unsafe_binder: implement v0 symbol mangling

[GrigorenkoPV]

Tracking issue: #130516
Fixes #154367

rustc_demangle also needs to be changed, but they are in a separate repo. I think I will put up a PR there once the suggested mangling is deemed OK.

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[mati865]

I'm not familiar with the process for modifying the mangling spec, but I'd guess this requires some kind of agreement from the Compiler team Maintainers (not just the members) and rustc book update.

Hopefully we get lucky on reroll, otherwise I'll ask on Zulip.
@rustbot reroll

[oli-obk]

Can we reuse the same trick as #122106 (comment) ?

Essentially turning unsafe binders into a magical symbol from libcore? Needs a bit more than what was done for f32, as there's data after that, but should be workable.

I should totally do the same for pattern types. Happy to do that first to try it out unless you wanna give it a shot for unsafe binders

[GrigorenkoPV]

If I understand correctly, the approach you are suggesting requires no changes to rustc_demangle, which is nice.

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I don't think I have enough time to work on that in the coming days, so I'm okay with you trying pattern types first or maybe even taking over this one too if I'm still busy by then.

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Your recent PRs regarding pattern types were a nice reference for me while working on this one and while trying to take a stab at #154871 today. Unfortunately, the latter is still more than what I'm competent enough for.

[jackh726]

@GrigorenkoPV I'm happy to take this over at some point if you're busy. (I have an open item for me to go through and finish up the unsafe binders implementation.)

[oli-obk]

I'll review that work. Marking this PR as blocked for now