[clang-tidy] check std::string_view and custom string-like classes in readability-string-compare (#88636)

This PR aims to expand the list of classes that are considered to be
"strings" by `readability-string-compare` check.

1. Currently only `std::string;:compare` is checked, but
`std::string_view` has a similar `compare` method. This PR enables
checking of `std::string_view::compare` by default.
2. Some codebases use custom string-like classes that have public
interfaces similar to `std::string` or `std::string_view`. Example:
[TStringBase](https://github.com/yandex/yatool/blob/main/util/generic/strbase.h#L38),
A new option, `readability-string-compare.StringClassNames`, is added to
allow specifying a custom list of string-like classes.

Related to, but does not solve #28396 (only adds support for custom
string-like classes, not custom functions)

clang-tools-extra/clang-tidy/readability/StringCompareCheck.cpp

@@ -7,12 +7,15 @@
 //===----------------------------------------------------------------------===//
 
 #include "StringCompareCheck.h"
-#include "../utils/FixItHintUtils.h"
+#include "../utils/OptionsUtils.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Tooling/FixIt.h"
+#include "llvm/ADT/StringRef.h"
 
 using namespace clang::ast_matchers;
+namespace optutils = clang::tidy::utils::options;
 
 namespace clang::tidy::readability {
 
@@ -20,11 +23,27 @@ static const StringRef CompareMessage = "do not use 'compare' to test equality "
                                         "of strings; use the string equality "
                                         "operator instead";
 
+static const StringRef DefaultStringLikeClasses = "::std::basic_string;"
+                                                  "::std::basic_string_view";
+
+StringCompareCheck::StringCompareCheck(StringRef Name,
+                                       ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      StringLikeClasses(optutils::parseStringList(
+          Options.get("StringLikeClasses", DefaultStringLikeClasses))) {}
+
+void StringCompareCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "StringLikeClasses",
+                optutils::serializeStringList(StringLikeClasses));
+}
+
 void StringCompareCheck::registerMatchers(MatchFinder *Finder) {
+  if (StringLikeClasses.empty()) {
+    return;
+  }
   const auto StrCompare = cxxMemberCallExpr(
-      callee(cxxMethodDecl(hasName("compare"),
-                           ofClass(classTemplateSpecializationDecl(
-                               hasName("::std::basic_string"))))),
+      callee(cxxMethodDecl(hasName("compare"), ofClass(cxxRecordDecl(hasAnyName(
+                                                   StringLikeClasses))))),
       hasArgument(0, expr().bind("str2")), argumentCountIs(1),
       callee(memberExpr().bind("str1")));
 

clang-tools-extra/clang-tidy/readability/StringCompareCheck.h

@@ -10,6 +10,7 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_STRINGCOMPARECHECK_H
 
 #include "../ClangTidyCheck.h"
+#include <vector>
 
 namespace clang::tidy::readability {
 
@@ -20,13 +21,18 @@ namespace clang::tidy::readability {
 /// http://clang.llvm.org/extra/clang-tidy/checks/readability/string-compare.html
 class StringCompareCheck : public ClangTidyCheck {
 public:
-  StringCompareCheck(StringRef Name, ClangTidyContext *Context)
-      : ClangTidyCheck(Name, Context) {}
+  StringCompareCheck(StringRef Name, ClangTidyContext *Context);
+
   bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
     return LangOpts.CPlusPlus;
   }
+
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+
+private:
+  const std::vector<StringRef> StringLikeClasses;
 };
 
 } // namespace clang::tidy::readability

clang-tools-extra/docs/ReleaseNotes.rst

@@ -362,6 +362,11 @@ Changes in existing checks
   check by resolving fix-it overlaps in template code by disregarding implicit
   instances.
 
+- Improved :doc:`readability-string-compare
+  <clang-tidy/checks/readability/string-compare>` check to also detect
+  usages of ``std::string_view::compare``. Added a `StringLikeClasses` option
+  to detect usages of ``compare`` method in custom string-like classes.
+
 Removed checks
 ^^^^^^^^^^^^^^
 

clang-tools-extra/docs/clang-tidy/checks/readability/string-compare.rst

@@ -14,10 +14,12 @@ recommended to avoid the risk of incorrect interpretation of the return value
 and to simplify the code. The string equality and inequality operators can
 also be faster than the ``compare`` method due to early termination.
 
