[clang-tidy] Add new modernize-use-starts-ends-with check (#72385)

Make a modernize version of abseil-string-find-startswith using the
available C++20 `std::string::starts_with` and
`std::string_view::starts_with`. Following up from
#72283.

clang-tools-extra/clang-tidy/abseil/StringFindStartswithCheck.h

@@ -21,7 +21,6 @@ namespace clang::tidy::abseil {
 
 // Find string.find(...) == 0 comparisons and suggest replacing with StartsWith.
 // FIXME(niko): Add similar check for EndsWith
-// FIXME(niko): Add equivalent modernize checks for C++20's std::starts_With
 class StringFindStartswithCheck : public ClangTidyCheck {
 public:
   using ClangTidyCheck::ClangTidyCheck;
@@ -31,6 +30,10 @@ class StringFindStartswithCheck : public ClangTidyCheck {
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
   void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    // Prefer modernize-use-starts-ends-with when C++20 is available.
+    return LangOpts.CPlusPlus && !LangOpts.CPlusPlus20;
+  }
 
 private:
   const std::vector<StringRef> StringLikeClasses;

clang-tools-extra/clang-tidy/modernize/CMakeLists.txt

@@ -38,6 +38,7 @@ add_clang_library(clangTidyModernizeModule
   UseNoexceptCheck.cpp
   UseNullptrCheck.cpp
   UseOverrideCheck.cpp
+  UseStartsEndsWithCheck.cpp
   UseStdPrintCheck.cpp
   UseTrailingReturnTypeCheck.cpp
   UseTransparentFunctorsCheck.cpp

clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp

@@ -39,6 +39,7 @@
 #include "UseNoexceptCheck.h"
 #include "UseNullptrCheck.h"
 #include "UseOverrideCheck.h"
+#include "UseStartsEndsWithCheck.h"
 #include "UseStdPrintCheck.h"
 #include "UseTrailingReturnTypeCheck.h"
 #include "UseTransparentFunctorsCheck.h"
@@ -66,6 +67,8 @@ class ModernizeModule : public ClangTidyModule {
     CheckFactories.registerCheck<MakeSharedCheck>("modernize-make-shared");
     CheckFactories.registerCheck<MakeUniqueCheck>("modernize-make-unique");
     CheckFactories.registerCheck<PassByValueCheck>("modernize-pass-by-value");
+    CheckFactories.registerCheck<UseStartsEndsWithCheck>(
+        "modernize-use-starts-ends-with");
     CheckFactories.registerCheck<UseStdPrintCheck>("modernize-use-std-print");
     CheckFactories.registerCheck<RawStringLiteralCheck>(
         "modernize-raw-string-literal");

