The money shot (wish this were included in the blog post):

  #   if defined(_MSC_VER) && !defined(__clang__)
  #      define Py_MUSTTAIL [[msvc::musttail]]
  #      define Py_PRESERVE_NONE_CC __preserve_none
  #   else
  #       define Py_MUSTTAIL __attribute__((musttail))
  #       define Py_PRESERVE_NONE_CC __attribute__((preserve_none))
  #   endif

Apparently(?) this also needs to be attached to the function declarator and does not work as a function specifier: `static void *__preserve_none slowpath();` and not `__preserve_none static void *slowpath();` (unlike GCC attribute syntax, which tends to be fairly gung-ho about this sort of thing, sometimes with confusing results).

Yay to getting undocumented MSVC features disclosed if Microsoft thinks you’re important enough :/