clang-format: [JS] support fields with case/switch/default labels.

[mprobst]

Summary

case: and default: would normally parse as labels for a switch block.
However in TypeScript, they can be used in field declarations, e.g.:

interface I {
  case: string;
}

This change special cases parsing them in declaration lines to avoid wrapping
them.

[djasper]

Looks good.

lib/Format/UnwrappedLineParser.cpp

do {
     case tok::r_brace:
       if (HasOpeningBrace)
         return;
       nextToken();
       addUnwrappedLine();
       break;
     case tok::kw_default:
     case tok::kw_case:
+      if (Style.Language == FormatStyle::LK_JavaScript && Line->MustBeDeclaration) {
+        // A 'case: string' style field declaration.
+        parseStructuralElement();
+        break;
+      }
       if (!SwitchLabelEncountered &&
           (Style.IndentCaseLabels || (Line->InPPDirective && Line->Level == 1)))
         ++Line->Level;
       SwitchLabelEncountered = true;
       parseStructuralElement();
       break;
     default:
       parseStructuralElement();
void UnwrappedLineParser::parseStructuralElement() {
   case tok::kw_for:
   case tok::kw_while:
     parseForOrWhileLoop();
     return;
   case tok::kw_do:
     parseDoWhile();
     return;
   case tok::kw_switch:
+    if (Style.Language == FormatStyle::LK_JavaScript && Line->MustBeDeclaration)
+      // 'switch: string' field declaration.
+      break;
     parseSwitch();
     return;
   case tok::kw_default:
+    if (Style.Language == FormatStyle::LK_JavaScript && Line->MustBeDeclaration)
+      // 'default: string' field declaration.
+      break;
     nextToken();
     parseLabel();
     return;
   case tok::kw_case:
+    if (Style.Language == FormatStyle::LK_JavaScript && Line->MustBeDeclaration)
+      // 'case: string' field declaration.
+      break;
     parseCaseLabel();
     return;
   case tok::kw_try:
   case tok::kw___try:
     parseTryCatch();
     return;
   case tok::kw_extern:
     nextToken();

unittests/Format/FormatTestJS.cpp

verifyFormat("x = {\n"
                "  interface: 1,\n"
                "  switch: 1,\n"
                "};");
   verifyFormat("var struct = 2;");
   verifyFormat("var union = 2;");
   verifyFormat("var interface = 2;");
   verifyFormat("interface = 2;");
   verifyFormat("x = interface instanceof y;");
+  verifyFormat("interface Test {\n"
+               "  x: string;\n"
+               "  switch: string;\n"
+               "  case: string;\n"
+               "  default: string;\n"
+               "}\n");
 }
 
 TEST_F(FormatTestJS, ReservedWordsMethods) {
   verifyFormat(
       "class X {\n"
       "  delete() {\n"
       "    x();\n"
       "  }\n"