clang-format: [JS] support new assignment operators.

Before:

    a && = b;

After:

    a &&= b;

These operators are new additions in ES2021.

Differential Revision: https://reviews.llvm.org/D91132

Author: mprobst

clang/lib/Format/FormatToken.h

@@ -68,6 +68,9 @@ namespace format {
   TYPE(JsTypeColon)                                                            \
   TYPE(JsTypeOperator)                                                         \
   TYPE(JsTypeOptionalQuestion)                                                 \
+  TYPE(JsAndAndEqual)                                                          \
+  TYPE(JsPipePipeEqual)                                                        \
+  TYPE(JsNullishCoalescingEqual)                                               \
   TYPE(LambdaArrow)                                                            \
   TYPE(LambdaLBrace)                                                           \
   TYPE(LambdaLSquare)                                                          \

clang/lib/Format/FormatTokenLexer.cpp

@@ -121,6 +121,10 @@ void FormatTokenLexer::tryMergePreviousTokens() {
                                                                tok::period};
     static const tok::TokenKind JSNullishOperator[] = {tok::question,
                                                        tok::question};
+    static const tok::TokenKind JSNullishEqual[] = {tok::question,
+                                                    tok::question, tok::equal};
+    static const tok::TokenKind JSPipePipeEqual[] = {tok::pipepipe, tok::equal};
+    static const tok::TokenKind JSAndAndEqual[] = {tok::ampamp, tok::equal};
 
     // FIXME: Investigate what token type gives the correct operator priority.
     if (tryMergeTokens(JSIdentity, TT_BinaryOperator))
@@ -148,6 +152,13 @@ void FormatTokenLexer::tryMergePreviousTokens() {
       Tokens.back()->Tok.setKind(tok::period);
       return;
     }
+    if (tryMergeTokens(JSAndAndEqual, TT_JsAndAndEqual) ||
+        tryMergeTokens(JSPipePipeEqual, TT_JsPipePipeEqual) ||
+        tryMergeTokens(JSNullishEqual, TT_JsNullishCoalescingEqual)) {
+      // Treat like the "=" assignment operator.
+      Tokens.back()->Tok.setKind(tok::equal);
+      return;
+    }
     if (tryMergeJSPrivateIdentifier())
       return;
   }

clang/unittests/Format/FormatTestJS.cpp

@@ -2424,6 +2424,15 @@ TEST_F(FormatTestJS, NullishCoalescingOperator) {
       getGoogleJSStyleWithColumns(40));
 }
 
+TEST_F(FormatTestJS, AssignmentOperators) {
+  verifyFormat("a &&= b;\n");
+  verifyFormat("a ||= b;\n");
+  // NB: need to split ? ?= to avoid it being interpreted by C++ as a trigraph
+  // for #.
+  verifyFormat("a ?"
+               "?= b;\n");
+}
+
 TEST_F(FormatTestJS, Conditional) {
   verifyFormat("y = x ? 1 : 2;");
   verifyFormat("x ? 1 : 2;");