[clang-tidy] Add readability-use-span-first-last check

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

@@ -58,6 +58,7 @@ add_clang_library(clangTidyReadabilityModule STATIC
   UppercaseLiteralSuffixCheck.cpp
   UseAnyOfAllOfCheck.cpp
   UseStdMinMaxCheck.cpp
+  UseSpanFirstLastCheck.cpp
 
   LINK_LIBS
   clangTidy

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

@@ -60,6 +60,7 @@
 #include "UniqueptrDeleteReleaseCheck.h"
 #include "UppercaseLiteralSuffixCheck.h"
 #include "UseAnyOfAllOfCheck.h"
+#include "UseSpanFirstLastCheck.h"
 #include "UseStdMinMaxCheck.h"
 
 namespace clang::tidy {
@@ -172,6 +173,8 @@ class ReadabilityModule : public ClangTidyModule {
         "readability-use-anyofallof");
     CheckFactories.registerCheck<UseStdMinMaxCheck>(
         "readability-use-std-min-max");
+    CheckFactories.registerCheck<UseSpanFirstLastCheck>(
+        "readability-use-span-first-last");
   }
 };
 

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

@@ -0,0 +1,110 @@
+//===--- UseSpanFirstLastCheck.cpp - 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "UseSpanFirstLastCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+void UseSpanFirstLastCheck::registerMatchers(MatchFinder *Finder) {
+  if (!getLangOpts().CPlusPlus20)
+    return;
+
+  // Match span::subspan calls
+  const auto HasSpanType =
+      hasType(hasUnqualifiedDesugaredType(recordType(hasDeclaration(
+          classTemplateSpecializationDecl(hasName("::std::span"))))));
+
+  Finder->addMatcher(cxxMemberCallExpr(callee(memberExpr(hasDeclaration(
+                                           cxxMethodDecl(hasName("subspan"))))),
+                                       on(expr(HasSpanType)))
+                         .bind("subspan"),
+                     this);
+}
+
+void UseSpanFirstLastCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *Call = Result.Nodes.getNodeAs<CXXMemberCallExpr>("subspan");
+  if (!Call)
+    return;
+
+  handleSubspanCall(Result, Call);
+}
+
+void UseSpanFirstLastCheck::handleSubspanCall(
+    const MatchFinder::MatchResult &Result, const CXXMemberCallExpr *Call) {
+  unsigned NumArgs = Call->getNumArgs();
+  if (NumArgs == 0 || NumArgs > 2)
+    return;
+
+  const Expr *Offset = Call->getArg(0);
+  const Expr *Count = NumArgs > 1 ? Call->getArg(1) : nullptr;
+  auto &Context = *Result.Context;
+
+  // Check if this is subspan(0, n) -> first(n)
+  bool IsZeroOffset = false;
+  const Expr *OffsetE = Offset->IgnoreImpCasts();
+  if (const auto *IL = dyn_cast<IntegerLiteral>(OffsetE)) {
+    IsZeroOffset = IL->getValue() == 0;
+  }
+
+  // Check if this is subspan(size() - n) -> last(n)
+  bool IsSizeMinusN = false;
+  const Expr *SizeMinusArg = nullptr;
+  if (const auto *BO = dyn_cast<BinaryOperator>(OffsetE)) {
+    if (BO->getOpcode() == BO_Sub) {
+      if (const auto *SizeCall = dyn_cast<CXXMemberCallExpr>(BO->getLHS())) {
+        if (SizeCall->getMethodDecl()->getName() == "size") {
+          IsSizeMinusN = true;
+          SizeMinusArg = BO->getRHS();
+        }
+      }
+    }
+  }
+
+  // Build replacement text
+  std::string Replacement;
+  if (IsZeroOffset && Count) {
+    // subspan(0, count) -> first(count)
+    auto CountStr = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(Count->getSourceRange()),
+        Context.getSourceManager(), Context.getLangOpts());
+    const auto *Base =
+        cast<CXXMemberCallExpr>(Call)->getImplicitObjectArgument();
+    auto BaseStr = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(Base->getSourceRange()),
+        Context.getSourceManager(), Context.getLangOpts());
+    Replacement = BaseStr.str() + ".first(" + CountStr.str() + ")";
+  } else if (IsSizeMinusN && SizeMinusArg) {
+    // subspan(size() - n) -> last(n)
+    auto ArgStr = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(SizeMinusArg->getSourceRange()),
+        Context.getSourceManager(), Context.getLangOpts());
+    const auto *Base =
+        cast<CXXMemberCallExpr>(Call)->getImplicitObjectArgument();
+    auto BaseStr = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(Base->getSourceRange()),
+        Context.getSourceManager(), Context.getLangOpts());
+    Replacement = BaseStr.str() + ".last(" + ArgStr.str() + ")";
+  }
+
+  if (!Replacement.empty()) {
+    if (IsZeroOffset && Count) {
+      diag(Call->getBeginLoc(), "prefer span::first() over subspan()")
+          << FixItHint::CreateReplacement(Call->getSourceRange(), Replacement);
+    } else {
+      diag(Call->getBeginLoc(), "prefer span::last() over subspan()")
+          << FixItHint::CreateReplacement(Call->getSourceRange(), Replacement);
+    }
+  }
+}
+} // namespace clang::tidy::readability

