[clang-tidy] Teach cppcoreguidelines-interfaces-global-init about constinit (#148334)

This check already understands how `constexpr` makes initialization
order problems impossible, and C++20's `constinit` provides the exact
same guarantees.

Author: localspook

clang-tools-extra/clang-tidy/cppcoreguidelines/InterfacesGlobalInitCheck.cpp

@@ -19,7 +19,7 @@ void InterfacesGlobalInitCheck::registerMatchers(MatchFinder *Finder) {
               hasDeclContext(anyOf(translationUnitDecl(), // Global scope.
                                    namespaceDecl(),       // Namespace scope.
                                    recordDecl())),        // Class scope.
-              unless(isConstexpr()));
+              unless(isConstexpr()), unless(isConstinit()));
 
   const auto ReferencesUndefinedGlobalVar = declRefExpr(hasDeclaration(
       varDecl(GlobalVarDecl, unless(isDefinition())).bind("referencee")));

clang-tools-extra/docs/ReleaseNotes.rst

@@ -232,6 +232,10 @@ Changes in existing checks
   <clang-tidy/checks/cppcoreguidelines/avoid-goto>` check by adding the option
   `IgnoreMacros` to ignore ``goto`` labels defined in macros.
 
+- Improved :doc:`cppcoreguidelines-interfaces-global-init
+  <clang-tidy/checks/cppcoreguidelines/interfaces-global-init>` check by
+  fixing false positives on uses of ``constinit`` variables.
+
 - Improved :doc:`cppcoreguidelines-missing-std-forward
   <clang-tidy/checks/cppcoreguidelines/missing-std-forward>` check by adding a
   flag to specify the function used for forwarding instead of ``std::forward``.

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/interfaces-global-init.cpp

@@ -14,11 +14,23 @@ static int GlobalScopeBadInit3 = takesIntPtr(&ExternGlobal);
 static int GlobalScopeBadInit4 = 3 * (ExternGlobal + 2);
 // CHECK-MESSAGES: [[@LINE-1]]:12: warning: initializing non-local variable with non-const expression depending on uninitialized non-local variable 'ExternGlobal'
 
+#if __cplusplus >= 202002L
+extern constinit int ExternConstinitGlobal;
+static int GlobalScopeConstinit1 = ExternConstinitGlobal;
+static int GlobalScopeConstinit2 = takesInt(ExternConstinitGlobal);
+static int GlobalScopeConstinit3 = takesIntPtr(&ExternConstinitGlobal);
+static int GlobalScopeConstinit4 = 3 * (ExternConstinitGlobal + 2);
+#endif
+
 namespace ns {
 static int NamespaceScope = makesInt();
 static int NamespaceScopeBadInit = takesInt(ExternGlobal);
 // CHECK-MESSAGES: [[@LINE-1]]:12: warning: initializing non-local variable with non-const expression depending on uninitialized non-local variable 'ExternGlobal'
 
+#if __cplusplus >= 202002L
+static int NamespaceScopeConstinit = takesInt(ExternConstinitGlobal);
+#endif
+
 struct A {
   static int ClassScope;
   static int ClassScopeBadInit;
@@ -29,6 +41,17 @@ int A::ClassScopeBadInit = takesInt(ExternGlobal);
 
 static int FromClassBadInit = takesInt(A::ClassScope);
 // CHECK-MESSAGES: [[@LINE-1]]:12: warning: initializing non-local variable with non-const expression depending on uninitialized non-local variable 'ClassScope'
+
+#if __cplusplus >= 202002L
+struct B {
+  static constinit int ClassScopeConstinit;
+  static int ClassScopeFromConstinit;
+};
+
+int B::ClassScopeFromConstinit = takesInt(ExternConstinitGlobal);
+static int FromClassScopeConstinit = takesInt(B::ClassScopeConstinit);
+#endif
+
 } // namespace ns
 
 // "const int B::I;" is fine, it just ODR-defines B::I. See [9.4.3] Static
@@ -42,6 +65,16 @@ const int B1::J;
 // CHECK-MESSAGES: [[@LINE-1]]:15: warning: initializing non-local variable with non-const expression depending on uninitialized non-local variable 'I'
 const int B1::I;
 
+#if __cplusplus >= 202002L
+class D {
+  static const constinit int I = 0;
+  static const int J = I;
+};
+
+const int D::J;
+const int D::I;
+#endif
+
 void f() {
   // This is fine, it's executed after dynamic initialization occurs.
   static int G = takesInt(ExternGlobal);
@@ -81,4 +114,3 @@ class B2 {
 };
 const int B2::I;
 const int B2::J;
-