Ensure code writer always has a StringBuilder

smithy-utils/src/main/java/software/amazon/smithy/utils/AbstractCodeWriter.java

@@ -1766,7 +1766,6 @@ private final class State {
         private CopyOnWriteRef<CodeWriterFormatterContainer> formatters;
         private CopyOnWriteRef<CodeInterceptorContainer<T>> interceptors;
 
-        /** This StringBuilder, if null, will only be created lazily when needed. */
         private StringBuilder builder;
 
         /**
@@ -1777,6 +1776,7 @@ private final class State {
         private boolean isInline;
 
         State() {
+            builder = new StringBuilder();
             isRoot = true;
             CodeWriterFormatterContainer formatterContainer = new CodeWriterFormatterContainer();
             DEFAULT_FORMATTERS.forEach(formatterContainer::putFormatter);
@@ -1810,13 +1810,10 @@ private void copyStateFrom(State copy) {
 
         @Override
         public String toString() {
-            return builder == null ? "" : builder.toString();
+            return getBuilder().toString();
         }
 
         StringBuilder getBuilder() {
-            if (builder == null) {
-                builder = new StringBuilder();
-            }
             return builder;
         }
 
@@ -1921,7 +1918,7 @@ private void makeInterceptableCodeSection(CodeSection section) {
             // level is reset back to the root, and the newline prefix is removed.
             // Indentation and prefixes are added automatically if/when the
             // captured text is written into the parent state.
-            currentState.builder = null;
+            currentState.builder = new StringBuilder();
             currentState.newlinePrefix = "";
             dedent(-1);
         }

smithy-utils/src/test/java/software/amazon/smithy/utils/CodeWriterTest.java

@@ -1030,4 +1030,22 @@ public void ensuresNewlineIsPresent() {
 
         assertThat(writer.toString(), equalTo("Foo\nBar\nBaz\nBam\n"));
     }
+
+    // Dynamically creating builders for a section doesn't work when pushing
+    // sections into captured sections that haven't written anything yet.
+    // It ends up swallowing the text written to sub-sections because the
+    // top-level section's builder wasn't written yet. This test makes sure
+    // that doesn't happen.
+    @Test
+    public void alwaysWritesToParentBuilders() {
+        SimpleCodeWriter writer = new SimpleCodeWriter();
+
+        writer.pushState("Hi");
+        writer.pushState();
+        writer.write("Hello");
+        writer.popState();
+        writer.popState();
+
+        assertThat(writer.toString(), equalTo("Hello\n"));
+    }
 }