[clangd] Add isKeyword function.

This will be used in rename for doing basic name validation.

Differential Revision: https://reviews.llvm.org/D88810

clang-tools-extra/clangd/SourceCode.cpp

@@ -633,6 +633,12 @@ std::vector<Range> collectIdentifierRanges(llvm::StringRef Identifier,
   return Ranges;
 }
 
+bool isKeyword(llvm::StringRef NewName, const LangOptions &LangOpts) {
+  // Keywords are initialized in constructor.
+  clang::IdentifierTable KeywordsTable(LangOpts);
+  return KeywordsTable.find(NewName) != KeywordsTable.end();
+}
+
 namespace {
 struct NamespaceEvent {
   enum {

clang-tools-extra/clangd/SourceCode.h

@@ -248,6 +248,10 @@ struct SpelledWord {
                                               const LangOptions &LangOpts);
 };
 
+/// Return true if the \p TokenName is in the list of reversed keywords of the
+/// language.
+bool isKeyword(llvm::StringRef TokenName, const LangOptions &LangOpts);
+
 /// Heuristically determine namespaces visible at a point, without parsing Code.
 /// This considers using-directives and enclosing namespace-declarations that
 /// are visible (and not obfuscated) in the file itself (not headers).

clang-tools-extra/clangd/unittests/SourceCodeTests.cpp

@@ -789,6 +789,19 @@ TEST(SourceCodeTests, isHeaderFile) {
   EXPECT_TRUE(isHeaderFile("header.h", LangOpts));
 }
 
+TEST(SourceCodeTests, isKeywords) {
+  LangOptions LangOpts;
+  LangOpts.CPlusPlus20 = true;
+  EXPECT_TRUE(isKeyword("int", LangOpts));
+  EXPECT_TRUE(isKeyword("return", LangOpts));
+  EXPECT_TRUE(isKeyword("co_await", LangOpts));
+
+  // these are identifiers (not keywords!) with special meaning in some
+  // contexts.
+  EXPECT_FALSE(isKeyword("final", LangOpts));
+  EXPECT_FALSE(isKeyword("override", LangOpts));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang