Add support for post_fields without buffer copy

[dovreshef]

Hi

I wanted to add support for uploading data without the buffer copy between curl and rust, and without copying it in parts into a read function.

Basically support CURLOPT_POSTFIELDS, but unlike today, without setting CURLOPT_COPYPOSTFIELDS.

I've added both the option to support uploading 'static data, which I think is simple.

I'm not so sure about my approach to solving it for Transfer. I'm open to any other approach really, if you have any pointers. This was for me high magic, and I hope I've got it right.

And, of course, all that is in case that is something that you're interested in adding support for.

[alexcrichton]

I think the reason I didn't originally bind this was the lifetime issue with data. I think we'll probably want to either make this unsafe or figure out a safe operation, because having &'static isn't too too helpful I think?

[dovreshef]

I think the reason I didn't originally bind this was the lifetime issue with data. I think we'll probably want to either make this unsafe or figure out a safe operation, because having &'static isn't too too helpful I think?

You're right. I mostly added it out of completion (to have all the options) and being useful for tests I guess.

What about tying it the lifetime in some way into a transfer object? Does it have to be unsafe?

I'm not sure that what I did is safe, but if not, surely there is some way to make sure that the upload buffer is set to null when the transfer goes out of scope?

[alexcrichton]

I'm not very familiar with the safety here or this option, and I don't have a ton of time to dig in right now unfortunately. It might be possible though to tie it to the Transfer object.