[clang] Add an off-by-default warning to complain about MSVC bitfield padding (PR #117428)

[via cfe-commits]

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@@ -633,6 +633,50 @@ def Packed : DiagGroup<"packed", [PackedNonPod]>;
 def PaddedBitField : DiagGroup<"padded-bitfield">;
 def Padded : DiagGroup<"padded", [PaddedBitField]>;
 def UnalignedAccess : DiagGroup<"unaligned-access">;
+def MSBitfieldCompatibility : DiagGroup<"ms-bitfield-compatibility"> {
+  code Documentation = [{
+    Under the MSVC ABI adjacemt bit-fields are not packed if the underlying type
+    has a different storage size. This warning indicates that a pair of adjacent
+    bit-fields may not pack in the same way due to this behavioural difference.
+
+    This can occur when mixing different types explicitly:
+
+    .. code-block:: c++
+
+      struct S {
+        uint16_t field1 : 1;
+        uint32_t field2 : 1;
+      };
+
+    or more subtly through enums
+
+    .. code-block:: c++
+
+      enum Enum1 { /* ... */ };
+      enum class Enum2 : unsigned char { /* ... */ };
+      struct S {
+        Enum1 field1 : 1;
+        Enum2 field2 : 1;
+      };
+
+    In each of these cases under the MSVC ABI the second bit-field will not be
+    packed with the preceding bit-field, and instead will be aligned as if the
+    fields were each separately defined integer fields of their respective storage
+    size. For binary compatibility this is obviously and observably incompatible,
+    however where bit-fields are being used solely for memory use reduction this
+    incomplete packing may silently increase the size of objects vs what is
+    expected.
+
+    This issue can be addressed by ensuring the storage type of each bit-field is
+    the same, either by explicitly using the same integer type, or in the case of
+    enum types declaring the enum types with the same storage size. For enum types
+    where you cannot specify the underlying type, the options are to either switch
+    to int sized storage for all specifiers or to resort to declaring the
+    bit-fields with explicit integer storage types and cast in and out of the field.
+    If such a solution is required the `preferred_type` attribute can be used to
+    convey the actual field type to debuggers and other tooling.
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Sirraide wrote:

Can we link to the documentation of that attribute here maybe?

https://github.com/llvm/llvm-project/pull/117428