptor.setSlotKind(slot, FrameSlotKind.Boolean);
-          targetFrame.setBoolean(targetSlot, sourceFrame.getBoolean(sourceSlot));
+          targetFrame.setBoolean(slot, sourceFrame.getBoolean(slot));
         }
         case Long -> {
-          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Long);
+          targetDescriptor.setSlotKind(slot, FrameSlotKind.Long);
-          targetFrame.setLong(targetSlot, sourceFrame.getLong(sourceSlot));
+          targetFrame.setLong(slot, sourceFrame.getLong(slot));
         }
         case Double -> {
-          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Double);
+          targetDescriptor.setSlotKind(slot, FrameSlotKind.Double);
-          targetFrame.setDouble(targetSlot, sourceFrame.getDouble(sourceSlot));
+          targetFrame.setDouble(slot, sourceFrame.getDouble(slot));
         }
         case Object -> {
-          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Object);
+          targetDescriptor.setSlotKind(slot, FrameSlotKind.Object);
           targetFrame.setObject(
-              targetSlot,
+              slot,
               sourceFrame instanceof MaterializedFrame
                   // Even though sourceDescriptor.getSlotKind is now Object,
                   // it may have been a primitive kind when `sourceFrame`'s local was written.
-                  // Hence we need to read the local with getValue() instead of getObject().
+                  // Hence, we need to read the local with getValue() instead of getObject().
-                  ? sourceFrame.getValue(sourceSlot)
+                  ? sourceFrame.getValue(slot)
-                  : sourceFrame.getObject(sourceSlot));
+                  : sourceFrame.getObject(slot));
         }
         default -> {
           CompilerDirectives.transferToInterpreter();
@@ -978,10 +970,9 @@ public final class VmUtils {
   }
 
   public static int findCustomThisSlot(VirtualFrame frame) {
-    return frame
+    var result = frame.getFrameDescriptor().getAuxiliarySlots().get(CustomThisScope.FRAME_SLOT_ID);
-        .getFrameDescriptor()
+    assert result != null;
-        .getAuxiliarySlots()
+    return result;
-        .getOrDefault(CustomThisScope.FRAME_SLOT_ID, -1);
   }
 
   @TruffleBoundary

pkl-core/src/main/java/org/pkl/core/util/ArrayUtils.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.util;
+
+import java.util.Arrays;
+
+public final class ArrayUtils {
+  private ArrayUtils() {}
+
+  public static final int[] EMPTY_INT_ARRAY = new int[0];
+
+  public static int[] append(int[] array, int elem) {
+    if (array.length == 0) {
+      return new int[] {elem};
+    }
+    var ret = Arrays.copyOf(array, array.length + 1);
+    ret[array.length] = elem;
+    return ret;
+  }
+
+  public static int[] append(int[] array, int elem1, int elem2) {
+    if (array.length == 0) {
+      return new int[] {elem1, elem2};
+    }
+    var ret = Arrays.copyOf(array, array.length + 2);
+    ret[array.length] = elem1;
+    ret[array.length + 1] = elem2;
+    return ret;
+  }
+
+  public static int[] concat(int[] array1, int[] array2) {
+    var result = new int[array1.length + array2.length];
+    System.arraycopy(array1, 0, result, 0, array1.length);
+    System.arraycopy(array2, 0, result, array1.length, array2.length);
+    return result;
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/input/basic/let2.pkl

@@ -0,0 +1,30 @@
+// let expr inside custom this scopes
+typealias Foo = String(let (x = this) this == x)
+
+typealias Foo2 = String(let (x = "foo") this.startsWith(x), let (y = "o") this.endsWith(y))
+
+typealias Foo3 = String(let (x = "foo") this.startsWith(x))(let (y = "o") this.endsWith(y))
+
+res1: Foo = "foo"
+
+res2: Foo2 = "foo"
+
+res3: Foo3 = "foo"
+
+hidden bar {
+  new {
+    name = "fooey"
+  }
+}
+
+res4 = (bar) {
+  [[let (x = "foo") this.name.startsWith(x)]] {
+    name = "bob"
+  }
+}
+
+res5 = (bar) {
+  [[let (x = this) x.name.startsWith("foo")]] {
+    name = "bob"
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/input/basic/let3.pkl

@@ -0,0 +1,53 @@
+res =
+  let (
+    x = new Listing {
+      for (elem in List(1, 2)) {
+        elem + 5
+      }
+    }
+  )
+    x
+
+res2 =
+  let (qux = 1)
+  let (
+    x = new Listing {
+      for (num in List(1, 2)) {
+        qux + num
+      }
+    }
+  )
+    x
+
+res3 =
+  let (qux = 1)
+  let (
+    x = new Listing {
+      for (num in List(1, 2)) {
+        qux + num
+      }
+    }
+  )
+  let (y = 5)
+  let (
+    z = new Listing {
+      for (nummm in List(1, 2)) {
+        when (qux + y == nummm) {
+          nummm + y
+        }
+      }
+    }
+  )
+    new { ...x; ...z }
+
+res4 = 
+  let (m: Mapping<String, String> = new Mapping {
+    ["foo"] = "bar"
+  })
+    new Mapping {
+      for (k, _ in Map("foo", "bar")) {
+        [k] {
+          m[k]
+        }
+      }
+    }

pkl-core/src/test/files/LanguageSnippetTests/input/basic/moduleRef4.pkl

@@ -0,0 +1,11 @@
+prop = "qux"
+
+res =
+  new Mapping {
+    default {
+      when (module.prop == "qux") {
+        myProp = "qux"
+      }
+    }
+    ["bar"] {}
+  }

