Improve Parser implementation (#1508)

- Make leaf AST classes final
- Make protected Lexer fields private and add getter
- Split Parser into Parser and ParserImpl
- Using a fresh ParserImpl instance per parse simplifies reasoning
(important) and makes the Parser API thread-safe (nice to have)
- Split GenericParser into GenericParser and GenericParserImpl
  - Same motivation as for Parser

Some of these changes will facilitate the move to JSpecify, which has
proven challenging for this package.

pkl-parser/src/main/java/org/pkl/parser/GenericParserError.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -17,7 +17,7 @@ package org.pkl.parser;
 
 import org.pkl.parser.syntax.generic.FullSpan;
 
-public class GenericParserError extends RuntimeException {
+public final class GenericParserError extends RuntimeException {
   private final FullSpan span;
 
   public GenericParserError(String msg, FullSpan span) {

pkl-parser/src/main/java/org/pkl/parser/Lexer.java

@@ -20,12 +20,12 @@ import java.util.Deque;
 import org.pkl.parser.syntax.generic.FullSpan;
 import org.pkl.parser.util.ErrorMessages;
 
-public class Lexer {
+public final class Lexer {
 
   private final char[] source;
   private final int size;
-  protected int cursor = 0;
+  private int cursor = 0;
-  protected int sCursor = 0;
+  private int sCursor = 0;
   private int line = 1;
   private int sLine = 1;
   private int col = 1;
@@ -36,7 +36,7 @@ public class Lexer {
   private boolean stringEnded = false;
   private boolean isEscape = false;
   // how many newlines exist between two subsequent tokens
-  protected int newLinesBetween = 0;
+  private int newLinesBetween = 0;
 
   private static final int EOF = -1;
 
@@ -73,6 +73,10 @@ public class Lexer {
     return cursor;
   }
 
+  public int getNewLinesBetween() {
+    return newLinesBetween;
+  }
+
   public char[] getSource() {
     return source;
   }

pkl-parser/src/main/java/org/pkl/parser/ParserError.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -18,7 +18,7 @@ package org.pkl.parser;
 import org.pkl.parser.syntax.Module;
 import org.pkl.parser.util.Nullable;
 
-public class ParserError extends RuntimeException {
+public final class ParserError extends RuntimeException {
   private final Span span;
   private @Nullable Module partialParseResult;
 

pkl-parser/src/main/java/org/pkl/parser/syntax/Annotation.java

@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class Annotation extends AbstractNode {
+public final class Annotation extends AbstractNode {
   public Annotation(List<Node> nodes, Span span) {
     super(span, nodes);
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/ArgumentList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ArgumentList extends AbstractNode {
+public final class ArgumentList extends AbstractNode {
 
   public ArgumentList(List<Expr> arguments, Span span) {
     super(span, arguments);

pkl-parser/src/main/java/org/pkl/parser/syntax/ClassBody.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -21,7 +21,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ClassBody extends AbstractNode {
+public final class ClassBody extends AbstractNode {
 
   public ClassBody(List<Node> nodes, Span span) {
     super(span, nodes);

pkl-parser/src/main/java/org/pkl/parser/syntax/ClassMethod.java

@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ClassMethod extends AbstractNode {
+public final class ClassMethod extends AbstractNode {
   private final int modifiersOffset;
   private final int nameOffset;
   private final Span headerSpan;

pkl-parser/src/main/java/org/pkl/parser/syntax/ExtendsOrAmendsClause.java

@@ -21,7 +21,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ExtendsOrAmendsClause extends AbstractNode {
+public final class ExtendsOrAmendsClause extends AbstractNode {
   private final Type type;
 
   public ExtendsOrAmendsClause(StringConstant url, Type type, Span span) {

pkl-parser/src/main/java/org/pkl/parser/syntax/Keyword.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -18,7 +18,7 @@ package org.pkl.parser.syntax;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 
-public class Keyword extends AbstractNode {
+public final class Keyword extends AbstractNode {
 
   public Keyword(Span span) {
     super(span, null);

pkl-parser/src/main/java/org/pkl/parser/syntax/ParameterList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ParameterList extends AbstractNode {
+public final class ParameterList extends AbstractNode {
   public ParameterList(List<Parameter> parameters, Span span) {
     super(span, parameters);
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/ReplInput.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class ReplInput extends AbstractNode {
+public final class ReplInput extends AbstractNode {
   public ReplInput(List<Node> nodes, Span span) {
     super(span, nodes);
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/StringConstant.java

@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class StringConstant extends AbstractNode {
+public final class StringConstant extends AbstractNode {
   private final String string;
 
   public StringConstant(String string, Span span) {

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeAnnotation.java

@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class TypeAnnotation extends AbstractNode {
+public final class TypeAnnotation extends AbstractNode {
   public TypeAnnotation(Type type, Span span) {
     super(span, List.of(type));
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeArgumentList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -19,7 +19,7 @@ import java.util.List;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 
-public class TypeArgumentList extends AbstractNode {
+public final class TypeArgumentList extends AbstractNode {
 
   public TypeArgumentList(List<Type> children, Span span) {
     super(span, children);

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeParameterList.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -20,7 +20,7 @@ import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 import org.pkl.parser.util.Nullable;
 
-public class TypeParameterList extends AbstractNode {
+public final class TypeParameterList extends AbstractNode {
   public TypeParameterList(List<TypeParameter> parameters, Span span) {
     super(span, parameters);
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/generic/Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -20,7 +20,7 @@ import java.util.List;
 import java.util.Objects;
 import org.pkl.parser.util.Nullable;
 
-public class Node {
+public final class Node {
   public final List<Node> children;
   public final FullSpan span;
   public final NodeType type;