Make Arc support DSTs

[ftxqxd]

No description provided.

[rust-highfive]

r? @huonw

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[huonw]

I wonder if it will be clearer and easier to handle to just duplicate the whole file with the old code in arc_stage0.rs and something like in lib.rs

#[cfg_attr(stage0, path = "arc_stage0.rs")]
mod arc;

[bors]

☔ The latest upstream changes (presumably #25460) made this pull request unmergeable. Please resolve the merge conflicts.

[ftxqxd]

@huonw I’ve updated it to duplicate the whole file instead. The diff is significantly larger, but I think it is much clearer and it should make making a snapshot easier in any case.

#[cfg_attr] and #[path] don’t seem to mix very well (it seems that mod statements are expanded before cfg attributes are considered) so I instead created a private arc_stage0 module and pub used all its items inside a public arc wrapper module.

[huonw]

@bors r+

[bors]

📌 Commit e840393 has been approved by huonw

[bors]

⌛ Testing commit e840393 with merge 4f83c4b...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt