Consider flag to mark unstable targets

[madsmtm]

(I wasn't really sure where to file this, so it's going here, but might be more relevant on the Cargo repo).

Currently, users of Tier 3 targets are (effectively) required to use a nightly compiler, by virtue of -Zbuild-std being unstable. As we move towards stabilizing -Zbuild-std, this will likely change, and users are going to use Tier 3 targets on stable.

As co-maintainer of the Apple target triples, I think there's value in some targets being unstable:

• Users of it are on nightly, so will get fixes faster, which is useful when the target is newly added (there's usually an iteration period before it gets well supported).
• It sends a clear signal that the target isn't as well supported (the name "Tier 3" does this too, but fewer people read the platform support docs).
• It makes it easier to do breaking changes like renaming the target.

Do you think it'd make sense to have a rustc/Cargo flag like -Zunstable-targets (or maybe merge under -Zunstable-options), and then mark certain targets as unstable, and require the use of this flag?

[Lokathor]

As the maintainer of two Tier 3 targets (that are the same target with different default cpu features), i would absolutely not want this to be applied to my own targets.

please remember that Nightly is no guarantee of getting fixes faster. I know plenty of people (including me) who use Nightly because they must, but still don't run rustup update until they hear about a new stable release and maybe remember to update the compiler.

[madsmtm]

I wouldn't want it for all Tier 3 targets either, just for newer ones that are still "in the works", and older ones that are mostly legacy and ill-supported.

[Lokathor]

the armv4t targets are quite possibly what you'd call "legacy", but I still don't want them marked unstable.