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

core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

@@ -1185,11 +1185,7 @@ public Void visitLambdaExpression(LambdaExpressionTree node, Void unused) {
     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(),
@@ -2793,7 +2789,13 @@ void addArguments(List<? extends ExpressionTree> arguments, Indent plusIndent) {
         builder.close();
         builder.close();
       } else {
-        builder.breakOp();
+        if (arguments.size() == 1) {
+          if (!(arguments.get(0) instanceof JCTree.JCLambda)) {
+            builder.breakOp();
+          }
+        } else {
+          builder.breakOp();
+        }
         argList(arguments);
       }
     }

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

@@ -89,19 +89,13 @@ class B20128760 {
   {
     Stream<ItemKey> itemIdsStream =
         stream(members)
-            .flatMap(
-                m ->
-                    m.getFieldValues()
+            .flatMap(m -> m.getFieldValues()
                         .entrySet()
                         .stream()
                         .filter(fv -> itemLinkFieldIds.contains(fv.getKey()))
-                        .flatMap(
-                            fv ->
-                                FieldDTO.deserializeStringToListOfStrings(fv.getValue())
+                        .flatMap(fv -> FieldDTO.deserializeStringToListOfStrings(fv.getValue())
                                     .stream()
-                                    .map(
-                                        id ->
-                                            new ItemKey(
+                                    .map(id -> new ItemKey(
                                                 fieldsById.get(fv.getKey()).getItemTypeId(), id))));
   }
 }

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

@@ -37,8 +37,7 @@ class B21305044 {
   {
     Function f = () -> moderatelyLongResult;
     Function f =
-        () ->
-            breakableResult
+        () -> breakableResult
                 + breakableResult
                 + breakableResult
                 + breakableResult
@@ -48,11 +47,10 @@ class B21305044 {
                 + breakableResult
                 + breakableResult;
     Function f =
-        () ->
-            System.err.println(
+        () -> System.err.println(
                 "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
     Function f =
-        (someParam) ->
-            System.err.println("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
+        (someParam) -> System.err.println(
+                "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
   }
 }

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

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

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

@@ -2,22 +2,19 @@ class B33358723 {
   {
     f(
         //
-        x ->
-            System.err.println(
+        x -> System.err.println(
                 //
                 "hello"));
     f(
         //
         ( //
-            x) ->
-            System.err.println(
+            x) -> System.err.println(
                 //
                 "hello"));
     f(
         //
         (int //
-                x) ->
-            System.err.println(
+                x) -> System.err.println(
                 //
                 "hello"));
   }