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

@@ -480,3 +480,16 @@ whenWithElse = new Dynamic {
     }
   }
 }
+
+local func = (it) -> new Listing {
+  for (elem1 in List(1, 2)) {
+    it
+    new Listing {
+      for (elem2 in List(1, 2)) {
+        elem1 + elem2
+      }
+    }
+  }
+}
+
+nestedForsWithinLambda = func.apply("hi")

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

@@ -475,3 +475,42 @@ withinLetExpr =
         [k] = v
       }
     }
+
+// object bodies that are children of for-generators inherit the frame descriptor of the parent object
+nestedObjectInsideFor {
+  for (idx, qux in List(1, 2)) {
+    [idx] {
+      for (baz in List(1, 2)) {
+        when (baz == qux) {
+          baz
+        }
+      }
+    }
+  }
+}
+
+// object bodies inherit the frame descriptor of the parent object
+doublyNestedObjectInsideFor {
+  for (idx, qux in List(1, 2)) {
+    [idx] {
+      new Listing {
+        for (baz in List(1, 2)) {
+          when (baz == qux) {
+            baz
+          }
+        }
+      }
+    }
+  }
+}
+
+insideObjectBodyWithParam: Mapping<String, Dynamic> = new {
+  default { key ->
+    for (key, value in Map("foo", List(new Dynamic { bar = 1 }))) {
+      [key] {
+        ...value
+      }
+    }
+  }
+  ["res"] {}
+}

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

@@ -1,7 +1,7 @@
 examples {
   local a = List("1", "2", "3", "4")
   local b = List("a", "b", "c", "d")
-  
+
   ["shadow key variable"] {
     new {
       for (key, outerValue in a) {

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

@@ -0,0 +1,5 @@
+res {
+  for (prefix in List("foo")) {
+    "foobar" as String(let (y = this) y.startsWith(prefix))
+  }
+}

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

@@ -43,7 +43,8 @@ res5 = new {
 
 // nested predicate
 res6 = (people) {
-  [[(people) { [[name == "Barn Owl"]] { age = 99 } }.toList().find((it) -> it.age == 99).name == name]] {
+  [[(people) { [[name == "Barn Owl"]] { age = 99 } }.toList().find((it) -> it.age == 99).name
+    == name]] {
     age = 55
   }
 }
@@ -86,3 +87,28 @@ res12 = (people) {
     [[name == "Pigeon"]] { age = 122 }
   }
 }
+
+res13 = (people) {
+  [[(() -> this.name == "Pigeon").apply()]] {
+    age = 99
+  }
+}
+
+res14 = (people) {
+  for (foo in List(1, 2)) {
+    [[foo == 1]] {
+      age = foo
+    }
+  }
+}
+
+res15 =
+  (
+    (it) -> (people) {
+      for (foo in List(1, 2)) {
+        [[foo == 1]] {
+          age = foo
+        }
+      }
+    }
+  ).apply("hi")

pkl-core/src/test/files/LanguageSnippetTests/output/basic/let2.pcf

@@ -0,0 +1,13 @@
+res1 = "foo"
+res2 = "foo"
+res3 = "foo"
+res4 {
+  new {
+    name = "bob"
+  }
+}
+res5 {
+  new {
+    name = "bob"
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/basic/let3.pcf

@@ -0,0 +1,17 @@
+res {
+  6
+  7
+}
+res2 {
+  2
+  3
+}
+res3 {
+  2
+  3
+}
+res4 {
+  ["foo"] {
+    "bar"
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/basic/moduleRef4.pcf

@@ -0,0 +1,6 @@
+prop = "qux"
+res {
+  ["bar"] {
+    myProp = "qux"
+  }
+}

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

@@ -335,3 +335,15 @@ whenWithElse {
   20
   40
 }
+nestedForsWithinLambda {
+  "hi"
+  new {
+    2
+    3
+  }
+  "hi"
+  new {
+    3
+    4
+  }
+}

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

@@ -323,3 +323,32 @@ withinLetExpr {
   ["a"] = 1
   ["b"] = 2
 }
+nestedObjectInsideFor {
+  [0] {
+    1
+  }
+  [1] {
+    2
+  }
+}
+doublyNestedObjectInsideFor {
+  [0] {
+    new {
+      1
+    }
+  }
+  [1] {
+    new {
+      2
+    }
+  }
+}
+insideObjectBodyWithParam {
+  ["res"] {
+    ["foo"] {
+      new {
+        bar = 1
+      }
+    }
+  }
+}

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

@@ -0,0 +1,3 @@
+res {
+  "foobar"
+}

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

@@ -153,3 +153,45 @@ res12 {
     age = 33
   }
 }
+res13 {
+  new {
+    name = "Pigeon"
+    age = 99
+  }
+  new {
+    name = "Barn Owl"
+    age = 21
+  }
+  new {
+    name = "Parrot"
+    age = 33
+  }
+}
+res14 {
+  new {
+    name = "Pigeon"
+    age = 1
+  }
+  new {
+    name = "Barn Owl"
+    age = 1
+  }
+  new {
+    name = "Parrot"
+    age = 1
+  }
+}
+res15 {
+  new {
+    name = "Pigeon"
+    age = 1
+  }
+  new {
+    name = "Barn Owl"
+    age = 1
+  }
+  new {
+    name = "Parrot"
+    age = 1
+  }
+}