R1CS: Closure of challenge phase is `Fn`.

[rubdos]

Since #222, one writes the challenge phase using a closure. This closure gets stored in a Vec<Box<Fn(...)>>, and then gets called once at the start of the second phase.

Ideally, this closure should be FnOnce, such that it's able to consume some of its context (e.g. a set of variables could be consumed instead of borrowed).

Note that it is currently impossible to call a Box<FnOnce>, because it would call a method with unsized self. Cfr. rust-lang/rust#28796

Nightly does support FnBox(), which is a temporary workaround. The above issue is discussion deprecation of FnBox() in favour of the unsized_locals feature (which makes Box<FnOnce> callable).

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Note that there does not seem to be a fix for stable Rust, but I thought this should be noted somewhere. I'm not sure whether you are in favour of keeping this issue open while waiting for unsized_locals stabilization.

[oleganza]

Thanks for opening this issue and explaining the context behind Box<Fn>. I think it makes total sense to keep it open until there is a fix for Box<FnOnce>.

[oleganza]

Since we have to use nightly for other reasons (non-lexical lifetimes in IPP nll), and that Box<FnOnce> is getting stabilized in Rust 1.35 (next release), we could probably change the API now and throw in a #![feature(unsized_locals)] until 1.35.

[cathieyun]

That sounds good to me! As long as we remember to remove unsized_locals when 1.35 comes out so we have fewer nightly dependencies.

[hdevalence]

+1 to all of the above. One thing we could do is to make the unsized_locals only enabled when the yoloproofs (which can be renamed to r1cs or something later if we decide it's not yolo), so that it's only pulled in for the r1cs code rather than for the rangeproof code. It might also be good to check on the NLL status.

[oleganza]

Seems like NLL is stable since 1.31: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#non-lexical-lifetimes