Preserve existing custom areas

11 files changed, 288 insertions, 120 deletions

diff --git a/examples/05-custom-code/hobgoblin.json5 b/examples/05-custom-code/hobgoblin.json5
new file mode 100644
index 0000000..19cd31b
--- /dev/null
+++ b/examples/05-custom-code/hobgoblin.json5
@@ -0,0 +1,9 @@
+{
+  outputs: [
+    {
+      generator: "java-types",
+      path: "output/java/src/main/java",
+      packageName: "org.zwobble.example.types"
+    },
+  ],
+}
diff --git a/examples/05-custom-code/output/java/.gitignore b/examples/05-custom-code/output/java/.gitignore
new file mode 100644
index 0000000..ff511d4
--- /dev/null
+++ b/examples/05-custom-code/output/java/.gitignore
@@ -0,0 +1,2 @@
+/src/gen/
+/target/
diff --git a/examples/05-custom-code/output/java/pom.xml b/examples/05-custom-code/output/java/pom.xml
new file mode 100644
index 0000000..7493667
--- /dev/null
+++ b/examples/05-custom-code/output/java/pom.xml
@@ -0,0 +1,39 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.zwobble.hobgoblin.examples</groupId>
+  <artifactId>example</artifactId>
+  <version>1.0-SNAPSHOT</version>
+
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <maven.compiler.source>25</maven.compiler.source>
+    <maven.compiler.target>25</maven.compiler.target>
+  </properties>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>build-helper-maven-plugin</artifactId>
+        <version>3.6.1</version>
+        <executions>
+          <execution>
+            <id>add-source</id>
+            <phase>generate-sources</phase>
+            <goals>
+              <goal>add-source</goal>
+            </goals>
+            <configuration>
+              <sources>
+                <source>src/gen/java</source>
+              </sources>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>
diff --git a/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/Main.java b/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/Main.java
new file mode 100644
index 0000000..2d2366d
--- /dev/null
+++ b/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/Main.java
@@ -0,0 +1,10 @@
+package org.zwobble.example;
+
+import org.zwobble.example.types.shapes.Square;
+
+public class Main {
+    public static void main() {
+        Square square = new Square(10);
+        assert square.area() == 100;
+    }
+}
diff --git a/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/types/shapes/Square.java b/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/types/shapes/Square.java
new file mode 100644
index 0000000..dd48603
--- /dev/null
+++ b/examples/05-custom-code/output/java/src/main/java/org/zwobble/example/types/shapes/Square.java
@@ -0,0 +1,28 @@
+package org.zwobble.example.types.shapes;
+
+public record Square(int width) {
+    public static org.zwobble.example.types.shapes.Square.Builder arbitrary() {
+        return new org.zwobble.example.types.shapes.Square.Builder(0);
+    }
+
+    public record Builder(int width) {
+        public org.zwobble.example.types.shapes.Square build() {
+            return new org.zwobble.example.types.shapes.Square(width);
+        }
+
+        public org.zwobble.example.types.shapes.Square.Builder withWidth(int width) {
+            return new org.zwobble.example.types.shapes.Square.Builder(width);
+        }
+
+        // Custom area start: org.zwobble.example.types.shapes.Square.Builder body
+        // Custom area end: org.zwobble.example.types.shapes.Square.Builder body
+    }
+
+    // Custom area start: org.zwobble.example.types.shapes.Square body
+
+    public int area() {
+        return this.width * this.width;
+    }
+
+    // Custom area end: org.zwobble.example.types.shapes.Square body
+}
diff --git a/examples/05-custom-code/src/shapes.hob b/examples/05-custom-code/src/shapes.hob
new file mode 100644
index 0000000..5dfe833
--- /dev/null
+++ b/examples/05-custom-code/src/shapes.hob
@@ -0,0 +1,3 @@
+struct Square {
+    field width: Int32;
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/CodeWriter.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/CodeWriter.java
index 3e08c85..b8b0709 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/CodeWriter.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/CodeWriter.java
@@ -4,21 +4,92 @@ import java.io.FileWriter;
 import java.io.IOException;
 import java.io.Writer;
 import java.nio.file.Files;
+import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
+import java.util.*;
 
