One may get away with defining __fastcall as empty and warning the user not to call the functions directly. __stdcall is obviously necessary because most Windows APIs use that.
@WalterBright created dlang/dmd pull request #13614 "fix Issue 22727 ImportC: add __stdcall Windows calling convention" fixing this issue: - fix Issue 22727 - ImportC: support for __stdcall and __fastcall is necessary for 32-bit Windows builds https://github.com/dlang/dmd/pull/13614
Not going to support __fastcall. It should be #define'd to nothing by the user.
dlang/dmd pull request #13614 "fix Issue 22727 ImportC: add __stdcall Windows calling convention" was merged into master: - 0193db3d11a9c218c4c00cb9532a572365d5688b by Walter Bright: fix Issue 22727 - ImportC: support for __stdcall and __fastcall is necessary for 32-bit Windows builds https://github.com/dlang/dmd/pull/13614