[clang-tidy] Add readability-string-view-substr check

Add a new check that suggests using string_view::remove_prefix() and
remove_suffix() instead of substr() when the intent is to remove
characters from either end of a string_view.

Author: hjanuschka

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

@@ -53,6 +53,7 @@ add_clang_library(clangTidyReadabilityModule STATIC
   StaticAccessedThroughInstanceCheck.cpp
   StaticDefinitionInAnonymousNamespaceCheck.cpp
   StringCompareCheck.cpp
+  StringViewSubstrCheck.cpp
   SuspiciousCallArgumentCheck.cpp
   UniqueptrDeleteReleaseCheck.cpp
   UppercaseLiteralSuffixCheck.cpp

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

@@ -56,6 +56,7 @@
 #include "StaticAccessedThroughInstanceCheck.h"
 #include "StaticDefinitionInAnonymousNamespaceCheck.h"
 #include "StringCompareCheck.h"
+#include "StringViewSubstrCheck.h"
 #include "SuspiciousCallArgumentCheck.h"
 #include "UniqueptrDeleteReleaseCheck.h"
 #include "UppercaseLiteralSuffixCheck.h"
@@ -146,6 +147,8 @@ class ReadabilityModule : public ClangTidyModule {
         "readability-static-definition-in-anonymous-namespace");
     CheckFactories.registerCheck<StringCompareCheck>(
         "readability-string-compare");
+    CheckFactories.registerCheck<StringViewSubstrCheck>(
+        "readability-stringview-substr");
     CheckFactories.registerCheck<readability::NamedParameterCheck>(
         "readability-named-parameter");
     CheckFactories.registerCheck<NonConstParameterCheck>(

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

@@ -0,0 +1,132 @@
+//===--- StringViewSubstrCheck.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 "StringViewSubstrCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+void StringViewSubstrCheck::registerMatchers(MatchFinder *Finder) {
+  const auto HasStringViewType = hasType(hasUnqualifiedDesugaredType(recordType(
+      hasDeclaration(recordDecl(hasName("::std::basic_string_view"))))));
+
+  // Match assignment to string_view's substr
+  Finder->addMatcher(
+      cxxOperatorCallExpr(
+          hasOverloadedOperatorName("="),
+          hasArgument(0, expr(HasStringViewType).bind("target")),
+          hasArgument(
+              1, cxxMemberCallExpr(callee(memberExpr(hasDeclaration(
+                                       cxxMethodDecl(hasName("substr"))))),
+                                   on(expr(HasStringViewType).bind("source")))
+                     .bind("substr_call")))
+          .bind("assignment"),
+      this);
+}
+
+void StringViewSubstrCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *Assignment =
+      Result.Nodes.getNodeAs<CXXOperatorCallExpr>("assignment");
+  const auto *Target = Result.Nodes.getNodeAs<Expr>("target");
+  const auto *Source = Result.Nodes.getNodeAs<Expr>("source");
+  const auto *SubstrCall =
+      Result.Nodes.getNodeAs<CXXMemberCallExpr>("substr_call");
+
+  if (!Assignment || !Target || !Source || !SubstrCall) {
+    return;
+  }
+
+  // Get the DeclRefExpr for the target and source to compare variables
+  const auto *TargetDRE = dyn_cast<DeclRefExpr>(Target->IgnoreParenImpCasts());
+  const auto *SourceDRE = dyn_cast<DeclRefExpr>(Source->IgnoreParenImpCasts());
+
+  // Only handle self-assignment cases
+  if (!TargetDRE || !SourceDRE ||
+      TargetDRE->getDecl() != SourceDRE->getDecl()) {
+    return;
+  }
+
+  const Expr *StartArg = SubstrCall->getArg(0);
+  const Expr *LengthArg =
+      SubstrCall->getNumArgs() > 1 ? SubstrCall->getArg(1) : nullptr;
+
+  // Get source text of first argument
+  std::string StartText =
+      Lexer::getSourceText(
+          CharSourceRange::getTokenRange(StartArg->getSourceRange()),
+          *Result.SourceManager, Result.Context->getLangOpts())
+          .str();
+
+  // Case 1: Check for remove_prefix pattern - only when the second arg is
+  // missing (uses npos)
+  if (!LengthArg || isa<CXXDefaultArgExpr>(LengthArg)) {
+    std::string Replacement = TargetDRE->getNameInfo().getAsString() +
+                              ".remove_prefix(" + StartText + ")";
+    diag(Assignment->getBeginLoc(), "prefer 'remove_prefix' over 'substr' for "
+                                    "removing characters from the start")
+        << FixItHint::CreateReplacement(Assignment->getSourceRange(),
+                                        Replacement);
+    return;
+  }
+
+  // Case 2: Check for remove_suffix pattern
+  if (StartText == "0") {
+    if (const auto *BinOp = dyn_cast<BinaryOperator>(LengthArg)) {
+      if (BinOp->getOpcode() == BO_Sub) {
+        const Expr *LHS = BinOp->getLHS();
+        const Expr *RHS = BinOp->getRHS();
+
+        // Check if LHS is a length() call on the same string_view
+        if (const auto *LengthCall = dyn_cast<CXXMemberCallExpr>(LHS)) {
+          if (const auto *LengthMethod =
+                  dyn_cast<CXXMethodDecl>(LengthCall->getDirectCallee())) {
+            if (LengthMethod->getName() == "length") {
+              // Verify the length() call is on the same string_view
+              const Expr *LengthObject =
+                  LengthCall->getImplicitObjectArgument();
+              const auto *LengthDRE =
+                  dyn_cast<DeclRefExpr>(LengthObject->IgnoreParenImpCasts());
+
+              if (!LengthDRE || LengthDRE->getDecl() != TargetDRE->getDecl()) {
+                return;
+              }
+
+              // Must be a simple non-zero integer literal
+              const auto *IL =
+                  dyn_cast<IntegerLiteral>(RHS->IgnoreParenImpCasts());
+              if (!IL || IL->getValue() == 0) {
+                return;
+              }
+
+              std::string RHSText =
+                  Lexer::getSourceText(
+                      CharSourceRange::getTokenRange(RHS->getSourceRange()),
+                      *Result.SourceManager, Result.Context->getLangOpts())
+                      .str();
+
+              std::string Replacement = TargetDRE->getNameInfo().getAsString() +
+                                        ".remove_suffix(" + RHSText + ")";
+              diag(Assignment->getBeginLoc(),
+                   "prefer 'remove_suffix' over 'substr' for removing "
+                   "characters from the end")
+                  << FixItHint::CreateReplacement(Assignment->getSourceRange(),
+                                                  Replacement);
+              return;
+            }
+          }
+        }
+      }
+    }
+  }
+}
+
+} // namespace clang::tidy::readability

