--show-span should be a -Z option

[nrc]

Not a breaking change since it is an unstable option.

[jas-on]

Is it already an unstable option? https://github.com/rust-lang/rust/blob/master/src/librustc_driver/lib.rs#L143

[sfackler]

@jas-on this issue is saying that --show-span should be changed to -Z show-span. It is already an unstable option.

[jas-on]

So show-span can accept expr|pat|ty as arguments. How would those be specified if show-span were changed to a -Z FLAG?

[nrc]

Hopefully exactly the same way, not sure how possible that is. If it is not possible right now, then this probably isn't an easy issue to fix. I can look into it next week.

[alexcrichton]

We've got kinda our own syntax for the -Z flags so where today you pass --show-span foo it could instead be -Z show-span=foo