Fix MSVC static assertion failure in SourceKit

[hughbe]

The following struct has a different size with MSVC and Clang/GCC.

enum class CodeCompletionDeclKind {};

struct SwiftSemanticToken {
  unsigned ByteOffset;
  unsigned Length : 24;
  CodeCompletionDeclKind Kind : 6;
  bool IsRef : 1;
  bool IsSystem : 1;
};
static_assert(sizeof(SwiftSemanticToken) == 8, "Too big");

int main()
{
    std::cout << "Hello, world!\n";
}

Clang/GCC report 8
MSVC reports 12

So, I'm not sure if this is really the right fix! I'd like to be able to make this struct size 8, as I don't like nerfing behaviour based on the compiler.

Please suggest something if anyone has ideas.

Using #pragma pack(1) with MSVC makes the struct size 9. Still not good enough!

[hughbe]

@compnerd if you have any ideas as our resident Windows compiler expert

[jrose-apple]

IIRC, MSVC won't combine signed and unsigned fields into the same unit, and bool is considered signed. (Not sure what CodeCompletionDeclKind counts as.) We usually make all bitfields unsigned int for this reason, and use accessors (with asserts) to convert back to the real type.

[hughbe]

You're certainly correct! This works

enum class CodeCompletionDeclKind {};

struct SwiftSemanticToken {
  unsigned ByteOffset;
  unsigned Length : 24;
  CodeCompletionDeclKind Kind : 6;
  unsigned IsRef : 1;
  unsigned IsSystem : 1;
};
static_assert(sizeof(SwiftSemanticToken) == 8, "Too big");

[hughbe]

@swift-ci please smoke test

[hughbe]

That's working - let me know if you have any other comments @jrose-apple

[jrose-apple]

Deferring back to @nkcsgexi.

[nkcsgexi]

looks good to me.