 public class CodeWriter implements AutoCloseable {
-    public static CodeWriter file(Path path) throws IOException {
+    public record CustomAreas(
+        Map<String, List<String>> existing,
+        String customAreaStart,
+        String customAreaEnd
+    ) {
+        public static CustomAreas read(
+            String existingString,
+            String customAreaStart,
+            String customAreaEnd
+        ) {
+            var existing = new HashMap<String, List<String>>();
+
+            Iterable<String> lines = () -> existingString.lines().iterator();
+            Optional<String> name = Optional.empty();
+            var customAreaLines = new ArrayList<String>();
+            for (var line : lines) {
+                var trimmedLine = line.trim();
+                if (trimmedLine.startsWith(customAreaStart)) {
+                    name = Optional.of(trimmedLine.substring(customAreaStart.length()));
+                } else if (name.isPresent() && trimmedLine.startsWith(customAreaEnd + name.get())) {
+                    existing.put(name.get(), customAreaLines);
+                    customAreaLines = new ArrayList<>();
+                    name = Optional.empty();
+                } else if (name.isPresent()) {
+                    customAreaLines.add(line);
+                }
+            }
+
+            return new CustomAreas(existing, customAreaStart, customAreaEnd);
+        }
+
+        public static CustomAreas empty(
+            String customAreaStart,
+            String customAreaEnd
+        ) {
+            return new CustomAreas(Map.of(), customAreaStart, customAreaEnd);
+        }
+
+        public Iterable<String> lines(String name) {
+            var lines = this.existing.get(name);
+            if (lines == null) {
+                return List.of();
+            } else {
+                return lines;
+            }
+        }
+    }
+
+    public static CodeWriter file(
+        Path path,
+        String customAreaStart,
+        String customAreaEnd
+    ) throws IOException {
+        var customAreas = readCustomAreas(path, customAreaStart, customAreaEnd);
         Files.createDirectories(path.getParent());
         var writer = new FileWriter(path.toFile());
-        return new CodeWriter(writer);
+        // TODO: warn if custom areas are unused?
+        return new CodeWriter(writer, customAreas);
+    }
+
+    private static CustomAreas readCustomAreas(
+        Path path,
+        String customAreaStart,
+        String customAreaEnd
+    ) throws IOException {
+        try {
+            var contents = Files.readString(path);
+            return CustomAreas.read(contents, customAreaStart, customAreaEnd);
+        } catch (NoSuchFileException exception) {
+            return CustomAreas.empty(customAreaStart, customAreaEnd);
+        }
     }
 
     private final Writer writer;
+    private final CustomAreas customAreas;
     private int indent;
     private boolean pendingIndent;
 
-    public CodeWriter(Writer writer) {
+    public CodeWriter(Writer writer, CustomAreas customAreas) {
         this.writer = writer;
+        this.customAreas = customAreas;
         this.indent = 0;
         this.pendingIndent = false;
     }
@@ -47,6 +118,21 @@ public class CodeWriter implements AutoCloseable {
         this.indent -= 1;
     }
 
+    public void writeCustomArea(String name) throws IOException {
+        this.write(this.customAreas.customAreaStart);
+        this.write(name);
+
+        for (var line : this.customAreas.lines(name)) {
+            this.writer.write("\n");
+            this.write(line);
+        }
+
+        this.newLine();
+        this.write(this.customAreas.customAreaEnd);
+        this.write(name);
+        this.newLine();
+    }
+
     @Override
     public void close() throws Exception {
         this.writer.close();
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
index 77c2ecd..9356519 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
@@ -4,7 +4,6 @@ import org.zwobble.hobgoblin.compiler.ast.DocComment;
 import org.zwobble.hobgoblin.compiler.ast.typed.*;
 import org.zwobble.hobgoblin.compiler.builtins.NativeTypes;
 import org.zwobble.hobgoblin.compiler.config.OutputConfig;
-import org.zwobble.hobgoblin.compiler.output.CodeWriter;
 import org.zwobble.hobgoblin.compiler.output.generators.Generator;
 import org.zwobble.hobgoblin.compiler.output.lang.java.JavaWriter;
 import org.zwobble.hobgoblin.compiler.output.lang.java.ast.*;
@@ -78,7 +77,7 @@ public class JavaTypesGenerator implements Generator {
                 var javaCompilationUnitPath = this.sourceRootDirectory
                     .resolve(String.join(File.separator, javaCompilationUnit.packageName()))
                     .resolve(javaCompilationUnit.typeDeclaration().name() + ".java");
-                try (var writer = new JavaWriter(CodeWriter.file(javaCompilationUnitPath))) {
+                try (var writer = JavaWriter.file(javaCompilationUnitPath)) {
                     writer.writeCompilationUnit(javaCompilationUnit);
                 } catch (Exception e) {
                     // TODO: better handling
@@ -115,7 +114,7 @@ public class JavaTypesGenerator implements Generator {
                             sumDefinition.variants().stream()
                                 .map(variant -> generateTypeRef(variant.type(), context))
                                 .toList(),
-                            JavaCustomArea.EMPTY,
+                            JavaCustomArea.type(generateTypeRef(sumDefinition.type(), context), "body"),
                             sumDefinition.docComment()
                         )
                     );
@@ -168,7 +167,7 @@ public class JavaTypesGenerator implements Generator {
                 ),
                 generateStructDefinitionBuilder(structDefinition, components, context)
             ),
-            JavaCustomArea.EMPTY,
+            JavaCustomArea.type(generateTypeRef(structDefinition.type(), context), "body"),
             structDefinition.docComment()
         );
     }
@@ -216,7 +215,7 @@ public class JavaTypesGenerator implements Generator {
             components,
             List.of(),
             body,
-            JavaCustomArea.EMPTY,
+            JavaCustomArea.type(builderJavaTypeRef, "body"),
             DocComment.EMPTY
         );
     }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
index 9f87e0f..82c2f9b 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
@@ -5,8 +5,18 @@ import org.zwobble.hobgoblin.compiler.output.CodeWriter;
 import org.zwobble.hobgoblin.compiler.output.lang.java.ast.*;
 
