Avoid mangling {@snippet ...}.

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocFormatter.java

@@ -66,6 +66,12 @@ private static String render(List<Token> input, int blockIndent) {
         case FOOTER_JAVADOC_TAG_START:
           output.writeFooterJavadocTagStart(token);
           break;
+        case SNIPPET_BEGIN:
+          output.writeSnippetBegin(token);
+          break;
+        case SNIPPET_END:
+          output.writeSnippetEnd(token);
+          break;
         case LIST_OPEN_TAG:
           output.writeListOpen(token);
           break;

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocLexer.java

@@ -42,6 +42,8 @@
 import static com.google.googlejavaformat.java.javadoc.Token.Type.PARAGRAPH_OPEN_TAG;
 import static com.google.googlejavaformat.java.javadoc.Token.Type.PRE_CLOSE_TAG;
 import static com.google.googlejavaformat.java.javadoc.Token.Type.PRE_OPEN_TAG;
+import static com.google.googlejavaformat.java.javadoc.Token.Type.SNIPPET_BEGIN;
+import static com.google.googlejavaformat.java.javadoc.Token.Type.SNIPPET_END;
 import static com.google.googlejavaformat.java.javadoc.Token.Type.TABLE_CLOSE_TAG;
 import static com.google.googlejavaformat.java.javadoc.Token.Type.TABLE_OPEN_TAG;
 import static com.google.googlejavaformat.java.javadoc.Token.Type.WHITESPACE;
@@ -97,6 +99,7 @@ private static String stripJavadocBeginAndEnd(String input) {
   private final NestingCounter preDepth = new NestingCounter();
   private final NestingCounter codeDepth = new NestingCounter();
   private final NestingCounter tableDepth = new NestingCounter();
+  private boolean outerInlineTagIsSnippet;
   private boolean somethingSinceNewline;
 
   private JavadocLexer(CharStream input) {
@@ -158,13 +161,26 @@ private Type consumeToken() throws LexException {
     }
     somethingSinceNewline = true;
 
-    if (input.tryConsumeRegex(INLINE_TAG_OPEN_PATTERN)) {
+    if (input.tryConsumeRegex(SNIPPET_TAG_OPEN_PATTERN)) {
+      if (braceDepth.value() == 0) {
+        braceDepth.increment();
+        outerInlineTagIsSnippet = true;
+        return SNIPPET_BEGIN;
+      }
+      braceDepth.increment();
+      return LITERAL;
+    } else if (input.tryConsumeRegex(INLINE_TAG_OPEN_PATTERN)) {
       braceDepth.increment();
       return LITERAL;
     } else if (input.tryConsume("{")) {
       braceDepth.incrementIfPositive();
       return LITERAL;
     } else if (input.tryConsume("}")) {
+      if (outerInlineTagIsSnippet && braceDepth.value() == 1) {
+        braceDepth.decrementIfPositive();
+        outerInlineTagIsSnippet = false;
+        return SNIPPET_END;
+      }
       braceDepth.decrementIfPositive();
       return LITERAL;
     }
@@ -239,7 +255,10 @@ private Type consumeToken() throws LexException {
   }
 
   private boolean preserveExistingFormatting() {
-    return preDepth.isPositive() || tableDepth.isPositive() || codeDepth.isPositive();
+    return preDepth.isPositive()
+        || tableDepth.isPositive()
+        || codeDepth.isPositive()
+        || outerInlineTagIsSnippet;
   }
 
   private void checkMatchingTags() throws LexException {
@@ -400,6 +419,7 @@ private static ImmutableList<Token> optionalizeSpacesAfterLinks(List<Token> inpu
    * <p>Also trim leading and trailing blank lines, and move the trailing `}` to its own line.
    */
   private static ImmutableList<Token> deindentPreCodeBlocks(List<Token> input) {
+    // TODO: b/323389829 - De-indent {@snippet ...} blocks, too.
     ImmutableList.Builder<Token> output = ImmutableList.builder();
     for (PeekingIterator<Token> tokens = peekingIterator(input.iterator()); tokens.hasNext(); ) {
       if (tokens.peek().getType() != PRE_OPEN_TAG) {
@@ -528,6 +548,7 @@ private static boolean hasMultipleNewlines(String s) {
   private static final Pattern BLOCKQUOTE_OPEN_PATTERN = openTagPattern("blockquote");
   private static final Pattern BLOCKQUOTE_CLOSE_PATTERN = closeTagPattern("blockquote");
   private static final Pattern BR_PATTERN = openTagPattern("br");
+  private static final Pattern SNIPPET_TAG_OPEN_PATTERN = compile("^[{]@snippet\\b");
   private static final Pattern INLINE_TAG_OPEN_PATTERN = compile("^[{]@\\w*");
   /*
    * We exclude < so that we don't swallow following HTML tags. This lets us fix up "foo<p>" (~400