clang-tools-extra/clang-tidy/modernize/UseStartsEndsWithCheck.cpp

@@ -0,0 +1,109 @@
+//===--- UseStartsEndsWithCheck.cpp - clang-tidy --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseStartsEndsWithCheck.h"
+
+#include "../utils/OptionsUtils.h"
+#include "clang/Lex/Lexer.h"
+
+#include <string>
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::modernize {
+
+UseStartsEndsWithCheck::UseStartsEndsWithCheck(StringRef Name,
+                                               ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context) {}
+
+void UseStartsEndsWithCheck::registerMatchers(MatchFinder *Finder) {
+  const auto ZeroLiteral = integerLiteral(equals(0));
+  const auto HasStartsWithMethodWithName = [](const std::string &Name) {
+    return hasMethod(
+        cxxMethodDecl(hasName(Name), isConst(), parameterCountIs(1))
+            .bind("starts_with_fun"));
+  };
+  const auto HasStartsWithMethod =
+      anyOf(HasStartsWithMethodWithName("starts_with"),
+            HasStartsWithMethodWithName("startsWith"),
+            HasStartsWithMethodWithName("startswith"));
+  const auto ClassWithStartsWithFunction = cxxRecordDecl(anyOf(
+      HasStartsWithMethod, hasAnyBase(hasType(hasCanonicalType(hasDeclaration(
+                               cxxRecordDecl(HasStartsWithMethod)))))));
+
+  const auto FindExpr = cxxMemberCallExpr(
+      // A method call with no second argument or the second argument is zero...
+      anyOf(argumentCountIs(1), hasArgument(1, ZeroLiteral)),
+      // ... named find...
+      callee(cxxMethodDecl(hasName("find")).bind("find_fun")),
+      // ... on a class with a starts_with function.
+      on(hasType(
+          hasCanonicalType(hasDeclaration(ClassWithStartsWithFunction)))));
+
+  const auto RFindExpr = cxxMemberCallExpr(
+      // A method call with a second argument of zero...
+      hasArgument(1, ZeroLiteral),
+      // ... named rfind...
+      callee(cxxMethodDecl(hasName("rfind")).bind("find_fun")),
+      // ... on a class with a starts_with function.
+      on(hasType(
+          hasCanonicalType(hasDeclaration(ClassWithStartsWithFunction)))));
+
+  const auto FindOrRFindExpr =
+      cxxMemberCallExpr(anyOf(FindExpr, RFindExpr)).bind("find_expr");
+
+  Finder->addMatcher(
+      // Match [=!]= with a zero on one side and a string.(r?)find on the other.
+      binaryOperator(hasAnyOperatorName("==", "!="),
+                     hasOperands(FindOrRFindExpr, ZeroLiteral))
+          .bind("expr"),
+      this);
+}
+
+void UseStartsEndsWithCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *ComparisonExpr = Result.Nodes.getNodeAs<BinaryOperator>("expr");
+  const auto *FindExpr = Result.Nodes.getNodeAs<CXXMemberCallExpr>("find_expr");
+  const auto *FindFun = Result.Nodes.getNodeAs<CXXMethodDecl>("find_fun");
+  const auto *StartsWithFunction =
+      Result.Nodes.getNodeAs<CXXMethodDecl>("starts_with_fun");
+
+  if (ComparisonExpr->getBeginLoc().isMacroID()) {
+    return;
+  }
+
+  const bool Neg = ComparisonExpr->getOpcode() == BO_NE;
+
+  auto Diagnostic =
+      diag(FindExpr->getBeginLoc(), "use %0 instead of %1() %select{==|!=}2 0")
+      << StartsWithFunction->getName() << FindFun->getName() << Neg;
+
+  // Remove possible zero second argument and ' [!=]= 0' suffix.
+  Diagnostic << FixItHint::CreateReplacement(
+      CharSourceRange::getTokenRange(
+          Lexer::getLocForEndOfToken(FindExpr->getArg(0)->getEndLoc(), 0,
+                                     *Result.SourceManager, getLangOpts()),
+          ComparisonExpr->getEndLoc()),
+      ")");
+
+  // Remove possible '0 [!=]= ' prefix.
+  Diagnostic << FixItHint::CreateRemoval(CharSourceRange::getCharRange(
+      ComparisonExpr->getBeginLoc(), FindExpr->getBeginLoc()));
+
+  // Replace '(r?)find' with 'starts_with'.
+  Diagnostic << FixItHint::CreateReplacement(
+      CharSourceRange::getTokenRange(FindExpr->getExprLoc(),
+                                     FindExpr->getExprLoc()),
+      StartsWithFunction->getName());
+
+  // Add possible negation '!'.
+  if (Neg) {
+    Diagnostic << FixItHint::CreateInsertion(FindExpr->getBeginLoc(), "!");
+  }
+}
+
+} // namespace clang::tidy::modernize

clang-tools-extra/clang-tidy/modernize/UseStartsEndsWithCheck.h

@@ -0,0 +1,37 @@
+//===--- UseStartsEndsWithCheck.h - clang-tidy ------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USESTARTSENDSWITHCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USESTARTSENDSWITHCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::modernize {
+
+/// Checks whether a ``find`` or ``rfind`` result is compared with 0 and
+/// suggests replacing with ``starts_with`` when the method exists in the class.
+/// Notably, this will work with ``std::string`` and ``std::string_view``.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/modernize/use-starts-ends-with.html
+class UseStartsEndsWithCheck : public ClangTidyCheck {
+public:
+  UseStartsEndsWithCheck(StringRef Name, ClangTidyContext *Context);
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+  bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
+    return LangOpts.CPlusPlus;
+  }
+  std::optional<TraversalKind> getCheckTraversalKind() const override {
+    return TK_IgnoreUnlessSpelledInSource;
+  }
+};
+
+} // namespace clang::tidy::modernize
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USESTARTSENDSWITHCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -186,6 +186,13 @@ New checks
 
   Replace ``enable_if`` with C++20 requires clauses.
 
+- New :doc:`modernize-use-starts-ends-with
+  <clang-tidy/checks/modernize/use-starts-ends-with>` check.
+
+  Checks whether a ``find`` or ``rfind`` result is compared with 0 and suggests
+  replacing with ``starts_with`` when the method exists in the class. Notably,
+  this will work with ``std::string`` and ``std::string_view``.
+
 - New :doc:`performance-enum-size
   <clang-tidy/checks/performance/enum-size>` check.
 

clang-tools-extra/docs/clang-tidy/checks/abseil/string-find-startswith.rst

@@ -8,6 +8,10 @@ corresponding ``std::string_view`` methods) result is compared with 0, and
 suggests replacing with ``absl::StartsWith()``. This is both a readability and
 performance issue.
 
