1 //===--- MakeMemberFunctionConstCheck.cpp - clang-tidy --------------------===//
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 #include "MakeMemberFunctionConstCheck.h"
10 #include "clang/AST/ASTContext.h"
11 #include "clang/AST/ParentMapContext.h"
12 #include "clang/AST/RecursiveASTVisitor.h"
13 #include "clang/ASTMatchers/ASTMatchFinder.h"
14 #include "clang/Lex/Lexer.h"
15 
16 using namespace clang::ast_matchers;
17 
18 namespace clang {
19 namespace tidy {
20 namespace readability {
21 
22 AST_MATCHER(CXXMethodDecl, isStatic) { return Node.isStatic(); }
23 
24 AST_MATCHER(CXXMethodDecl, hasTrivialBody) { return Node.hasTrivialBody(); }
25 
26 AST_MATCHER(CXXRecordDecl, hasAnyDependentBases) {
27   return Node.hasAnyDependentBases();
28 }
29 
30 AST_MATCHER(CXXMethodDecl, isTemplate) {
31   return Node.getTemplatedKind() != FunctionDecl::TK_NonTemplate;
32 }
33 
34 AST_MATCHER(CXXMethodDecl, isDependentContext) {
35   return Node.isDependentContext();
36 }
37 
38 AST_MATCHER(CXXMethodDecl, isInsideMacroDefinition) {
39   const ASTContext &Ctxt = Finder->getASTContext();
40   return clang::Lexer::makeFileCharRange(
41              clang::CharSourceRange::getCharRange(
42                  Node.getTypeSourceInfo()->getTypeLoc().getSourceRange()),
43              Ctxt.getSourceManager(), Ctxt.getLangOpts())
44       .isInvalid();
45 }
46 
47 AST_MATCHER_P(CXXMethodDecl, hasCanonicalDecl,
48               ast_matchers::internal::Matcher<CXXMethodDecl>, InnerMatcher) {
49   return InnerMatcher.matches(*Node.getCanonicalDecl(), Finder, Builder);
50 }
51 
52 enum UsageKind { Unused, Const, NonConst };
53 
54 class FindUsageOfThis : public RecursiveASTVisitor<FindUsageOfThis> {
55   ASTContext &Ctxt;
56 
57 public:
58   FindUsageOfThis(ASTContext &Ctxt) : Ctxt(Ctxt) {}
59   UsageKind Usage = Unused;
60 
61   template <class T> const T *getParent(const Expr *E) {
62     DynTypedNodeList Parents = Ctxt.getParents(*E);
63     if (Parents.size() != 1)
64       return nullptr;
65 
66     return Parents.begin()->get<T>();
67   }
68 
69   bool VisitUnresolvedMemberExpr(const UnresolvedMemberExpr *) {
70     // An UnresolvedMemberExpr might resolve to a non-const non-static
71     // member function.
72     Usage = NonConst;
73     return false; // Stop traversal.
74   }
75 
76   bool VisitCXXConstCastExpr(const CXXConstCastExpr *) {
77     // Workaround to support the pattern
78     // class C {
79     //   const S *get() const;
80     //   S* get() {
81     //     return const_cast<S*>(const_cast<const C*>(this)->get());
82     //   }
83     // };
84     // Here, we don't want to make the second 'get' const even though
85     // it only calls a const member function on this.
86     Usage = NonConst;
87     return false; // Stop traversal.
88   }
89 
90   // Our AST is
91   //  `-ImplicitCastExpr
92   //  (possibly `-UnaryOperator Deref)
93   //        `-CXXThisExpr 'S *' this
94   bool visitUser(const ImplicitCastExpr *Cast) {
95     if (Cast->getCastKind() != CK_NoOp)
96       return false; // Stop traversal.
97 
98     // Only allow NoOp cast to 'const S' or 'const S *'.
99     QualType QT = Cast->getType();
100     if (QT->isPointerType())
101       QT = QT->getPointeeType();
102 
103     if (!QT.isConstQualified())
104       return false; // Stop traversal.
105 
106     const auto *Parent = getParent<Stmt>(Cast);
107     if (!Parent)
108       return false; // Stop traversal.
109 
110     if (isa<ReturnStmt>(Parent))
111       return true; // return (const S*)this;
112 
113     if (isa<CallExpr>(Parent))
114       return true; // use((const S*)this);
115 
116     // ((const S*)this)->Member
117     if (const auto *Member = dyn_cast<MemberExpr>(Parent))
118       return visitUser(Member, /*OnConstObject=*/true);
119 
120     return false; // Stop traversal.
121   }
122 
123   // If OnConstObject is true, then this is a MemberExpr using
124   // a constant this, i.e. 'const S' or 'const S *'.
125   bool visitUser(const MemberExpr *Member, bool OnConstObject) {
126     if (Member->isBoundMemberFunction(Ctxt)) {
127       if (!OnConstObject || Member->getFoundDecl().getAccess() != AS_public) {
128         // Non-public non-static member functions might not preserve the
129         // logical constness. E.g. in
130         // class C {
131         //   int &data() const;
132         // public:
133         //   int &get() { return data(); }
134         // };
135         // get() uses a private const method, but must not be made const
136         // itself.
