1 //===--- UnusedUsingDeclsCheck.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 "UnusedUsingDeclsCheck.h"
10 #include "clang/AST/ASTContext.h"
11 #include "clang/ASTMatchers/ASTMatchFinder.h"
12 #include "clang/Lex/Lexer.h"
13 
14 using namespace clang::ast_matchers;
15 
16 namespace clang {
17 namespace tidy {
18 namespace misc {
19 
20 namespace {
21 
22 AST_MATCHER_P(DeducedTemplateSpecializationType, refsToTemplatedDecl,
23               clang::ast_matchers::internal::Matcher<NamedDecl>, DeclMatcher) {
24   if (const auto *TD = Node.getTemplateName().getAsTemplateDecl())
25     return DeclMatcher.matches(*TD, Finder, Builder);
26   return false;
27 }
28 
29 } // namespace
30 
31 // A function that helps to tell whether a TargetDecl in a UsingDecl will be
32 // checked. Only variable, function, function template, class template, class,
33 // enum declaration and enum constant declaration are considered.
34 static bool shouldCheckDecl(const Decl *TargetDecl) {
35   return isa<RecordDecl>(TargetDecl) || isa<ClassTemplateDecl>(TargetDecl) ||
36          isa<FunctionDecl>(TargetDecl) || isa<VarDecl>(TargetDecl) ||
37          isa<FunctionTemplateDecl>(TargetDecl) || isa<EnumDecl>(TargetDecl) ||
38          isa<EnumConstantDecl>(TargetDecl);
39 }
40 
41 void UnusedUsingDeclsCheck::registerMatchers(MatchFinder *Finder) {
42   Finder->addMatcher(usingDecl(isExpansionInMainFile()).bind("using"), this);
43   auto DeclMatcher = hasDeclaration(namedDecl().bind("used"));
44   Finder->addMatcher(loc(templateSpecializationType(DeclMatcher)), this);
45   Finder->addMatcher(loc(deducedTemplateSpecializationType(
46                          refsToTemplatedDecl(namedDecl().bind("used")))),
47                      this);
48   Finder->addMatcher(callExpr(callee(unresolvedLookupExpr().bind("used"))),
49                      this);
50   Finder->addMatcher(
51       callExpr(hasDeclaration(functionDecl(
52           forEachTemplateArgument(templateArgument().bind("used"))))),
53       this);
54   Finder->addMatcher(loc(templateSpecializationType(forEachTemplateArgument(
55                          templateArgument().bind("used")))),
56                      this);
57   // Cases where we can identify the UsingShadowDecl directly, rather than
58   // just its target.
59   // FIXME: cover more cases in this way, as the AST supports it.
60   auto ThroughShadowMatcher = throughUsingDecl(namedDecl().bind("usedShadow"));
61   Finder->addMatcher(declRefExpr(ThroughShadowMatcher), this);
62   Finder->addMatcher(loc(usingType(ThroughShadowMatcher)), this);
63 }
64 
65 void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
66   if (Result.Context->getDiagnostics().hasUncompilableErrorOccurred())
67     return;
68 
69   if (const auto *Using = Result.Nodes.getNodeAs<UsingDecl>("using")) {
70     // Ignores using-declarations defined in macros.
71     if (Using->getLocation().isMacroID())
72       return;
73 
74     // Ignores using-declarations defined in class definition.
75     if (isa<CXXRecordDecl>(Using->getDeclContext()))
76       return;
77 
78     // FIXME: We ignore using-decls defined in function definitions at the
79     // moment because of false positives caused by ADL and different function
80     // scopes.
81     if (isa<FunctionDecl>(Using->getDeclContext()))
82       return;
83 
84     UsingDeclContext Context(Using);
85     Context.UsingDeclRange = CharSourceRange::getCharRange(
86         Using->getBeginLoc(),
87         Lexer::findLocationAfterToken(
88             Using->getEndLoc(), tok::semi, *Result.SourceManager, getLangOpts(),
89             /*SkipTrailingWhitespaceAndNewLine=*/true));
90     for (const auto *UsingShadow : Using->shadows()) {
91       const auto *TargetDecl = UsingShadow->getTargetDecl()->getCanonicalDecl();
92       if (shouldCheckDecl(TargetDecl))
93         Context.UsingTargetDecls.insert(TargetDecl);
94     }
95     if (!Context.UsingTargetDecls.empty())
96       Contexts.push_back(Context);
97     return;
98   }
99 
100   // Mark a corresponding using declaration as used.
