[clang-tools-extra] [clang-tidy] add new check readability-enum-initial-value (PR #86129)

[Congcong Cai via cfe-commits]

================
@@ -0,0 +1,82 @@
+//===--- EnumInitialValueCheck.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 "EnumInitialValueCheck.h"
+#include "../utils/LexerUtils.h"
+#include "clang/AST/Decl.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Basic/Diagnostic.h"
+#include "llvm/ADT/SmallString.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+namespace {
+
+AST_MATCHER(EnumDecl, isNoneEnumeratorsInitialized) {
+  return llvm::all_of(Node.enumerators(), [](EnumConstantDecl const *ECD) {
+    return ECD->getInitExpr() == nullptr;
+  });
+}
+
+AST_MATCHER(EnumDecl, isOnlyFirstEnumeratorsInitialized) {
+  for (EnumConstantDecl const *ECD : Node.enumerators())
+    if (ECD == *Node.enumerator_begin()) {
+      if (ECD->getInitExpr() == nullptr)
+        return false;
+    } else {
+      if (ECD->getInitExpr() != nullptr)
+        return false;
+    }
+  return true;
+}
+
+AST_MATCHER(EnumDecl, isAllEnumeratorsInitialized) {
+  return llvm::all_of(Node.enumerators(), [](EnumConstantDecl const *ECD) {
+    return ECD->getInitExpr() != nullptr;
+  });
+}
+
+} // namespace
+
+void EnumInitialValueCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(enumDecl(unless(anyOf(isNoneEnumeratorsInitialized(),
+                                           isOnlyFirstEnumeratorsInitialized(),
+                                           isAllEnumeratorsInitialized())))
----------------
HerrCai0907 wrote:

I will add some options to make it possible to let it strict.
But I don't find the code guideline about "all enums are initialized but with consecutive values, in such case those inits could be removed and only first could be left". Inferring the usage of enum is hard and impossible, I think adding this check will create lots of false positive.
For example 
```c++
enum A {
a = 0,
b = 1,
c = 2,
}
```
can also be a bitmap.

https://github.com/llvm/llvm-project/pull/86129