core/src/main/java/com/google/googlejavaformat/java/javadoc/JavadocWriter.java

@@ -123,6 +123,38 @@ void writeFooterJavadocTagStart(Token token) {
     continuingFooterTag = true;
   }
 
+  void writeSnippetBegin(Token token) {
+    requestBlankLine();
+    writeToken(token);
+    /*
+     * We don't request a newline here because we should have at least a colon following on this
+     * line, and we may have attributes after that.
+     *
+     * (If we find it convenient, we could instead consume the entire rest of the line as part of
+     * the same token as `{@snippet` itself. But we already would never split the rest of the line
+     * across lines (because we preserve whitespace), so that might not accomplish anything. Plus,
+     * we'd probably want to be careful not to swallow an expectedly early closing `}`.)
+     */
+  }
+
+  void writeSnippetEnd(Token token) {
+    /*
+     * We don't request a newline here because we have preserved all newlines that existed in the
+     * input. TODO: b/323389829 - Improve upon that. Specifically:
+     *
+     * - If there is not yet a newline, we should add one.
+     *
+     * - If there are multiple newlines, we should probably collapse them.
+     *
+     * - If the closing brace isn't indented as we'd want (as in the link below, in which the whole
+     *   @apiNote isn't indented), we should indent it.
+     *
+     * https://github.com/openjdk/jdk/blob/1ebf2cf639300728ffc024784f5dc1704317b0b3/src/java.base/share/classes/java/util/Collections.java#L5993-L6006
+     */
+    writeToken(token);
+    requestBlankLine();
+  }
+
   void writeListOpen(Token token) {
     requestBlankLine();
 

core/src/main/java/com/google/googlejavaformat/java/javadoc/Token.java

@@ -41,6 +41,10 @@ enum Type {
     END_JAVADOC,
     /** The {@code @foo} that begins a block Javadoc tag like {@code @throws}. */
     FOOTER_JAVADOC_TAG_START,
+    /** The opening {@code ｛@snippet} of a code snippet. */
+    SNIPPET_BEGIN,
+    /** The closing {@code ｝} of a code snippet. */
+    SNIPPET_END,
     LIST_OPEN_TAG,
     LIST_CLOSE_TAG,
     LIST_ITEM_OPEN_TAG,

core/src/test/java/com/google/googlejavaformat/java/JavadocFormattingTest.java

@@ -906,6 +906,48 @@ public void unicodeCharacterCountArguableBug() {
     doFormatTest(input, expected);
   }
 
+  @Test
+  public void blankLinesAroundSnippetAndNoMangling() {
+    String[] input = {
+      "/**", //
+      " * hello world",
+      " * {@snippet :",
+      " * public class Foo {",
+      " *   private String s;",
+      " * }",
+      " * }",
+      " * hello again",
+      " */",
+      "class Test {}",
+    };
+    String[] expected = {
+      "/**", //
+      " * hello world",
+      " *",
+      " * {@snippet :",
+      " * public class Foo {",
+      " *   private String s;",
+      " * }",
+      " * }",
+      " *",
+      " * hello again",
+      " */",
+      "class Test {}",
+    };
+    doFormatTest(input, expected);
+  }
+
+  @Test
+  public void notASnippetUnlessOuterTag() {
+    String[] input = {
+      "/** I would like to tell you about the {@code {@snippet ...}} tag. */", "class Test {}",
+    };
+    String[] expected = {
+      "/** I would like to tell you about the {@code {@snippet ...}} tag. */", "class Test {}",
+    };
+    doFormatTest(input, expected);
+  }
+
   @Test
   public void blankLineBeforeParams() {
     String[] input = {