library/test: always enable unstable features for miri

[cuviper]

The unstable features of the test crate used to be default-enabled,
and manually disabled when building the beta and stable channels. Commit
dae8ea9 flipped that to default-disabled, only enabled for nightly
and dev channels.

However, this broke miri testing on the beta/stable channels, because it
also uses unstable features -- which should be fine to enable just for
its own sysroot build. Now the test build script makes that happen by
noticing the MIRI_CALLED_FROM_SETUP environment variable.

[rustbot]

r? @Mark-Simulacrum

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[cuviper]

1.95-beta is currently blocked on this problem: #153314 (comment)

@rustbot label +beta-nominated

[jieyouxu]

This looks reasonable to me (but am neither a libs reviewer nor miri reviewer).

View changes since this review

[Kobzol]

I looked at this again and I guess that this is a natural consequence of flipping the default, before we used to use unstable features in some places "by accident", such as in Miri, while they need an explicit opt-in. Would be nicer to set this for Miri tests from bootstrap, rather than in the build probe, but I guess that this would still break miri when executed outside of bootstrap?

I think that as a hotfix for beta this is good enough.

You can r=me if you tested that this unblocks the beta build (the miri test on stable/beta).

[cuviper]

I had tested that locally, but I went ahead and added this to #153314 for beta to confirm, and it did pass miri's PR CI. Also, T-libs folks are showing approval for the backport on zulip.

@rustbot label +beta-accepted
@bors r=Kobzol

[rust-bors]

📌 Commit 30d7ed4 has been approved by Kobzol

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

[cuviper]

Would be nicer to set this for Miri tests from bootstrap, rather than in the build probe, but I guess that this would still break miri when executed outside of bootstrap?

Yes, I think a bootstrap-only fix would break miri elsewhere -- except we don't ship miri on stable or beta anyway. So maybe that's fine, although I'm not sure how else bootstrap would help this, because it has to do with how miri builds test in its own target sysroot.
cc @rust-lang/miri