[clang] Respect the [[clang::unsafe_buffer_usage]] attribute for field and constructor initializers (PR #91991)

[Dana Jansens via cfe-commits]

================
@@ -3328,3 +3300,63 @@ void clang::checkUnsafeBufferUsage(const Decl *D,
     }
   }
 }
+
+void clang::checkUnsafeBufferUsage(const Decl *D,
+                                   UnsafeBufferUsageHandler &Handler,
+                                   bool EmitSuggestions) {
+#ifndef NDEBUG
+  Handler.clearDebugNotes();
+#endif
+
+  assert(D);
+
+  SmallVector<Stmt *> Stmts;
+
+  // We do not want to visit a Lambda expression defined inside a method
+  // independently. Instead, it should be visited along with the outer method.
+  // FIXME: do we want to do the same thing for `BlockDecl`s?
+  if (const auto *fd = dyn_cast<CXXMethodDecl>(D)) {
+    if (fd->getParent()->isLambda() && fd->getParent()->isLocalClass())
+      return;
+  }
+
+  // Do not emit fixit suggestions for functions declared in an
+  // extern "C" block.
+  if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
+    for (FunctionDecl *FReDecl : FD->redecls()) {
+      if (FReDecl->isExternC()) {
+        EmitSuggestions = false;
+        break;
+      }
+    }
+
+    Stmts.push_back(FD->getBody());
+
+    if (const auto *ID = dyn_cast<CXXConstructorDecl>(D)) {
+      for (const CXXCtorInitializer *CI : ID->inits()) {
+        Stmts.push_back(CI->getInit());
+      }
+    }
+  }
+
+  if (const auto *FD = dyn_cast<FieldDecl>(D)) {
----------------
danakj wrote:

I see. This test has `TraverseCXXDefaultInitExpr` occur:

```cc
// RUN: %clang_cc1 -std=c++20 -Wunsafe-buffer-usage \
// RUN:            -fsafe-buffer-usage-suggestions -verify %s

class UnsafeMembers {
public:
    [[clang::unsafe_buffer_usage]]
    UnsafeMembers(int) {}
};

struct HoldsUnsafeMembers {
    HoldsUnsafeMembers() {}
    
    UnsafeMembers FromField{3};  // expected-warning{{function introduces unsafe buffer manipulation}}
};
```

But if the explicit ctor is omitted, then there is no such `TraverseCXXDefaultInitExpr` and the FieldDecl must be visited, unless there's something else to anchor it on.

https://github.com/llvm/llvm-project/pull/91991