Add documentation and a release note

Author: DKLoehr

clang/docs/ReleaseNotes.rst

@@ -103,6 +103,10 @@ Attribute Changes in Clang
 Improvements to Clang's diagnostics
 -----------------------------------
 
+- The ``-Wunique-object-duplication`` warning has been added to warn about objects
+  which are supposed to only exist once per program, but may get duplicated when
+  built into a shared library.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -696,7 +696,36 @@ def SuspiciousMemaccess : DiagGroup<"suspicious-memaccess",
    NonTrivialMemaccess, MemsetTransposedArgs, SuspiciousBzero]>;
 def StaticInInline : DiagGroup<"static-in-inline">;
 def StaticLocalInInline : DiagGroup<"static-local-in-inline">;
-def UniqueObjectDuplication : DiagGroup<"unique-object-duplication">;
+def UniqueObjectDuplication : DiagGroup<"unique-object-duplication"> {
+  code Documentation = [{
+Warns when objects which are supposed to be globally unique might get duplicated
+when built into a shared library.
+
+If an object with hidden visibility is built into a shared library, each instance
+of the library will get its own copy. This can cause very subtle bugs if there was
+only supposed to be one copy of the object in question: singletons aren't single,
+changes to one object won't affect the others, the object's initializer will run
+once per copy, etc.
+
+Specifically, this warning fires when it detects an object which:
+  1. Appears in a header file (so it might get compiled into multiple libaries), and
+  2. Has external linkage (otherwise it's supposed to be duplicated), and
+  3. Has hidden visibility.
+
+As well as one of the following:
+  1. The object is mutable, or
+  2. The object's initializer definitely has side effects.
+
+The warning is best resolved by making the object ``const`` (if possible), or by explicitly
+giving the object non-hidden visibility, e.g. using ``__attribute((visibility("default")))``.
+Note that all levels of a pointer variable must be constant; ``const int*`` will
+trigger the warning because the pointer itself is mutable.
+
+This warning is currently disabled on Windows since it uses import/export rules
+instead of visibility.
+}];
+}
+
 def GNUStaticFloatInit : DiagGroup<"gnu-static-float-init">;
 def StaticFloatInit : DiagGroup<"static-float-init", [GNUStaticFloatInit]>;
 // Allow differentiation between GNU statement expressions in a macro versus