Make structopt-derive's dependency on structopt optional

[killercup]

So, I have a very weird request. I'm trying to re-export stuctopt-derive in a crate, but since this code

structopt/structopt-derive/src/lib.rs

Lines 907 to 909 in cc6dfd2

	const #dummy_const: () = {
	extern crate structopt as _structopt;
	use structopt::StructOpt;

expands to extern crate structopt, my consumers would have to add structopt to their dependencies, which I want to avoid. Is there another option of doing this? E.g., making this part of the derive optional (with a feature flag).

cc killercup/quicli#9

[TeXitoi]

This trick was taken to serde. Thus how do you manage serde case ?

Then, if you have the trick to only depend on structopt without depending on structopt-derive I'm interested by the trick that may simplify this particular use case.

[TeXitoi]

The feature of this trick is to compile without the user needing to extern crate structopt

[killercup]

Thus how do you manage serde case ?

By opening the same issue there as well, of course ;) serde-rs/serde#1147

Since there are already some ideas in this thread, maybe we can discuss options there? Otherwise I'll let you know if you find a solution :)

The feature of this trick is to compile without the user needing to extern crate structopt

And it's a great trick :) The thing is that my crate already re-exports structopt::StructOpt;, and users of quicli should always glob import the prelude that contains StructOpt. In such a case, the user doesn't need extern crate structopt;.