1 //===--- ImplicitConversionInLoopCheck.cpp - clang-tidy--------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #include "ImplicitConversionInLoopCheck.h"
11 
12 #include "clang/AST/ASTContext.h"
13 #include "clang/AST/Decl.h"
14 #include "clang/ASTMatchers/ASTMatchFinder.h"
15 #include "clang/ASTMatchers/ASTMatchers.h"
16 #include "clang/Lex/Lexer.h"
17 
18 using namespace clang::ast_matchers;
19 
20 namespace clang {
21 namespace tidy {
22 namespace performance {
23 
24 // Checks if the stmt is a ImplicitCastExpr with a CastKind that is not a NoOp.
25 // The subtelty is that in some cases (user defined conversions), we can
26 // get to ImplicitCastExpr inside each other, with the outer one a NoOp. In this
27 // case we skip the first cast expr.
28 static bool IsNonTrivialImplicitCast(const Stmt *ST) {
29   if (const auto *ICE = dyn_cast<ImplicitCastExpr>(ST)) {
30     return (ICE->getCastKind() != CK_NoOp) ||
31             IsNonTrivialImplicitCast(ICE->getSubExpr());
32   }
33   return false;
34 }
35 
36 void ImplicitConversionInLoopCheck::registerMatchers(MatchFinder *Finder) {
37   // We look for const ref loop variables that (optionally inside an
38   // ExprWithCleanup) materialize a temporary, and contain a implicit
39   // conversion. The check on the implicit conversion is done in check() because
40   // we can't access implicit conversion subnode via matchers: has() skips casts
41   // and materialize! We also bind on the call to operator* to get the proper
42   // type in the diagnostic message. We use both cxxOperatorCallExpr for user
43   // defined operator and unaryOperator when the iterator is a pointer, like
44   // for arrays or std::array.
45   //
46   // Note that when the implicit conversion is done through a user defined
47   // conversion operator, the node is a CXXMemberCallExpr, not a
48   // CXXOperatorCallExpr, so it should not get caught by the
49   // cxxOperatorCallExpr() matcher.
50   Finder->addMatcher(
51       cxxForRangeStmt(hasLoopVariable(
52           varDecl(
53               hasType(qualType(references(qualType(isConstQualified())))),
54               hasInitializer(
55                   expr(anyOf(hasDescendant(
56                                  cxxOperatorCallExpr().bind("operator-call")),
57                              hasDescendant(unaryOperator(hasOperatorName("*"))
58                                                .bind("operator-call"))))
59                       .bind("init")))
60               .bind("faulty-var"))),
61       this);
62 }
63 
64 void ImplicitConversionInLoopCheck::check(
65     const MatchFinder::MatchResult &Result) {
66   const auto *VD = Result.Nodes.getNodeAs<VarDecl>("faulty-var");
67   const auto *Init = Result.Nodes.getNodeAs<Expr>("init");
68   const auto *OperatorCall =
69       Result.Nodes.getNodeAs<Expr>("operator-call");
70 
71   if (const auto *Cleanup = dyn_cast<ExprWithCleanups>(Init))
72     Init = Cleanup->getSubExpr();
73 
74   const auto *Materialized = dyn_cast<MaterializeTemporaryExpr>(Init);
75   if (!Materialized)
76     return;
77 
78   // We ignore NoOp casts. Those are generated if the * operator on the
79   // iterator returns a value instead of a reference, and the loop variable
80   // is a reference. This situation is fine (it probably produces the same
81   // code at the end).
82   if (IsNonTrivialImplicitCast(Materialized->getTemporary()))
83     ReportAndFix(Result.Context, VD, OperatorCall);
84 }
85 
86 void ImplicitConversionInLoopCheck::ReportAndFix(
87     const ASTContext *Context, const VarDecl *VD,
88     const Expr *OperatorCall) {
89   // We only match on const ref, so we should print a const ref version of the
90   // type.
91   QualType ConstType = OperatorCall->getType().withConst();
92   QualType ConstRefType = Context->getLValueReferenceType(ConstType);
93   const char Message[] =
94       "the type of the loop variable %0 is different from the one returned "
95       "by the iterator and generates an implicit conversion; you can either "
96       "change the type to the matching one (%1 but 'const auto&' is always a "
97       "valid option) or remove the reference to make it explicit that you are "
98       "creating a new value";
99   diag(VD->getLocStart(), Message) << VD << ConstRefType;
100 }
101 
102 } // namespace performance
103 } // namespace tidy
104 } // namespace clang
105