Yes, omitting "stdcall", although for subtle reason.
If `extern "stdcall"` is given, rustc produces llvm's function declaration with callconv=X86StdcallCallConv, even if win64 does not have such convention.
Then llvm makes it worse: it *mangles* function name e.g. "MessageBoxA" -> "_MessageBox@16" but win64 only has "MessageBoxA", so link fails.
I think llvm is wrong, but don't know rustc is wrong or not.