1 //===- RedundantStringCStrCheck.cpp - Check for redundant c_str calls -----===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 //  This file implements a check for redundant calls of c_str() on strings.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "RedundantStringCStrCheck.h"
14 #include "clang/Lex/Lexer.h"
15 #include "clang/Tooling/FixIt.h"
16 
17 using namespace clang::ast_matchers;
18 
19 namespace clang {
20 namespace tidy {
21 namespace readability {
22 
23 namespace {
24 
25 // Return true if expr needs to be put in parens when it is an argument of a
26 // prefix unary operator, e.g. when it is a binary or ternary operator
27 // syntactically.
28 bool needParensAfterUnaryOperator(const Expr &ExprNode) {
29   if (isa<clang::BinaryOperator>(&ExprNode) ||
30       isa<clang::ConditionalOperator>(&ExprNode)) {
31     return true;
32   }
33   if (const auto *Op = dyn_cast<CXXOperatorCallExpr>(&ExprNode)) {
34     return Op->getNumArgs() == 2 && Op->getOperator() != OO_PlusPlus &&
35            Op->getOperator() != OO_MinusMinus && Op->getOperator() != OO_Call &&
36            Op->getOperator() != OO_Subscript;
37   }
38   return false;
39 }
40 
41 // Format a pointer to an expression: prefix with '*' but simplify
42 // when it already begins with '&'.  Return empty string on failure.
43 std::string
44 formatDereference(const ast_matchers::MatchFinder::MatchResult &Result,
45                   const Expr &ExprNode) {
46   if (const auto *Op = dyn_cast<clang::UnaryOperator>(&ExprNode)) {
47     if (Op->getOpcode() == UO_AddrOf) {
48       // Strip leading '&'.
49       return tooling::fixit::getText(*Op->getSubExpr()->IgnoreParens(),
50                                      *Result.Context);
51     }
52   }
53   StringRef Text = tooling::fixit::getText(ExprNode, *Result.Context);
54 
55   if (Text.empty())
56     return std::string();
57   // Add leading '*'.
58   if (needParensAfterUnaryOperator(ExprNode)) {
59     return (llvm::Twine("*(") + Text + ")").str();
60   }
61   return (llvm::Twine("*") + Text).str();
62 }
63 
64 } // end namespace
65 
66 void RedundantStringCStrCheck::registerMatchers(
67     ast_matchers::MatchFinder *Finder) {
68   // Only register the matchers for C++; the functionality currently does not
69   // provide any benefit to other languages, despite being benign.
70   if (!getLangOpts().CPlusPlus)
71     return;
72 
73   // Match expressions of type 'string' or 'string*'.
74   const auto StringDecl = type(hasUnqualifiedDesugaredType(recordType(
75       hasDeclaration(cxxRecordDecl(hasName("::std::basic_string"))))));
76   const auto StringExpr =
77       expr(anyOf(hasType(StringDecl), hasType(qualType(pointsTo(StringDecl)))));
78 
79   // Match string constructor.
80   const auto StringConstructorExpr = expr(anyOf(
81       cxxConstructExpr(argumentCountIs(1),
82                        hasDeclaration(cxxMethodDecl(hasName("basic_string")))),
83       cxxConstructExpr(
84           argumentCountIs(2),
85           hasDeclaration(cxxMethodDecl(hasName("basic_string"))),
86           // If present, the second argument is the alloc object which must not
87           // be present explicitly.
88           hasArgument(1, cxxDefaultArgExpr()))));
89 
90   // Match a call to the string 'c_str()' method.
91   const auto StringCStrCallExpr =
92       cxxMemberCallExpr(on(StringExpr.bind("arg")),
93                         callee(memberExpr().bind("member")),
94                         callee(cxxMethodDecl(hasAnyName("c_str", "data"))))
95           .bind("call");
96 
97   // Detect redundant 'c_str()' calls through a string constructor.
