Allow to easily box an Input value

[soenkehahn]

Sometimes it's convenient to conditionally provide an input to a command or not. Currently that is difficult due to the Input values being typed. So for example this doesn't work:

cmd_unit!("ls", if true { "some-file" } else { () });

Currently this can be solved with something like:

cmd_unit!("ls", if true { vec!["some-file"] } else { vec![] });

But you could imagine situations where it's not that easy to find a workaround like this. So maybe it'd be good to have a BoxedInput type:

cmd_unit!("ls", if true { BoxedInput("some-file") } else { BoxedInput(()) });

[casey]

Another way to do this would be to have something like OptionInput, analogous to OptionFuture.

Something like:

struct OptionInput<I: Input>(Option<I>);

// With a boolean condition:
cmd_unit!("ls", OptionInput(condition.then("some-file")));

// With an option:
cmd_unit!("ls", OptionInput(Some("some-file")));

[soenkehahn]

If we'd have a BoxedInput, then users could also do something like:

let mut args: Vec<BoxedInput> = vec!["foo".boxed(), "bar".boxed(), LogCommand.boxed()];
if condition {
   args.push(Stdin("input").boxed());
}
args.run();

With OptionInput that wouldn't be possible, since the inner type would still leak out into the wrapper type. OptionInput would of course also be useful, but BoxedInput seems more general.

[casey]

I think they're probably both useful, I wouldn't mind having both.