[clang-tools-extra] [clang-tidy] Add performance-substr-self-assignment check (PR #209657)

[Yanzuo Liu via cfe-commits]

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@@ -0,0 +1,129 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "SubstrSelfAssignmentCheck.h"
+#include "../utils/Matchers.h"
+#include "../utils/OptionsUtils.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/ExprCXX.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Lex/Lexer.h"
+#include <optional>
+#include <string>
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::performance {
+
+SubstrSelfAssignmentCheck::SubstrSelfAssignmentCheck(StringRef Name,
+                                                     ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      StringLikeClasses(utils::options::parseStringList(
+          Options.get("StringLikeClasses", "::std::basic_string"))) {}
+
+void SubstrSelfAssignmentCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "StringLikeClasses",
+                utils::options::serializeStringList(StringLikeClasses));
+}
+
+void SubstrSelfAssignmentCheck::registerMatchers(MatchFinder *Finder) {
+  const auto VarRef =
+      ignoringParens(declRefExpr(to(varDecl().bind("var"))).bind("lhs"));
+
+  const auto StringClass =
+      cxxRecordDecl(hasAnyName(StringLikeClasses)).bind("string-class");
+
+  // The static member 'npos' of the matched string class. A bare name
+  // comparison is not enough: a local variable or parameter that happens to
+  // be called 'npos' carries a real count, which has no single-'erase'
+  // rewrite.
+  const auto NposDecl =
+      varDecl(hasName("npos"),
+              hasDeclContext(cxxRecordDecl(equalsBoundNode("string-class"))));
+
+  const auto Npos =
+      expr(ignoringParenImpCasts(anyOf(declRefExpr(hasDeclaration(NposDecl)),
+                                       memberExpr(hasDeclaration(NposDecl)))));
+
+  const auto SubstrCall =
+      cxxMemberCallExpr(
+          callee(cxxMethodDecl(hasName("substr"), ofClass(StringClass))),
+          on(ignoringParens(declRefExpr(to(varDecl(equalsBoundNode("var")))))),
+          optionally(hasArgument(1, Npos.bind("npos-count"))))
+          .bind("substr");
+
+  // Match: s = s.substr(...), except in unevaluated contexts such as
+  // decltype or sizeof, where no temporary is ever materialized.
+  Finder->addMatcher(
+      cxxOperatorCallExpr(
+          hasOperatorName("="), hasArgument(0, VarRef),
+          hasArgument(1, SubstrCall),
+          unless(anyOf(hasAncestor(typeLoc()),
+                       hasAncestor(expr(matchers::hasUnevaluatedContext())))))
+          .bind("assign"),
+      this);
+}
+
+void SubstrSelfAssignmentCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *AssignExpr =
+      Result.Nodes.getNodeAs<CXXOperatorCallExpr>("assign");
+  const auto *LHS = Result.Nodes.getNodeAs<DeclRefExpr>("lhs");
+  const auto *SubstrExpr = Result.Nodes.getNodeAs<CXXMemberCallExpr>("substr");
+  const SourceManager &SM = *Result.SourceManager;
+  const LangOptions &LangOpts = Result.Context->getLangOpts();
+
+  // Count only explicitly-written arguments (exclude CXXDefaultArgExpr).
+  unsigned NumExplicitArgs = 0;
+  for (const Expr *Arg : SubstrExpr->arguments())
+    if (!isa<CXXDefaultArgExpr>(Arg))
+      ++NumExplicitArgs;
+
+  // s = s.substr(pos) and s = s.substr(pos, npos) strip a prefix and can be
+  // rewritten as s.erase(0, pos). The truncation form s = s.substr(0, count)
+  // is diagnosed without a fix-it: s.erase(count) throws std::out_of_range
+  // for count > s.size() where substr merely clamps. The general
+  // two-argument form has no single-'erase' equivalent and is ignored.
+  // TODO: Also emit a fix-it for the truncation form when
+  // `count <= size()` can be proven.
+  const bool IsPrefixStrip =
+      NumExplicitArgs == 1 ||
+      (NumExplicitArgs == 2 &&
+       Result.Nodes.getNodeAs<Expr>("npos-count") != nullptr);
+  bool IsTruncation = false;
+  if (!IsPrefixStrip && NumExplicitArgs == 2) {
+    const auto *PosLiteral =
+        dyn_cast<IntegerLiteral>(SubstrExpr->getArg(0)->IgnoreParenImpCasts());
+    IsTruncation = PosLiteral && PosLiteral->getValue() == 0;
+  }
+  if (!IsPrefixStrip && !IsTruncation)
+    return;
+
+  // Rewriting a macro expansion is unsafe; emit the warning without a fix-it.
+  // TODO: Only emit this fix-it when `pos <= size()` can be proven.
+  std::optional<std::string> Replacement;
+  if (IsPrefixStrip && !AssignExpr->getBeginLoc().isMacroID() &&
+      !AssignExpr->getEndLoc().isMacroID()) {
+    StringRef VarName = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(LHS->getSourceRange()), SM, LangOpts);
+    StringRef PosText = Lexer::getSourceText(
+        CharSourceRange::getTokenRange(SubstrExpr->getArg(0)->getSourceRange()),
+        SM, LangOpts);
+    if (!VarName.empty() && !PosText.empty())
+      Replacement = (VarName + ".erase(0, " + PosText + ")").str();
+  }
+
+  auto Diag = diag(AssignExpr->getOperatorLoc(),
+                   "inefficient self-assignment via 'substr'; use 'erase' to "
+                   "modify the string in-place");
+  if (Replacement)
+    Diag << FixItHint::CreateReplacement(AssignExpr->getSourceRange(),
+                                         *Replacement);
----------------
zwuis wrote:

It's up to you, but we can use functions in `clang/Tooling/FixIt.h`.

https://github.com/llvm/llvm-project/pull/209657