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

@@ -0,0 +1,44 @@
+//===--- UseSpanFirstLastCheck.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_READABILITY_USESPANFIRSTLASTCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_USESPANFIRSTLASTCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::readability {
+
+/// Suggests using clearer std::span member functions first()/last() instead of
+/// equivalent subspan() calls where applicable.
+///
+/// For example:
+/// \code
+///   std::span<int> s = ...;
+///   auto sub1 = s.subspan(0, n);           // ->  auto sub1 = s.first(n);
+///   auto sub2 = s.subspan(s.size() - n);   // ->  auto sub2 = s.last(n);
+///   auto sub3 = s.subspan(1, n);           // not changed
+///   auto sub4 = s.subspan(n);              // not changed
+/// \endcode
+///
+/// The check is only active in C++20 mode.
+class UseSpanFirstLastCheck : public ClangTidyCheck {
+public:
+  UseSpanFirstLastCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  void handleSubspanCall(const ast_matchers::MatchFinder::MatchResult &Result,
+                         const CXXMemberCallExpr *Call);
+};
+
+} // namespace clang::tidy::readability
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_USESPANFIRSTLASTCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -145,6 +145,10 @@ New checks
 New check aliases
 ^^^^^^^^^^^^^^^^^
 
+- New check `readability-use-span-first-last` has been added that suggests using
+  ``std::span::first()`` and ``std::span::last()`` member functions instead of
+  equivalent ``subspan()``.
+
 - New alias :doc:`cert-arr39-c <clang-tidy/checks/cert/arr39-c>` to
   :doc:`bugprone-sizeof-expression
   <clang-tidy/checks/bugprone/sizeof-expression>` was added.

clang-tools-extra/docs/clang-tidy/checks/readability/use-span-first-last.rst

@@ -0,0 +1,23 @@
+.. title:: clang-tidy - readability-use-span-first-last
+
+readability-use-span-first-last
+===============================
+
+Checks for uses of ``std::span::subspan()`` that can be replaced with clearer
+``first()`` or ``last()`` member functions. These dedicated methods were added 
+to C++20 to provide more expressive alternatives to common subspan operations.
+
+Covered scenarios:
+
+==================================== ==================================
+Expression                           Replacement
+------------------------------------ ----------------------------------
+``s.subspan(0, n)``                  ``s.first(n)``
+``s.subspan(s.size() - n)``          ``s.last(n)``
+==================================== ==================================
+
+Non-zero offset with count (like ``subspan(1, n)``) or offset-only calls 
+(like ``subspan(n)``) have no clearer equivalent using ``first()`` or 
+``last()``, so these cases are not transformed.
+
+This check is only active when C++20 or later is used.

clang-tools-extra/test/clang-tidy/checkers/readability/use-span-first-last.cpp

@@ -0,0 +1,51 @@
+// RUN: %check_clang_tidy -std=c++20 %s readability-use-span-first-last %t
+
+namespace std {
+template <typename T>
+class span {
+  T* ptr;
+  __SIZE_TYPE__ len;
+
+public:
+  span(T* p, __SIZE_TYPE__ l) : ptr(p), len(l) {}
+
+  span<T> subspan(__SIZE_TYPE__ offset) const {
+    return span(ptr + offset, len - offset);
+  }
+
+  span<T> subspan(__SIZE_TYPE__ offset, __SIZE_TYPE__ count) const {
+    return span(ptr + offset, count);
+  }
+
+  span<T> first(__SIZE_TYPE__ count) const {
+    return span(ptr, count);
+  }
+
+  span<T> last(__SIZE_TYPE__ count) const {
+    return span(ptr + (len - count), count);
+  }
+
+  __SIZE_TYPE__ size() const { return len; }
+};
+} // namespace std
+
+void test() {
+  int arr[] = {1, 2, 3, 4, 5};
+  std::span<int> s(arr, 5);
+
+  auto sub1 = s.subspan(0, 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer span::first() over subspan()
+  // CHECK-FIXES: auto sub1 = s.first(3);
+
+  auto sub2 = s.subspan(s.size() - 2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer span::last() over subspan()
+  // CHECK-FIXES: auto sub2 = s.last(2);
+
+  __SIZE_TYPE__ n = 2;
+  auto sub3 = s.subspan(0, n);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer span::first() over subspan()
+  // CHECK-FIXES: auto sub3 = s.first(n);
+
+  auto sub4 = s.subspan(1, 2);  // No warning
+  auto sub5 = s.subspan(2);     // No warning
+}