+``starts_with`` was added as a built-in function on those types in C++20. If
+available, prefer enabling :doc:`modernize-use-starts-ends-with
+<../modernize/use-starts-ends-with>` instead of this check.
+
 .. code-block:: c++
 
   string s = "...";

clang-tools-extra/docs/clang-tidy/checks/list.rst

@@ -292,6 +292,7 @@ Clang-Tidy Checks
    :doc:`modernize-use-noexcept <modernize/use-noexcept>`, "Yes"
    :doc:`modernize-use-nullptr <modernize/use-nullptr>`, "Yes"
    :doc:`modernize-use-override <modernize/use-override>`, "Yes"
+   :doc:`modernize-use-starts-ends-with <modernize/use-starts-ends-with>`, "Yes"
    :doc:`modernize-use-std-print <modernize/use-std-print>`, "Yes"
    :doc:`modernize-use-trailing-return-type <modernize/use-trailing-return-type>`, "Yes"
    :doc:`modernize-use-transparent-functors <modernize/use-transparent-functors>`, "Yes"

clang-tools-extra/docs/clang-tidy/checks/modernize/use-starts-ends-with.rst

@@ -0,0 +1,22 @@
+.. title:: clang-tidy - modernize-use-starts-ends-with
+
+modernize-use-starts-ends-with
+==============================
+
+Checks whether a ``find`` or ``rfind`` result is compared with 0 and suggests
+replacing with ``starts_with`` when the method exists in the class. Notably,
+this will work with ``std::string`` and ``std::string_view``.
+
+.. code-block:: c++
+
+  std::string s = "...";
+  if (s.find("prefix") == 0) { /* do something */ }
+  if (s.rfind("prefix", 0) == 0) { /* do something */ }
+
+becomes
+
+.. code-block:: c++
+
+  std::string s = "...";
+  if (s.starts_with("prefix")) { /* do something */ }
+  if (s.starts_with("prefix")) { /* do something */ }

clang-tools-extra/test/clang-tidy/checkers/Inputs/Headers/stddef.h

@@ -12,4 +12,4 @@
 typedef __PTRDIFF_TYPE__ ptrdiff_t;
 typedef __SIZE_TYPE__ size_t;
 
-#endif _STDDEF_H_
+#endif // _STDDEF_H_

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

@@ -10,8 +10,13 @@ typedef unsigned __INT32_TYPE__ char32;
 namespace std {
 template <typename T>
 class allocator {};
+
 template <typename T>
 class char_traits {};
+
+template <typename C, typename T = char_traits<C>>
+struct basic_string_view;
+
 template <typename C, typename T = char_traits<C>, typename A = allocator<C>>
 struct basic_string {
   typedef size_t size_type;
@@ -52,6 +57,10 @@ struct basic_string {
   _Type& insert(size_type pos, const C* s);
   _Type& insert(size_type pos, const C* s, size_type n);
 
+  constexpr bool starts_with(std::basic_string_view<C, T> sv) const noexcept;
+  constexpr bool starts_with(C ch) const noexcept;
+  constexpr bool starts_with(const C* s) const;
+
   _Type& operator[](size_type);
   const _Type& operator[](size_type) const;
 
@@ -68,7 +77,7 @@ typedef basic_string<wchar_t> wstring;
 typedef basic_string<char16> u16string;
 typedef basic_string<char32> u32string;
 
-template <typename C, typename T = char_traits<C>>
+template <typename C, typename T>
 struct basic_string_view {
   typedef size_t size_type;
   typedef basic_string_view<C, T> _Type;
@@ -86,8 +95,13 @@ struct basic_string_view {
   size_type rfind(const C* s, size_type pos, size_type count) const;
   size_type rfind(const C* s, size_type pos = npos) const;
 
+  constexpr bool starts_with(basic_string_view sv) const noexcept;
+  constexpr bool starts_with(C ch) const noexcept;
+  constexpr bool starts_with(const C* s) const;
+
   static constexpr size_t npos = -1;
 };
+
 typedef basic_string_view<char> string_view;
 typedef basic_string_view<wchar_t> wstring_view;
 typedef basic_string_view<char16> u16string_view;

clang-tools-extra/test/clang-tidy/checkers/abseil/string-find-startswith.cpp

@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy %s abseil-string-find-startswith %t -- \
+// RUN: %check_clang_tidy -std=c++17 %s abseil-string-find-startswith %t -- \
 // RUN:   -config="{CheckOptions: \
 // RUN:     {abseil-string-find-startswith.StringLikeClasses: \
 // RUN:       '::std::basic_string;::std::basic_string_view;::basic_string'}}" \