clang-format: [JS] whitespace after async in arrow functions.

[mprobst]

Summary

Async arrow functions should be marked with a whitespace after the async keyword, before the parameter list:

x = async () => foo();

Before:

x = async() => foo();

This makes it easier to tell apart an async arrow function from a call to a function called async.

[bkramer]

lg, but I don't really know JS ;)

[djasper]

lib/Format/TokenAnnotator.cpp

2229

As getNextNonComment() walks linearly over the tokens, I'd pull it out into a variable.

[mprobst]

• Extract local variable for next non comment.

lib/Format/TokenAnnotator.cpp

if (Right.is(tok::period) &&
                      Keywords.kw_repeated, Keywords.kw_extend))
       return true;
     if (Right.is(tok::l_paren) &&
         Left.isOneOf(Keywords.kw_returns, Keywords.kw_option))
       return true;
   } else if (Style.Language == FormatStyle::LK_JavaScript) {
     if (Left.is(TT_JsFatArrow))
       return true;
+    if (Left.is(Keywords.kw_async) && Right.is(tok::l_paren) &&
+        Right.MatchingParen) {
+      const FormatToken *Next = Right.MatchingParen->getNextNonComment();
+      // An async arrow function, for example: `x = async () => foo();`,
+      // as opposed to calling a function called async: `x = async();`
+      if (Next && Next->is(TT_JsFatArrow))
+        return true;
+    }
     if ((Left.is(TT_TemplateString) && Left.TokenText.endswith("${")) ||
         (Right.is(TT_TemplateString) && Right.TokenText.startswith("}")))
       return false;
     if (Left.is(tok::identifier) && Right.is(TT_TemplateString))
       return false;
     if (Right.is(tok::star) &&
         Left.isOneOf(Keywords.kw_function, Keywords.kw_yield))
       return false;

unittests/Format/FormatTestJS.cpp

verifyFormat("async function f() {\n"
                "  return fetch(x);\n"
                "}");
   verifyFormat("async function* f() {\n"
                "  yield fetch(x);\n"
                "}");
   verifyFormat("export async function f() {\n"
                "  return fetch(x);\n"
                "}");
+  verifyFormat("let x = async () => f();");
+  verifyFormat("let x = async();");
   verifyFormat("class X {\n"
                "  async asyncMethod() {\n"
                "    return fetch(1);\n"
                "  }\n"
                "}");
   verifyFormat("function initialize() {\n"
                "  // Comment.\n"
                "  return async.then();\n"