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

@@ -0,0 +1,39 @@
+//===--- StringViewSubstrCheck.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_STRINGVIEWSUBSTRCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_STRINGVIEWSUBSTRCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::readability {
+
+/// Finds string_view substr() calls that can be replaced with remove_prefix()
+/// or remove_suffix().
+///
+/// For the user-facing documentation see:
+/// https://clang.llvm.org/extra/clang-tidy/checks/readability/string-view-substr.html
+///
+/// The check matches two patterns:
+///   sv = sv.substr(N) -> sv.remove_prefix(N)
+///   sv = sv.substr(0, sv.length() - N) -> sv.remove_suffix(N)
+///
+/// These replacements make the intent clearer and are more efficient as they
+/// modify the string_view in place rather than creating a new one.
+class StringViewSubstrCheck : public ClangTidyCheck {
+public:
+  StringViewSubstrCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace clang::tidy::readability
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_STRINGVIEWSUBSTRCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -136,6 +136,13 @@ New checks
   Gives warnings for tagged unions, where the number of tags is
   different from the number of data members inside the union.
 
+- New :doc:`readability-string-view-substr 
+  <clang-tidy/checks/readability/string-view-substr>` check.
+
+  Finds ``std::string_view::substr()`` calls that can be replaced with clearer 
+  alternatives using ``remove_prefix()`` or ``remove_suffix()``. This makes the 
+  intent clearer and is more efficient as it modifies the string_view in place.
+
 - New :doc:`portability-template-virtual-member-function
   <clang-tidy/checks/portability/template-virtual-member-function>` check.
 

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

@@ -0,0 +1,16 @@
+.. title:: clang-tidy - readability-string-view-substr
+
+readability-string-view-substr
+==============================
+
+Finds ``string_view substr()`` calls that can be replaced with clearer alternatives
+using ``remove_prefix()`` or ``remove_suffix()``.
+
+The check suggests the following transformations:
+
+===========================================  =======================
+Expression                                   Replacement
+===========================================  =======================
+``sv = sv.substr(n)``                        ``sv.remove_prefix(n)``
+``sv = sv.substr(0, sv.length()-n)``         ``sv.remove_suffix(n)``
+===========================================  =======================

clang-tools-extra/test/clang-tidy/checkers/readability/stringview_substr.cpp

@@ -0,0 +1,55 @@
+// RUN: %check_clang_tidy %s readability-stringview-substr %t
+
+namespace std {
+template <typename T>
+class basic_string_view {
+public:
+  using size_type = unsigned long;
+  static constexpr size_type npos = -1;
+
+  basic_string_view(const char*) {}
+  basic_string_view substr(size_type pos, size_type count = npos) const { return *this; }
+  void remove_prefix(size_type n) {}
+  void remove_suffix(size_type n) {}
+  size_type length() const { return 0; }
+
+  // Needed for assignment operator
+  basic_string_view& operator=(const basic_string_view&) { return *this; }
+};
+
+using string_view = basic_string_view<char>;
+}
+
+void test() {
+  std::string_view sv("test");
+  std::string_view sv1("test");
+  std::string_view sv2("other");
+
+  // Should match: Removing N characters from start
+  sv = sv.substr(5);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: prefer 'remove_prefix' over 'substr' for removing characters from the start [readability-stringview-substr]
+  // CHECK-FIXES: sv.remove_prefix(5)
+
+  // Should match: Removing N characters from end
+  sv = sv.substr(0, sv.length() - 3);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: prefer 'remove_suffix' over 'substr' for removing characters from the end [readability-stringview-substr]
+  // CHECK-FIXES: sv.remove_suffix(3)
+
+  // Should not match: Basic substring operations
+  sv = sv.substr(0, 3);  // No warning - taking first N characters
+  sv = sv.substr(1, 3);  // No warning - taking N characters from position 1
+  
+  // Should not match: Operations on different string_views
+  sv = sv2.substr(0, sv2.length() - 3);  // No warning - not self-assignment
+  sv = sv.substr(0, sv2.length() - 3);   // No warning - length() from different string_view
+  sv1 = sv1.substr(0, sv2.length() - 3); // No warning - length() from different string_view
+  sv = sv1.substr(0, sv1.length() - 3);  // No warning - not self-assignment
+
+  // Should not match: Not actually removing from end
+  sv = sv.substr(0, sv.length());       // No warning - keeping entire string
+  sv = sv.substr(0, sv.length() - 0);   // No warning - subtracting zero
+  
+  // Should not match: Complex expressions
+  sv = sv.substr(0, sv.length() - (3 + 2));  // No warning - complex arithmetic
+  sv = sv.substr(1 + 2, sv.length() - 3);    // No warning - complex start position
+}