-Examples:
+Example
+-------
 
 .. code-block:: c++
 
+  // The same rules apply to std::string_view.
   std::string str1{"a"};
   std::string str2{"b"};
 
@@ -50,5 +52,36 @@ Examples:
   }
 
 The above code examples show the list of if-statements that this check will
-give a warning for. All of them uses ``compare`` to check if equality or
+give a warning for. All of them use ``compare`` to check equality or
 inequality of two strings instead of using the correct operators.
+
+Options
+-------
+
+.. option:: StringLikeClasses
+
+   A string containing semicolon-separated names of string-like classes.
+   By default contains only ``::std::basic_string``
+   and ``::std::basic_string_view``. If a class from this list has
+   a ``compare`` method similar to that of ``std::string``, it will be checked
+   in the same way.
+
+Example
+^^^^^^^
+
+.. code-block:: c++
+
+  struct CustomString {
+  public:
+    int compare (const CustomString& other) const;
+  }
+
+  CustomString str1;
+  CustomString str2;
+
+  // use str1 != str2 instead.
+  if (str1.compare(str2)) {
+  }
+
+If `StringLikeClasses` contains ``CustomString``, the check will suggest
+replacing ``compare`` with equality operator.

clang-tools-extra/test/clang-tidy/checkers/Inputs/Headers/string

@@ -108,6 +108,8 @@ struct basic_string_view {
   constexpr bool starts_with(C ch) const noexcept;
   constexpr bool starts_with(const C* s) const;
 
+  constexpr int compare(basic_string_view sv) const noexcept;
+
   static constexpr size_t npos = -1;
 };
 
@@ -132,6 +134,14 @@ bool operator==(const std::wstring&, const std::wstring&);
 bool operator==(const std::wstring&, const wchar_t*);
 bool operator==(const wchar_t*, const std::wstring&);
 
+bool operator==(const std::string_view&, const std::string_view&);
+bool operator==(const std::string_view&, const char*);
+bool operator==(const char*, const std::string_view&);
+
+bool operator!=(const std::string_view&, const std::string_view&);
+bool operator!=(const std::string_view&, const char*);
+bool operator!=(const char*, const std::string_view&);
+
 size_t strlen(const char* str);
 }
 

clang-tools-extra/test/clang-tidy/checkers/readability/string-compare-custom-string-classes.cpp

@@ -0,0 +1,35 @@
+// RUN: %check_clang_tidy %s readability-string-compare %t -- -config='{CheckOptions: {readability-string-compare.StringLikeClasses: "CustomStringTemplateBase;CustomStringNonTemplateBase"}}' -- -isystem %clang_tidy_headers
+#include <string>
+
+struct CustomStringNonTemplateBase {
+  int compare(const CustomStringNonTemplateBase& Other) const {
+    return 123;  // value is not important for check
+  }
+};
+
+template <typename T>
+struct CustomStringTemplateBase {
+  int compare(const CustomStringTemplateBase& Other) const {
+    return 123;
+  }
+};
+
+struct CustomString1 : CustomStringNonTemplateBase {};
+struct CustomString2 : CustomStringTemplateBase<char> {};
+
+void CustomStringClasses() {
+  std::string_view sv1("a");
+  std::string_view sv2("b");
+  if (sv1.compare(sv2)) {  // No warning - if a std class is not listed in StringLikeClasses, it won't be checked.
+  }
+
+  CustomString1 custom1;
+  if (custom1.compare(custom1)) {
+  }
+  // CHECK-MESSAGES: [[@LINE-2]]:7: warning: do not use 'compare' to test equality of strings; use the string equality operator instead [readability-string-compare]
+
+  CustomString2 custom2;
+  if (custom2.compare(custom2)) {
+  }
+  // CHECK-MESSAGES: [[@LINE-2]]:7: warning: do not use 'compare' to test equality of strings; use the string equality operator instead [readability-string-compare]
+}

clang-tools-extra/test/clang-tidy/checkers/readability/string-compare.cpp

@@ -67,11 +67,27 @@ void Test() {
   if (str1.compare(comp())) {
   }
   // CHECK-MESSAGES: [[@LINE-2]]:7: warning: do not use 'compare' to test equality of strings;
+
+  std::string_view sv1("a");
+  std::string_view sv2("b");
+  if (sv1.compare(sv2)) {
+  }
+  // CHECK-MESSAGES: [[@LINE-2]]:7: warning: do not use 'compare' to test equality of strings; use the string equality operator instead [readability-string-compare]
+}
+
+struct DerivedFromStdString : std::string {};
+
+void TestDerivedClass() {
+  DerivedFromStdString derived;
+  if (derived.compare(derived)) {
+  }
+  // CHECK-MESSAGES: [[@LINE-2]]:7: warning: do not use 'compare' to test equality of strings; use the string equality operator instead [readability-string-compare]
 }
 
 void Valid() {
   std::string str1("a", 1);
   std::string str2("b", 1);
+
   if (str1 == str2) {
   }
   if (str1 != str2) {
@@ -96,4 +112,11 @@ void Valid() {
   }
   if (str1.compare(str2) == -1) {
   }
+
+  std::string_view sv1("a");
+  std::string_view sv2("b");
+  if (sv1 == sv2) {
+  }
+  if (sv1.compare(sv2) > 0) {
+  }
 }