[Sema] Add -Wpointer-to-enum-cast and -Wvoid-pointer-to-enum-cast

[nathanchance]

Summary

GCC does not warn on casts from pointers to enumerators, while clang
currently does: https://godbolt.org/z/3DFDVG

This causes a bunch of extra warnings in the Linux kernel, where
certain structs contain a void pointer to avoid using a gigantic
union for all of the various types of driver data, such as
versions.

Add a diagnostic that allows certain projects like the kernel to
disable the warning just for enums, which allows those projects to
keep full compatibility with GCC but keeps the intention of treating
casts to integers and enumerators the same by default so that other
projects have the opportunity to catch issues not noticed before (or
follow suite and disable the warning).

Link: https://github.com/ClangBuiltLinux/linux/issues/887

[manojgupta]

Thanks, Verified that this fixes the kernel warnings in my local builds with https://gist.github.com/nathanchance/767cccf4d093c1342e1994083518815e!

[xbolva00]

clang/include/clang/Basic/DiagnosticSemaKinds.td

3672

You can reuse warning text:

warn_pointer_to_int_cast.Text

I think..

[nathanchance]

clang/include/clang/Basic/DiagnosticSemaKinds.td

3672

Thanks, that is indeed cleaner!

[nathanchance]

Thanks for the review and accepting. I do not have commit rights, would you mind doing that on my behalf?

[MaskRay]

clang/lib/Sema/SemaCast.cpp

2783

A ternary operator is better here. I'll fix it.

clang/include/clang/Basic/DiagnosticGroups.td

 
 def TypeSafety : DiagGroup<"type-safety">;
 
 def IncompatibleExceptionSpec : DiagGroup<"incompatible-exception-spec">;
 
 def IntToVoidPointerCast : DiagGroup<"int-to-void-pointer-cast">;
 def IntToPointerCast : DiagGroup<"int-to-pointer-cast",
                                  [IntToVoidPointerCast]>;
-def VoidPointerToIntCast : DiagGroup<"void-pointer-to-int-cast">;
+def VoidPointerToEnumCast : DiagGroup<"void-pointer-to-enum-cast">;
+def VoidPointerToIntCast : DiagGroup<"void-pointer-to-int-cast",
+                                     [VoidPointerToEnumCast]>;
+def PointerToEnumCast : DiagGroup<"pointer-to-enum-cast",
+                                  [VoidPointerToEnumCast]>;
 def PointerToIntCast : DiagGroup<"pointer-to-int-cast",
-                                 [VoidPointerToIntCast]>;
+                                 [PointerToEnumCast, VoidPointerToIntCast]>;
 
 def Move : DiagGroup<"move", [
     PessimizingMove,
     RedundantMove,
     ReturnStdMove,
     SelfMove
   ]>;
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

def warn_int_to_pointer_cast : Warning<
   "cast to %1 from smaller integer type %0">,
   InGroup<IntToPointerCast>;
 def warn_int_to_void_pointer_cast : Warning<
   "cast to %1 from smaller integer type %0">,
   InGroup<IntToVoidPointerCast>;
 def warn_pointer_to_int_cast : Warning<
   "cast to smaller integer type %1 from %0">,
   InGroup<PointerToIntCast>;
+def warn_pointer_to_enum_cast : Warning<
+  warn_pointer_to_int_cast.Text>,
+  InGroup<PointerToEnumCast>;
 def warn_void_pointer_to_int_cast : Warning<
   "cast to smaller integer type %1 from %0">,
   InGroup<VoidPointerToIntCast>;
+def warn_void_pointer_to_enum_cast : Warning<
+  warn_void_pointer_to_int_cast.Text>,
+  InGroup<VoidPointerToEnumCast>;
 def ext_ms_pointer_to_int_cast : ExtWarn<
   "cast to smaller integer type %1 from %0 is a Microsoft extension">,
 InGroup<MicrosoftCast>;
 
 def warn_attribute_ignored_for_field_of_type : Warning<
   "%0 attribute ignored for field of type %1">,
   InGroup<IgnoredAttributes>;
 def warn_no_underlying_type_specified_for_enum_bitfield : Warning<

clang/lib/Sema/SemaCast.cpp

if (!DestType->isArithmeticType()) {
     if ((Self.Context.getTypeSize(SrcType) >
          Self.Context.getTypeSize(DestType)) &&
         !DestType->isBooleanType()) {
       // C 6.3.2.3p6: Any pointer type may be converted to an integer type.
       // Except as previously specified, the result is implementation-defined.
       // If the result cannot be represented in the integer type, the behavior
       // is undefined. The result need not be in the range of values of any
       // integer type.
-      unsigned Diag = Self.getLangOpts().MicrosoftExt
-                          ? diag::ext_ms_pointer_to_int_cast
-                          : SrcType->isVoidPointerType()
-                                ? diag::warn_void_pointer_to_int_cast
-                                : diag::warn_pointer_to_int_cast;
+      unsigned Diag;
+      if (Self.getLangOpts().MicrosoftExt)
+        Diag = diag::ext_ms_pointer_to_int_cast;
+      else if (SrcType->isVoidPointerType())
+        Diag = DestType->isEnumeralType() ? diag::warn_void_pointer_to_enum_cast
+                                          : diag::warn_void_pointer_to_int_cast;
+      else if (DestType->isEnumeralType())
+        Diag = diag::warn_pointer_to_enum_cast;
+      else
+        Diag = diag::warn_pointer_to_int_cast;
       Self.Diag(OpRange.getBegin(), Diag) << SrcType << DestType << OpRange;
     }
   }
 
   if (Self.getLangOpts().OpenCL &&
       !Self.getOpenCLOptions().isEnabled("cl_khr_fp16")) {
     if (DestType->isHalfType()) {
       Self.Diag(SrcExpr.get()->getBeginLoc(), diag::err_opencl_cast_to_half)

clang/test/Sema/cast.c

 typedef enum { x_a, x_b } X;
 void *intToPointerCast2(X x) {
   return (void*)x;
 }
 
 void *intToPointerCast3() {
   return (void*)(1 + 3);
 }
+
+void voidPointerToEnumCast(VoidPtr v) {
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'VoidPtr' (aka 'void *')}}
+  // Test that casts to void* can be controlled separately
+  // from other -Wpointer-to-enum-cast warnings.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wvoid-pointer-to-enum-cast"
+  (void)(X) v; // no-warning
+#pragma clang diagnostic pop
+}
+
+void pointerToEnumCast(CharPtr v) {
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'CharPtr' (aka 'char *')}}
+  // Test that casts to void* can be controlled separately
+  // from other -Wpointer-to-enum-cast warnings.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wvoid-pointer-to-enum-cast"
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'CharPtr' (aka 'char *')}}
+#pragma clang diagnostic pop
+}