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Um, what? You've linked an RFC for Rust, but the CPP Reference article for C++ So yeah, the Rust RFC documents a proposed change, and the C++ reference documents an implemented feature, but you could equally link the C++ Proposal document and the Rust library docs to make the opposite point if you wanted.

Rust's https://doc.rust-lang.org/std/primitive.f32.html#method.next... https://doc.rust-lang.org/std/primitive.f32.html#method.next... of course exist, they're even actually constant expressions (the C++ functions are constexpr since 2023 but of course you're not promised they actually work as constant expressions because C++ is a stupid language and "constexpr" means almost nothing)

You can also rely on the fact (not promised in C++) that these are actually the IEEE floats and so they have all the resulting properties you can (entirely in safe Rust) just ask for the integers with the same bit pattern, compare integers and because of how IEEE is designed that tells you how far away in some proportional sense, the two values are.

On an actual CPU manufactured this century that's almost free because the type system evaporates during compilation -- for example f32::to_bits is literally zero CPU instructions.

by colechristensen10 hours ago|

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Oh, my research was wrong and the line from the RFC doc...

>Currently it is not possible to answer the question ‘which floating point value comes after x’ in Rust without intimate knowledge of the IEEE 754 standard.

So nevermind on it not being present in Rust I guess I was finding old documentation

by tialaramex3 hours ago|

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Yeah, the RFC is explaining what they proposed in 2021. In 2022 that work landed in "nightly" Rust, which means you could see it in the documentation (unless you've turned off seeing unstable features entirely) but to actually use it in software you need the nightly compiler mode and a feature flag in your source #![feature(float_next_up_down)].

By 2025 every remaining question about edge cases or real world experience was resolved and in April 2025 the finished feature was stabilized in release 1.86, so it just works in Rust since about a year.

For future reference you can follow separate links from a Rust RFC document to see whether the project took this RFC (anybody can write one, not everything gets accepted) and then also how far along the implementation work is. Can I use this in nightly? Maybe there's an outstanding question I can help answer. Or, maybe it's writing a stabilization report and this is my last chance to say "Hey, I am an expert on this and your API is a bit wrong".