[clang-tidy] Add check for classes missing -hash ⚠️

Summary:
Apple documentation states that:
"If two objects are equal, they must have the same hash value. This last
point is particularly important if you define isEqual: in a subclass and
intend to put instances of that subclass into a collection. Make sure
you also define hash in your subclass."
https://developer.apple.com/documentation/objectivec/1418956-nsobject/1418795-isequal?language=objc

In many or all versions of libobjc, -[NSObject isEqual:] is a pointer
equality check and -[NSObject hash] returns the messaged object's
pointer. A relatively common form of developer error is for a developer to
override -isEqual: in a subclass without overriding -hash to ensure that
hashes are equal for objects that are equal.

It is assumed that an override of -isEqual: is a strong signal for
changing the object's equality operator to something other than pointer
equality which implies that a missing override of -hash could result in
distinct objects being equal but having distinct hashes because they are
independent instances. This added check flags classes that override
-isEqual: but inherit NSObject's implementation of -hash to warn of the
potential for unexpected behavior.

The proper implementation of -hash is the responsibility of the
developer and the check will only verify that the developer made an
effort to properly implement -hash. Developers can set up unit tests
to verify that their implementation of -hash is appropriate.

Test Notes:
Ran check-clang-tools.

Reviewers: aaron.ballman, benhamilton

Reviewed By: aaron.ballman

Subscribers: Eugene.Zelenko, mgorny, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67737

llvm-svn: 372445

4 files changed, 101 insertions, 0 deletions

diff --git a/clang-tools-extra/clang-tidy/objc/CMakeLists.txt b/clang-tools-extra/clang-tidy/objc/CMakeLists.txt
index 4eeb14844a5..0a12e4a850d 100644
--- a/clang-tools-extra/clang-tidy/objc/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/objc/CMakeLists.txt
@@ -4,6 +4,7 @@ add_clang_library(clangTidyObjCModule
   AvoidNSErrorInitCheck.cpp
   AvoidSpinlockCheck.cpp
   ForbiddenSubclassingCheck.cpp
+  MissingHashCheck.cpp
   ObjCTidyModule.cpp
   PropertyDeclarationCheck.cpp
   SuperSelfCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/objc/MissingHashCheck.cpp b/clang-tools-extra/clang-tidy/objc/MissingHashCheck.cpp
new file mode 100644
index 00000000000..0da5571b2f2
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/objc/MissingHashCheck.cpp
@@ -0,0 +1,62 @@
+//===--- MissingHashCheck.cpp - clang-tidy --------------------------------===//
+//
+// 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 "MissingHashCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace objc {
+
+namespace {
+
+AST_MATCHER_P(ObjCImplementationDecl, hasInterface,
+              ast_matchers::internal::Matcher<ObjCInterfaceDecl>, Base) {
+  const ObjCInterfaceDecl *InterfaceDecl = Node.getClassInterface();
+  return Base.matches(*InterfaceDecl, Finder, Builder);
+}
+
+AST_MATCHER_P(ObjCContainerDecl, hasInstanceMethod,
+              ast_matchers::internal::Matcher<ObjCMethodDecl>, Base) {
+  // Check each instance method against the provided matcher.
+  for (const auto *I : Node.instance_methods()) {
+    if (Base.matches(*I, Finder, Builder))
+      return true;
+  }
+  return false;
+}
+
+} // namespace
+
+void MissingHashCheck::registerMatchers(MatchFinder *Finder) {
+  // This check should only be applied to Objective-C sources.
+  if (!getLangOpts().ObjC)
+    return;
+
+  Finder->addMatcher(
+      objcMethodDecl(
+          hasName("isEqual:"), isInstanceMethod(),
+          hasDeclContext(objcImplementationDecl(
+                             hasInterface(isDirectlyDerivedFrom("NSObject")),
+                             unless(hasInstanceMethod(hasName("hash"))))
+                             .bind("impl"))),
+      this);
+}
+
+void MissingHashCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *ID = Result.Nodes.getNodeAs<ObjCImplementationDecl>("impl");
+  diag(ID->getLocation(), "%0 implements -isEqual: without implementing -hash")
+      << ID;
+}
+
+} // namespace objc
+} // namespace tidy
+} // namespace clang
diff --git a/clang-tools-extra/clang-tidy/objc/MissingHashCheck.h b/clang-tools-extra/clang-tidy/objc/MissingHashCheck.h
new file mode 100644
index 00000000000..4ac74d58652
--- /dev/null
+++ b/clang-tools-extra/clang-tidy/objc/MissingHashCheck.h
@@ -0,0 +1,35 @@
+//===--- MissingHashCheck.h - clang-tidy ------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_MISSINGHASHCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_MISSINGHASHCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang {
+namespace tidy {
+namespace objc {
+
+/// Finds Objective-C implementations that implement -isEqual: without also
+/// appropriately implementing -hash.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/objc-missing-hash.html
+class MissingHashCheck : public ClangTidyCheck {
+public:
+  MissingHashCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace objc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_OBJC_MISSINGHASHCHECK_H
diff --git a/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp b/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
index 636e2c02af6..c9b57d5e2c8 100644
--- a/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
+++ b/clang-tools-extra/clang-tidy/objc/ObjCTidyModule.cpp
@@ -12,6 +12,7 @@
 #include "AvoidNSErrorInitCheck.h"
 #include "AvoidSpinlockCheck.h"
 #include "ForbiddenSubclassingCheck.h"
+#include "MissingHashCheck.h"
 #include "PropertyDeclarationCheck.h"
 #include "SuperSelfCheck.h"
 
@@ -30,6 +31,8 @@ public:
         "objc-avoid-spinlock");
     CheckFactories.registerCheck<ForbiddenSubclassingCheck>(
         "objc-forbidden-subclassing");
+    CheckFactories.registerCheck<MissingHashCheck>(
+        "objc-missing-hash");
     CheckFactories.registerCheck<PropertyDeclarationCheck>(
         "objc-property-declaration");
     CheckFactories.registerCheck<SuperSelfCheck>(