Improve lambda formatting

#19

The style guide says:
"A line is never broken adjacent to the arrow in a lambda, except that
a break may come immediately after the arrow if the body of the lambda
consists of a single unbraced expression."

There are two changes here:
1. Don't put a newline after the arrow.
2. When the only argument to a function is a lambda, don't put
a newline after the open-paren of the function.  I think
this newline was going in because a lambda is a single expression
that is longer than (the remainder of) a line.  But generally, it's
prettier to break inside the lambda.

Author: novalis

Diff for: core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

@@ -1163,7 +1163,6 @@ public Void visitLabeledStatement(LabeledStatementTree node, Void unused) {
   @Override
   public Void visitLambdaExpression(LambdaExpressionTree node, Void unused) {
     sync(node);
-    boolean statementBody = node.getBodyKind() == LambdaExpressionTree.BodyKind.STATEMENT;
     boolean parens = builder.peekToken().equals(Optional.of("("));
     builder.open(parens ? plusFour : ZERO);
     if (parens) {
@@ -1184,12 +1183,7 @@ public Void visitLambdaExpression(LambdaExpressionTree node, Void unused) {
     builder.close();
     builder.space();
     builder.op("->");
-    builder.open(statementBody ? ZERO : plusFour);
-    if (statementBody) {
-      builder.space();
-    } else {
-      builder.breakOp(" ");
-    }
+    builder.space();
     if (node.getBody().getKind() == Tree.Kind.BLOCK) {
       visitBlock(
           (BlockTree) node.getBody(),
@@ -1199,7 +1193,6 @@ public Void visitLambdaExpression(LambdaExpressionTree node, Void unused) {
     } else {
       scan(node.getBody(), null);
     }
-    builder.close();
     return null;
   }
 
@@ -2759,7 +2752,10 @@ void addTypeArguments(List<? extends Tree> typeArguments, Indent plusIndent) {
    * @param plusIndent the extra indent for the arguments
    */
   void addArguments(List<? extends ExpressionTree> arguments, Indent plusIndent) {
-    builder.open(plusIndent);
+    boolean singleLambdaArg = arguments.size() == 1 && arguments.get(0) instanceof JCTree.JCLambda;
+    if (!singleLambdaArg) {
+      builder.open(plusIndent);
+    }
     token("(");
     if (!arguments.isEmpty()) {
       if (arguments.size() % 2 == 0 && argumentsAreTabular(arguments) == 2) {
@@ -2793,12 +2789,16 @@ void addArguments(List<? extends ExpressionTree> arguments, Indent plusIndent) {
         builder.close();
         builder.close();
       } else {
-        builder.breakOp();
+        if (!singleLambdaArg) {
+          builder.breakOp();
+        }
         argList(arguments);
       }
     }
     token(")");
-    builder.close();
+    if (!singleLambdaArg) {
+      builder.close();
+    }
   }
 
   private void argList(List<? extends ExpressionTree> arguments) {

Diff for: core/src/test/resources/com/google/googlejavaformat/java/testdata/B20128760.output

@@ -89,19 +89,12 @@ class B20128760 {
   {
     Stream<ItemKey> itemIdsStream =
         stream(members)
-            .flatMap(
-                m ->
-                    m.getFieldValues()
-                        .entrySet()
-                        .stream()
-                        .filter(fv -> itemLinkFieldIds.contains(fv.getKey()))
-                        .flatMap(
-                            fv ->
-                                FieldDTO.deserializeStringToListOfStrings(fv.getValue())
-                                    .stream()
-                                    .map(
-                                        id ->
-                                            new ItemKey(
-                                                fieldsById.get(fv.getKey()).getItemTypeId(), id))));
+            .flatMap(m -> m.getFieldValues()
+                .entrySet()
+                .stream()
+                .filter(fv -> itemLinkFieldIds.contains(fv.getKey()))
+                .flatMap(fv -> FieldDTO.deserializeStringToListOfStrings(fv.getValue())
+                    .stream()
+                    .map(id -> new ItemKey(fieldsById.get(fv.getKey()).getItemTypeId(), id))));
   }
 }

Diff for: core/src/test/resources/com/google/googlejavaformat/java/testdata/B21305044.output

@@ -37,22 +37,20 @@ class B21305044 {
   {
     Function f = () -> moderatelyLongResult;
     Function f =
-        () ->
-            breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult
-                + breakableResult;
+        () -> breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult
+            + breakableResult;
     Function f =
-        () ->
-            System.err.println(
-                "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
+        () -> System.err.println(
+            "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
     Function f =
-        (someParam) ->
-            System.err.println("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
+        (someParam) -> System.err.println(
+            "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
   }
 }

Diff for: core/src/test/resources/com/google/googlejavaformat/java/testdata/B22873322.output

@@ -1,21 +1,15 @@
 class B22873322 {
   {
-    f(
-        param ->
-            veryLooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooongExpr(
-                param));
-    f(
-        (param1, param2) ->
-            veryLooooooooooooooooooooooooooooooooooooooooooooooooongExpr(param1, param2));
-    f(
-        (int param) ->
-            veryLooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooongExpr(param));
-    f(
-        (param1, param2) -> {
+    f(param -> veryLooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooongExpr(
+            param));
+    f((param1, param2) -> veryLooooooooooooooooooooooooooooooooooooooooooooooooongExpr(
+            param1, param2));
+    f((int param) -> veryLooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooongExpr(
+            param));
+    f((param1, param2) -> {
           return expr(param1, param2);
         });
-    f(
-        (param1, param2) -> {
+    f((param1, param2) -> {
           Object foo = expr(param1, param2);
           return foo;
         });

Diff for: core/src/test/resources/com/google/googlejavaformat/java/testdata/B33358723.output

@@ -2,23 +2,20 @@ class B33358723 {
   {
     f(
         //
-        x ->
-            System.err.println(
-                //
-                "hello"));
+        x -> System.err.println(
+            //
+            "hello"));
     f(
         //
         ( //
-            x) ->
-            System.err.println(
-                //
-                "hello"));
+            x) -> System.err.println(
+            //
+            "hello"));
     f(
         //
         (int //
-                x) ->
-            System.err.println(
-                //
-                "hello"));
+                x) -> System.err.println(
+            //
+            "hello"));
   }
 }