 import java.io.IOException;
+import java.io.StringWriter;
+import java.nio.file.Path;
 
 public class JavaWriter implements AutoCloseable {
+    public static JavaWriter file(Path path) throws IOException {
+        return new JavaWriter(CodeWriter.file(
+            path,
+            "// Custom area start: ",
+            "// Custom area end: "
+        ));
+    }
+
     private final CodeWriter writer;
 
     public JavaWriter(CodeWriter writer) {
@@ -84,7 +94,7 @@ public class JavaWriter implements AutoCloseable {
         }
 
         this.writer.newLine();
-        this.writeCustomArea("body", classDeclaration.customArea());
+        this.writeCustomArea(classDeclaration.customArea());
 
         this.writer.dedent();
         this.writer.write("}");
@@ -102,19 +112,12 @@ public class JavaWriter implements AutoCloseable {
         }
     }
 
-    private void writeCustomArea(String name, JavaCustomArea javaCustomArea) throws IOException {
-        this.writer.write("// Custom area start: ");
-        this.writer.write(name);
-        this.writer.newLine();
-
-        for (var line : javaCustomArea.lines()) {
-            this.writer.write(line);
-            this.writer.newLine();
-        }
-
-        this.writer.write("// Custom area end: ");
-        this.writer.write(name);
-        this.writer.newLine();
+    private void writeCustomArea(JavaCustomArea customArea) throws IOException {
+        var customAreaName = new StringWriter();
+        writeTypeRef(customArea.type(), customAreaName::write);
+        customAreaName.write(" ");
+        customAreaName.write(customArea.name());
+        this.writer.writeCustomArea(customAreaName.toString());
     }
 
     void writeExpression(JavaExpression expression) throws IOException {
@@ -165,7 +168,7 @@ public class JavaWriter implements AutoCloseable {
         this.writer.write("{");
         this.writer.indent();
         this.writer.newLine();
-        this.writeCustomArea("body", interfaceDeclaration.customArea());
+        this.writeCustomArea(interfaceDeclaration.customArea());
         this.writer.dedent();
         this.writer.write("}");
     }
@@ -271,7 +274,7 @@ public class JavaWriter implements AutoCloseable {
         }
 
         this.writer.newLine();
-        this.writeCustomArea("body", recordDeclaration.customArea());
+        this.writeCustomArea(recordDeclaration.customArea());
 
         this.writer.dedent();
         this.writer.write("}");
@@ -301,6 +304,10 @@ public class JavaWriter implements AutoCloseable {
     }
 
     void writeTypeRef(JavaTypeRef type) throws IOException {
+        writeTypeRef(type, writer::write);
+    }
+
+    private static void writeTypeRef(JavaTypeRef type, WriteString writer) throws IOException {
         for (var part : type.packageName()) {
             writer.write(part);
             writer.write(".");
@@ -316,14 +323,14 @@ public class JavaWriter implements AutoCloseable {
             writer.write("<");
             writeWithSeparator(
                 type.args(),
-                this::writeTypeRef,
+                arg -> writeTypeRef(arg, writer),
                 () -> writer.write(", ")
             );
             writer.write(">");
         }
     }
 
