External ABI by-value parameter can't be demoded

[brson]

std::net::tcp has crust functions like

extern fn on_tcp_read_cb(stream: *uv::ll::uv_stream_t,                                                                                                                        
                    nread: libc::ssize_t,                                                                                                                                     
                    ++buf: uv::ll::uv_buf_t) unsafe { 

I can't figure out a way to make that ++buf argument go away.

[nikomatsakis]

Yes, right now + mode is not necessarily following the C ABI. My "master plan" for this was that extern "C" fns would follow C's ABI precisely, whereas Rust ones might not... though perhaps it'd be easier to just make everything follow C ABI.