98   Finder->addMatcher(cxxConstructExpr(StringConstructorExpr,
99                                       hasArgument(0, StringCStrCallExpr)),
100                      this);
101 
102   // Detect: 's == str.c_str()'  ->  's == str'
103   Finder->addMatcher(
104       cxxOperatorCallExpr(
105           anyOf(
106               hasOverloadedOperatorName("<"), hasOverloadedOperatorName(">"),
107               hasOverloadedOperatorName(">="), hasOverloadedOperatorName("<="),
108               hasOverloadedOperatorName("!="), hasOverloadedOperatorName("=="),
109               hasOverloadedOperatorName("+")),
110           anyOf(allOf(hasArgument(0, StringExpr),
111                       hasArgument(1, StringCStrCallExpr)),
112                 allOf(hasArgument(0, StringCStrCallExpr),
113                       hasArgument(1, StringExpr)))),
114       this);
115 
116   // Detect: 'dst += str.c_str()'  ->  'dst += str'
117   // Detect: 's = str.c_str()'  ->  's = str'
118   Finder->addMatcher(cxxOperatorCallExpr(anyOf(hasOverloadedOperatorName("="),
119                                                hasOverloadedOperatorName("+=")),
120                                          hasArgument(0, StringExpr),
121                                          hasArgument(1, StringCStrCallExpr)),
122                      this);
123 
124   // Detect: 'dst.append(str.c_str())'  ->  'dst.append(str)'
125   Finder->addMatcher(
126       cxxMemberCallExpr(on(StringExpr), callee(decl(cxxMethodDecl(hasAnyName(
127                                             "append", "assign", "compare")))),
128                         argumentCountIs(1), hasArgument(0, StringCStrCallExpr)),
129       this);
130 
131   // Detect: 'dst.compare(p, n, str.c_str())'  ->  'dst.compare(p, n, str)'
132   Finder->addMatcher(
133       cxxMemberCallExpr(on(StringExpr),
134                         callee(decl(cxxMethodDecl(hasName("compare")))),
135                         argumentCountIs(3), hasArgument(2, StringCStrCallExpr)),
136       this);
137 
138   // Detect: 'dst.find(str.c_str())'  ->  'dst.find(str)'
139   Finder->addMatcher(
140       cxxMemberCallExpr(on(StringExpr),
141                         callee(decl(cxxMethodDecl(hasAnyName(
142                             "find", "find_first_not_of", "find_first_of",
143                             "find_last_not_of", "find_last_of", "rfind")))),
144                         anyOf(argumentCountIs(1), argumentCountIs(2)),
145                         hasArgument(0, StringCStrCallExpr)),
146       this);
147 
148   // Detect: 'dst.insert(pos, str.c_str())'  ->  'dst.insert(pos, str)'
149   Finder->addMatcher(
150       cxxMemberCallExpr(on(StringExpr),
151                         callee(decl(cxxMethodDecl(hasName("insert")))),
152                         argumentCountIs(2), hasArgument(1, StringCStrCallExpr)),
153       this);
154 
155   // Detect redundant 'c_str()' calls through a StringRef constructor.
156   Finder->addMatcher(
157       cxxConstructExpr(
158           // Implicit constructors of these classes are overloaded
159           // wrt. string types and they internally make a StringRef
160           // referring to the argument.  Passing a string directly to
161           // them is preferred to passing a char pointer.
162           hasDeclaration(cxxMethodDecl(hasAnyName(
163               "::llvm::StringRef::StringRef", "::llvm::Twine::Twine"))),
164           argumentCountIs(1),
165           // The only argument must have the form x.c_str() or p->c_str()
166           // where the method is string::c_str().  StringRef also has
167           // a constructor from string which is more efficient (avoids
168           // strlen), so we can construct StringRef from the string
169           // directly.
170           hasArgument(0, StringCStrCallExpr)),
171       this);
172 }
173 
174 void RedundantStringCStrCheck::check(const MatchFinder::MatchResult &Result) {
175   const auto *Call = Result.Nodes.getNodeAs<CallExpr>("call");
176   const auto *Arg = Result.Nodes.getNodeAs<Expr>("arg");
177   const auto *Member = Result.Nodes.getNodeAs<MemberExpr>("member");
178   bool Arrow = Member->isArrow();
179   // Replace the "call" node with the "arg" node, prefixed with '*'
180   // if the call was using '->' rather than '.'.
181   std::string ArgText =
182       Arrow ? formatDereference(Result, *Arg)
183             : tooling::fixit::getText(*Arg, *Result.Context).str();
184   if (ArgText.empty())
185     return;
186 
187   diag(Call->getBeginLoc(), "redundant call to %0")
188       << Member->getMemberDecl()
189       << FixItHint::CreateReplacement(Call->getSourceRange(), ArgText);
190 }
191 
192 } // namespace readability
193 } // namespace tidy
194 } // namespace clang
195