Merge pull request MicrosoftDocs#3614 from ktoliver/public-3187

[PUBLIC_MOVE] commit from public PR 3187

Author: ttorble

docs/code-quality/c6389.md

@@ -0,0 +1,57 @@
+---
+title: C6389
+description: "Describes the Microsoft C/C++ code analysis warning C6389, its causes, and how to address it."
+ms.date: 06/09/2021
+f1_keywords: ["C6389"]
+helpviewer_keywords: ["C6389"]
+---
+
+# C6389: MARK_INTERNAL_OR_MISSING_COMMON_DECL
+
+This check is intended to help reduce the visibility of certain symbols and to modularize the code. In multi-file C++ projects, each declaration should be either local to a C++ file (part of the anonymous namespace) or declared in a common header file that's included by multiple C++ files.
+
+When this check flags a declaration, either it should be moved to an anonymous namespace or a forward declaration should be moved to a header file, depending on the scope of the symbol.
+
+The rule is an experimental rule that must be explicitly enabled in a rule set file to work. For more information about rule sets, see [Use rule sets to group code analysis rules](/visualstudio/code-quality/using-rule-sets-to-group-code-analysis-rules).
+
+## Example
+
+```cpp
+// A.h
+struct X;
+```
+
+```cpp
+// A.cpp
+#include "A.h"
+
+// Not flagged, declared in a header file.
+struct X { int x; };
+
+struct Y { double y; }; // warning: Move 'Y' to anonymous namespace or put a forward declaration in a common header included in this file.
+
+void f();  // warning: Move 'f' to anonymous namespace or put a forward declaration in a common header included in this file.
+```
+
+One way to resolve these issues is to move `struct Y` into an anonymous namespace, and move the declaration of `f` into a header:
+
+```cpp
+// A.h
+struct X;
+void f();
+```
+
+```cpp
+// A.cpp
+#include "A.h"
+
+// Not flagged, declared in a header file.
+struct X { int x; };
+
+namespace {
+  struct Y { double y; };
+} // anonymous namespace
+
+// Not flagged, declared in a header file.
+void f(); 
+```

docs/code-quality/toc.yml

@@ -462,6 +462,8 @@
     href: ../code-quality/c6387.md
   - name: C6388
     href: ../code-quality/c6388.md
+  - name: C6389
+    href: ../code-quality/c6389.md
   - name: C6400
     href: ../code-quality/c6400.md
   - name: C6401