137         return false; // Stop traversal.
138       }
139       // Using a public non-static const member function.
140       return true;
141     }
142 
143     const auto *Parent = getParent<Expr>(Member);
144 
145     if (const auto *Cast = dyn_cast_or_null<ImplicitCastExpr>(Parent)) {
146       // A read access to a member is safe when the member either
147       // 1) has builtin type (a 'const int' cannot be modified),
148       // 2) or it's a public member (the pointee of a public 'int * const' can
149       // can be modified by any user of the class).
150       if (Member->getFoundDecl().getAccess() != AS_public &&
151           !Cast->getType()->isBuiltinType())
152         return false;
153 
154       if (Cast->getCastKind() == CK_LValueToRValue)
155         return true;
156 
157       if (Cast->getCastKind() == CK_NoOp && Cast->getType().isConstQualified())
158         return true;
159     }
160 
161     if (const auto *M = dyn_cast_or_null<MemberExpr>(Parent))
162       return visitUser(M, /*OnConstObject=*/false);
163 
164     return false; // Stop traversal.
165   }
166 
167   bool VisitCXXThisExpr(const CXXThisExpr *E) {
168     Usage = Const;
169 
170     const auto *Parent = getParent<Expr>(E);
171 
172     // Look through deref of this.
173     if (const auto *UnOp = dyn_cast_or_null<UnaryOperator>(Parent)) {
174       if (UnOp->getOpcode() == UO_Deref) {
175         Parent = getParent<Expr>(UnOp);
176       }
177     }
178 
179     // It's okay to
180     //  return (const S*)this;
181     //  use((const S*)this);
182     //  ((const S*)this)->f()
183     // when 'f' is a public member function.
184     if (const auto *Cast = dyn_cast_or_null<ImplicitCastExpr>(Parent)) {
185       if (visitUser(Cast))
186         return true;
187 
188       // And it's also okay to
189       //   (const T)(S->t)
190       //   (LValueToRValue)(S->t)
191       // when 't' is either of builtin type or a public member.
192     } else if (const auto *Member = dyn_cast_or_null<MemberExpr>(Parent)) {
193       if (visitUser(Member, /*OnConstObject=*/false))
194         return true;
195     }
196 
197     // Unknown user of this.
198     Usage = NonConst;
199     return false; // Stop traversal.
200   }
201 };
202 
203 AST_MATCHER(CXXMethodDecl, usesThisAsConst) {
204   FindUsageOfThis UsageOfThis(Finder->getASTContext());
205 
206   // TraverseStmt does not modify its argument.
207   UsageOfThis.TraverseStmt(const_cast<Stmt *>(Node.getBody()));
208 
209   return UsageOfThis.Usage == Const;
210 }
211 
212 void MakeMemberFunctionConstCheck::registerMatchers(MatchFinder *Finder) {
213   Finder->addMatcher(
214       traverse(
215           TK_AsIs,
216           cxxMethodDecl(
217               isDefinition(), isUserProvided(),
218               unless(anyOf(
219                   isExpansionInSystemHeader(), isVirtual(), isConst(),
220                   isStatic(), hasTrivialBody(), cxxConstructorDecl(),
221                   cxxDestructorDecl(), isTemplate(), isDependentContext(),
222                   ofClass(anyOf(isLambda(),
223                                 hasAnyDependentBases()) // Method might become
224                                                         // virtual depending on
225                                                         // template base class.
226                           ),
227                   isInsideMacroDefinition(),
228                   hasCanonicalDecl(isInsideMacroDefinition()))),
229               usesThisAsConst())
230               .bind("x")),
231       this);
232 }
233 
234 static SourceLocation getConstInsertionPoint(const CXXMethodDecl *M) {
235   TypeSourceInfo *TSI = M->getTypeSourceInfo();
236   if (!TSI)
237     return {};
238 
239   FunctionTypeLoc FTL =
240       TSI->getTypeLoc().IgnoreParens().getAs<FunctionTypeLoc>();
241   if (!FTL)
242     return {};
243 
244   return FTL.getRParenLoc().getLocWithOffset(1);
245 }
246 
247 void MakeMemberFunctionConstCheck::check(
248     const MatchFinder::MatchResult &Result) {
249   const auto *Definition = Result.Nodes.getNodeAs<CXXMethodDecl>("x");
250 
251   const auto *Declaration = Definition->getCanonicalDecl();
252 
253   auto Diag = diag(Definition->getLocation(), "method %0 can be made const")
254               << Definition
255               << FixItHint::CreateInsertion(getConstInsertionPoint(Definition),
256                                             " const");
257   if (Declaration != Definition) {
258     Diag << FixItHint::CreateInsertion(getConstInsertionPoint(Declaration),
259                                        " const");
260   }
261 }
262 
263 } // namespace readability
264 } // namespace tidy
265 } // namespace clang
266