101   auto RemoveNamedDecl = [&](const NamedDecl *Used) {
102     removeFromFoundDecls(Used);
103     // Also remove variants of Used.
104     if (const auto *FD = dyn_cast<FunctionDecl>(Used)) {
105       removeFromFoundDecls(FD->getPrimaryTemplate());
106     } else if (const auto *Specialization =
107                    dyn_cast<ClassTemplateSpecializationDecl>(Used)) {
108       removeFromFoundDecls(Specialization->getSpecializedTemplate());
109     } else if (const auto *FD = dyn_cast<FunctionDecl>(Used)) {
110       if (const auto *FDT = FD->getPrimaryTemplate())
111         removeFromFoundDecls(FDT);
112     } else if (const auto *ECD = dyn_cast<EnumConstantDecl>(Used)) {
113       if (const auto *ET = ECD->getType()->getAs<EnumType>())
114         removeFromFoundDecls(ET->getDecl());
115     }
116   };
117   // We rely on the fact that the clang AST is walked in order, usages are only
118   // marked after a corresponding using decl has been found.
119   if (const auto *Used = Result.Nodes.getNodeAs<NamedDecl>("used")) {
120     RemoveNamedDecl(Used);
121     return;
122   }
123 
124   if (const auto *UsedShadow =
125           Result.Nodes.getNodeAs<UsingShadowDecl>("usedShadow")) {
126     removeFromFoundDecls(UsedShadow->getTargetDecl());
127     return;
128   }
129 
130   if (const auto *Used = Result.Nodes.getNodeAs<TemplateArgument>("used")) {
131     if (Used->getKind() == TemplateArgument::Template) {
132       if (const auto *TD = Used->getAsTemplate().getAsTemplateDecl())
133         removeFromFoundDecls(TD);
134     } else if (Used->getKind() == TemplateArgument::Type) {
135       if (auto *RD = Used->getAsType()->getAsCXXRecordDecl())
136         removeFromFoundDecls(RD);
137     } else if (Used->getKind() == TemplateArgument::Declaration) {
138       RemoveNamedDecl(Used->getAsDecl());
139     }
140     return;
141   }
142 
143   if (const auto *DRE = Result.Nodes.getNodeAs<DeclRefExpr>("used")) {
144     RemoveNamedDecl(DRE->getDecl());
145     return;
146   }
147   // Check the uninstantiated template function usage.
148   if (const auto *ULE = Result.Nodes.getNodeAs<UnresolvedLookupExpr>("used")) {
149     for (const NamedDecl *ND : ULE->decls()) {
150       if (const auto *USD = dyn_cast<UsingShadowDecl>(ND))
151         removeFromFoundDecls(USD->getTargetDecl()->getCanonicalDecl());
152     }
153   }
154 }
155 
156 void UnusedUsingDeclsCheck::removeFromFoundDecls(const Decl *D) {
157   if (!D)
158     return;
159   // FIXME: Currently, we don't handle the using-decls being used in different
160   // scopes (such as different namespaces, different functions). Instead of
161   // giving an incorrect message, we mark all of them as used.
162   //
163   // FIXME: Use a more efficient way to find a matching context.
164   for (auto &Context : Contexts) {
165     if (Context.UsingTargetDecls.contains(D->getCanonicalDecl()))
166       Context.IsUsed = true;
167   }
168 }
169 
170 void UnusedUsingDeclsCheck::onEndOfTranslationUnit() {
171   for (const auto &Context : Contexts) {
172     if (!Context.IsUsed) {
173       diag(Context.FoundUsingDecl->getLocation(), "using decl %0 is unused")
174           << Context.FoundUsingDecl;
175       // Emit a fix and a fix description of the check;
176       diag(Context.FoundUsingDecl->getLocation(),
177            /*Description=*/"remove the using", DiagnosticIDs::Note)
178           << FixItHint::CreateRemoval(Context.UsingDeclRange);
179     }
180   }
181   Contexts.clear();
182 }
183 
184 } // namespace misc
185 } // namespace tidy
186 } // namespace clang
187