-    private <T> void writeWithSeparator(
+    private static <T> void writeWithSeparator(
         Iterable<T> iterable,
         WriteElement<T> writeElement,
         WriteSeparator writeSeparator
@@ -349,4 +356,8 @@ public class JavaWriter implements AutoCloseable {
     interface WriteSeparator {
         void write() throws IOException;
     }
+
+    interface WriteString {
+        void write(String string) throws IOException;
+    }
 }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaCustomArea.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaCustomArea.java
index 42d38d1..51e3096 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaCustomArea.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaCustomArea.java
@@ -1,7 +1,7 @@
 package org.zwobble.hobgoblin.compiler.output.lang.java.ast;
 
-import java.util.List;
-
-public record JavaCustomArea(List<String> lines) {
-    public static final JavaCustomArea EMPTY = new JavaCustomArea(List.of());
+public record JavaCustomArea(JavaTypeRef type, String name) {
+    public static JavaCustomArea type(JavaTypeRef type, String name) {
+        return new JavaCustomArea(type, name);
+    }
 }
diff --git a/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java b/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java
index e13c3dd..beef2ca 100644
--- a/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java
+++ b/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java
@@ -8,6 +8,7 @@ import org.zwobble.hobgoblin.compiler.output.lang.java.ast.*;
 import java.io.IOException;
 import java.io.StringWriter;
 import java.util.List;
+import java.util.Optional;
 
 import static org.zwobble.precisely.AssertThat.assertThat;
 import static org.zwobble.precisely.Matchers.equalTo;
@@ -18,7 +19,7 @@ public class JavaWriterTests {
         var java = new JavaClassDeclaration(
             "Rectangle",
             List.of(),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Rectangle"), "body"),
             DocComment.EMPTY
         );
 
@@ -26,8 +27,8 @@ public class JavaWriterTests {
 
         assertThat(string, equalTo("""
             public class Rectangle {
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Rectangle body
+                // Custom area end: Rectangle body
             }"""
         ));
     }
@@ -52,7 +53,7 @@ public class JavaWriterTests {
                     JavaBlock.EMPTY
                 )
             ),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Rectangle"), "body"),
             DocComment.EMPTY
         );
 
@@ -66,28 +67,8 @@ public class JavaWriterTests {
                 public void b() {
                 }
 
-                // Custom area start: body
-                // Custom area end: body
-            }"""
-        ));
-    }
-
-    @Test
-    public void classDeclarationWithCustomArea() throws IOException {
-        var java = new JavaClassDeclaration(
-            "Point",
-            List.of(),
-            new JavaCustomArea(List.of("// <custom area>")),
-            DocComment.EMPTY
-        );
-
-        var string = write(writer -> writer.writeTypeDeclaration(java));
-
-        assertThat(string, equalTo("""
-            public class Point {
-                // Custom area start: body
-                // <custom area>
-                // Custom area end: body
+                // Custom area start: Rectangle body
+                // Custom area end: Rectangle body
             }"""
         ));
     }
@@ -97,7 +78,7 @@ public class JavaWriterTests {
         var java = new JavaClassDeclaration(
             "Point",
             List.of(),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Point"), "body"),
             new DocComment("A 2D point.")
         );
 
@@ -106,8 +87,8 @@ public class JavaWriterTests {
         assertThat(string, equalTo("""
             /// A 2D point.
             public class Point {
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Point body
+                // Custom area end: Point body
             }"""
         ));
     }
@@ -117,7 +98,7 @@ public class JavaWriterTests {
         var java = new JavaInterfaceDeclaration(
             "Shape",
             List.of(),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Shape"), "body"),
             DocComment.EMPTY
         );
 
@@ -125,8 +106,8 @@ public class JavaWriterTests {
 
         assertThat(string, equalTo("""
             public sealed interface Shape {
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Shape body
+                // Custom area end: Shape body
             }"""
         ));
     }
@@ -139,7 +120,7 @@ public class JavaWriterTests {
                 JavaTypeRef.topLevel(List.of("abc", "def"), "One"),
                 JavaTypeRef.topLevel(List.of("abc", "def"), "Two")
             ),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Shape"), "body"),
             DocComment.EMPTY
         );
 
