compiletest: Automatically specify `-Zunstable-options` when the `edition` header is set to an unstable edition

[ehuss]

During each new edition, it is a bit of a hassle to add/remove -Zunstable-options headers for the next edition. I think it would be helpful to automatically specify -Zunstable-options when an edition header specifies an unstable edition. This means there would roughly be one place to make the switch (compiletest itself).

[jieyouxu]

Tagging as E-needs-investigation in case doing this makes writing tests difficult / introduces an implicit behavior that's not obvious to disable.
@rustbot label +E-needs-investigation

[ehuss]

Can you say more about what might seem difficult? When would it need to be disabled? If you want a test that circumvents that, it can do compile-flags: --edition=..., though I'm not sure when you would want to do that (we could have a test to verify that the flag is indeed unstable, but we haven't bothered with that).

[jieyouxu]

Can you say more about what might seem difficult? When would it need to be disabled? If you want a test that circumvents that, it can do compile-flags: --edition=..., though I'm not sure when you would want to do that (we could have a test to verify that the flag is indeed unstable, but we haven't bothered with that).

I tagged this as needs-investigation out of caution mostly to signify that if someone wants to implement this, they should double-check if doing so improves the UX of some tests but regresses others. I haven't done this specific research myself so I don't know for sure.