[clang-tools-extra] [clang-tidy] Add modernize-use-span-first-last check (PR #118074)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-tidy

Author: Helmut Januschka (hjanuschka)

<details>
<summary>Changes</summary>

Add new clang-tidy check that suggests using std::span's more expressive first() and last() member functions instead of equivalent subspan() calls.

The check modernizes code by replacing:
- subspan(0, n) -> first(n)
- subspan(n) -> last(size() - n)

These dedicated methods were added to the standard to provide clearer alternatives to common subspan operations. They improve readability by better expressing intent and are less error-prone by eliminating manual offset calculations.

For example:
```cpp
std::span<int> s = ...;
auto sub1 = s.subspan(0, n);     // transforms to: auto sub1 = s.first(n);
auto sub2 = s.subspan(n);        // transforms to: auto sub2 = s.last(s.size() - n);
auto sub3 = s.subspan(1, n);     // not transformed, no direct equivalent
```


not sure if this is a readability or a modernize check ❓ 

---
Full diff: https://github.com/llvm/llvm-project/pull/118074.diff


7 Files Affected:

- (modified) clang-tools-extra/clang-tidy/modernize/CMakeLists.txt (+1) 
- (modified) clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp (+3) 
- (added) clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.cpp (+98) 
- (added) clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.h (+40) 
- (modified) clang-tools-extra/docs/ReleaseNotes.rst (+4) 
- (added) clang-tools-extra/docs/clang-tidy/checks/modernize/use-span-first-last.rst (+19) 
- (added) clang-tools-extra/test/clang-tidy/checkers/modernize/modernize-subspan-conversion.cpp (+50) 


``````````diff
diff --git a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
index c919d49b42873a..47dd12a2640b6c 100644
--- a/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/modernize/CMakeLists.txt
@@ -49,6 +49,7 @@ add_clang_library(clangTidyModernizeModule STATIC
   UseTransparentFunctorsCheck.cpp
   UseUncaughtExceptionsCheck.cpp
   UseUsingCheck.cpp
+  UseSpanFirstLastCheck.cpp
 
   LINK_LIBS
   clangTidy
diff --git a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
index 18607593320635..6fc5de5aad20b7 100644
--- a/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp
@@ -42,6 +42,7 @@
 #include "UseNullptrCheck.h"
 #include "UseOverrideCheck.h"
 #include "UseRangesCheck.h"
+#include "UseSpanFirstLastCheck.h"
 #include "UseStartsEndsWithCheck.h"
 #include "UseStdFormatCheck.h"
 #include "UseStdNumbersCheck.h"
@@ -122,6 +123,8 @@ class ModernizeModule : public ClangTidyModule {
     CheckFactories.registerCheck<UseUncaughtExceptionsCheck>(
         "modernize-use-uncaught-exceptions");
     CheckFactories.registerCheck<UseUsingCheck>("modernize-use-using");
+    CheckFactories.registerCheck<UseSpanFirstLastCheck>("modernize-use-span-first-last");
+
   }
 };
 
diff --git a/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.cpp
new file mode 100644
index 00000000000000..6cf01386b0c3fb
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.cpp
@@ -0,0 +1,98 @@
+//===--- 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::modernize {
+
+void UseSpanFirstLastCheck::registerMatchers(MatchFinder *Finder) {
+  // 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) {
+  // Get arguments
+  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;
+  bool IsZeroOffset = false;
+
+  // Check if offset is zero through any implicit casts
+  const Expr *OffsetE = Offset->IgnoreImpCasts();
+  if (const auto *IL = dyn_cast<IntegerLiteral>(OffsetE)) {
+    IsZeroOffset = IL->getValue() == 0;
+  }
+
+  // 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 (NumArgs == 1) {
+    // subspan(n) -> last(size() - n)
+    auto OffsetStr = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(Offset->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(" + BaseStr.str() + ".size() - " +
+                  OffsetStr.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::modernize
\ No newline at end of file
diff --git a/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.h b/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.h
new file mode 100644
index 00000000000000..8d4c6035f7ec76
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/modernize/UseSpanFirstLastCheck.h
@@ -0,0 +1,40 @@
+//===--- 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_MODERNIZE_USESPANFIRSTLASTCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USESPANFIRSTLASTCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::modernize {
+
+/// Converts std::span::subspan() calls to the more modern first()/last()
+/// methods where applicable.
+///
+/// For example:
+/// \code
+///   std::span<int> s = ...;
+///   auto sub = s.subspan(0, n);    // ->  auto sub = s.first(n);
+///   auto sub2 = s.subspan(n);      // ->  auto sub2 = s.last(s.size() - n);
+/// \endcode
+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::modernize
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MODERNIZE_USESPANFIRSTLASTCHECK_H
\ No newline at end of file
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index f050391110385e..04a45d002c0d1d 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -145,6 +145,10 @@ New checks
 New check aliases
 ^^^^^^^^^^^^^^^^^
 
+- New check `modernize-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.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-span-first-last.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-span-first-last.rst
new file mode 100644
index 00000000000000..e8aad59bb2264f
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-span-first-last.rst
@@ -0,0 +1,19 @@
+.. title:: clang-tidy - modernize-use-span-first-last
+
+modernize-use-span-first-last
+============================
+
+Checks for uses of ``std::span::subspan()`` that can be replaced with clearer
+``first()`` or ``last()`` member functions.
+
+Covered scenarios:
+
+==================================== ==================================
+Expression                           Replacement
+------------------------------------ ----------------------------------
+``s.subspan(0, n)``                  ``s.first(n)``
+``s.subspan(n)``                     ``s.last(s.size() - n)``
+==================================== ==================================
+
+Non-zero offset with count (like ``subspan(1, n)``) has no direct equivalent
+using ``first()`` or ``last()``, so these cases are not transformed.
\ No newline at end of file
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/modernize-subspan-conversion.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/modernize-subspan-conversion.cpp
new file mode 100644
index 00000000000000..cb78bc02f22d4f
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/modernize-subspan-conversion.cpp
@@ -0,0 +1,50 @@
+// RUN: %check_clang_tidy %s modernize-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(2);
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: prefer span::last() over subspan()
+  // CHECK-FIXES: auto sub2 = s.last(s.size() - 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
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/118074