@@ -147,28 +128,8 @@ public class JavaWriterTests {
 
         assertThat(string, equalTo("""
             public sealed interface Shape permits abc.def.One, abc.def.Two {
-                // Custom area start: body
-                // Custom area end: body
-            }"""
-        ));
-    }
-
-    @Test
-    public void interfaceDeclarationWithCustomArea() throws IOException {
-        var java = new JavaInterfaceDeclaration(
-            "Shape",
-            List.of(),
-            new JavaCustomArea(List.of("// <custom area>")),
-            DocComment.EMPTY
-        );
-
-        var string = write(writer -> writer.writeTypeDeclaration(java));
-
-        assertThat(string, equalTo("""
-            public sealed interface Shape {
-                // Custom area start: body
-                // <custom area>
-                // Custom area end: body
+                // Custom area start: Shape body
+                // Custom area end: Shape body
             }"""
         ));
     }
@@ -178,7 +139,7 @@ public class JavaWriterTests {
         var java = new JavaInterfaceDeclaration(
             "Shape",
             List.of(),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Shape"), "body"),
             new DocComment("A 2D shape.")
         );
 
@@ -187,8 +148,8 @@ public class JavaWriterTests {
         assertThat(string, equalTo("""
             /// A 2D shape.
             public sealed interface Shape {
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Shape body
+                // Custom area end: Shape body
             }"""
         ));
     }
@@ -200,28 +161,7 @@ public class JavaWriterTests {
             List.of(),
             List.of(),
             List.of(),
-            JavaCustomArea.EMPTY,
-            DocComment.EMPTY
-        );
-
-        var string = write(writer -> writer.writeTypeDeclaration(java));
-
-        assertThat(string, equalTo("""
-            public record Point() {
-                // Custom area start: body
-                // Custom area end: body
-            }"""
-        ));
-    }
-
-    @Test
-    public void recordDeclarationWithCustomArea() throws IOException {
-        var java = new JavaRecordDeclaration(
-            "Point",
-            List.of(),
-            List.of(),
-            List.of(),
-            new JavaCustomArea(List.of("// <custom area>")),
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Point"), "body"),
             DocComment.EMPTY
         );
 
@@ -229,9 +169,8 @@ public class JavaWriterTests {
 
         assertThat(string, equalTo("""
             public record Point() {
-                // Custom area start: body
-                // <custom area>
-                // Custom area end: body
+                // Custom area start: Point body
+                // Custom area end: Point body
             }"""
         ));
     }
@@ -243,7 +182,7 @@ public class JavaWriterTests {
             List.of(),
             List.of(),
             List.of(),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Point"), "body"),
             new DocComment("A 2D point.")
         );
 
@@ -252,8 +191,8 @@ public class JavaWriterTests {
         assertThat(string, equalTo("""
             /// A 2D point.
             public record Point() {
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Point body
+                // Custom area end: Point body
             }"""
         ));
     }
@@ -280,7 +219,7 @@ public class JavaWriterTests {
                     JavaBlock.EMPTY
                 )
             ),
-            JavaCustomArea.EMPTY,
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Rectangle"), "body"),
             DocComment.EMPTY
         );
 
@@ -294,8 +233,8 @@ public class JavaWriterTests {
                 public void b() {
                 }
 
-                // Custom area start: body
-                // Custom area end: body
+                // Custom area start: Rectangle body
+                // Custom area end: Rectangle body
             }"""
         ));
     }
@@ -463,9 +402,51 @@ public class JavaWriterTests {
         assertThat(string, equalTo("abc.def.One.Two"));
     }
 
+    @Test
+    public void customAreaContentsArePreserved() throws IOException {
+        var java = new JavaRecordDeclaration(
+            "Point",
+            List.of(),
+            List.of(),
+            List.of(),
+            new JavaCustomArea(JavaTypeRef.topLevel(List.of(), "Point"), "body"),
+            DocComment.EMPTY
+        );
+
+        var string = write(
+            writer -> writer.writeTypeDeclaration(java),
+            """
+                public class Point {
+                // Custom area start: Point body
+                // <custom area>
+                // Custom area end: Point body
+            }
+            """
+        );
+
+        assertThat(string, equalTo("""
+            public record Point() {
+                // Custom area start: Point body
+                // <custom area>
+                // Custom area end: Point body
+            }"""
+        ));
+    }
+
     private String write(Write write) throws IOException {
+        return write(write, Optional.empty());
+    }
+
+    private String write(Write write, String originalContents) throws IOException {
+        return write(write, Optional.of(originalContents));
+    }
+
+    private String write(Write write, Optional<String> originalContents) throws IOException {
         var stringWriter = new StringWriter();
-        var javaWriter = new JavaWriter(new CodeWriter(stringWriter));
+        var javaWriter = new JavaWriter(new CodeWriter(
+            stringWriter,
+            CodeWriter.CustomAreas.read(originalContents.orElse(""), "// Custom area start: ", "// Custom area end: ")
+        ));
         write.write(javaWriter);
         return stringWriter.toString();
     }