Tracking Issue for `NonZero*::count_ones`

[WaffleLapkin]

Feature gate: #![feature(non_zero_count_ones)]

This is a tracking issue for NonZero*::count_ones family of methods.

Public API

// core::num

impl NonZero* {
    pub fn count_ones(self) -> NonZeroU32;
}

Steps / History

• Implementation: Add NonZero*::count_ones #118326
• Final comment period (FCP)¹: Tracking Issue for NonZero*::count_ones #120287 (comment)
• Stabilization PR: Stabilize NonZero::count_ones #136663

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[tyilo]

Anything holding this back from getting stabilized?

[WaffleLapkin]

@tyilo nothing that I'm aware of.

@rust-lang/libs-api I would like to propose this feature for stabilization. This feature adds a very straightforward function:

impl NonZero* {
    pub fn count_ones(self) -> NonZeroU32;
}

The benefit here, as opposed to writing .get().count_ones(), is that NonZero::count_ones returns NonZeroU32 and not u32 (a non-zero number must have at least one bit set!). The implementation is very simple and hasn't changed since the original addition in 1.77.

[tyilo]

I understand that this returns NonZeroU32 because <integer>::count_ones returns u32.

But why u32 and not u8 or u16?

u8 would work for all current primitive integers, but not for the potential u256 and i256.
u16 would first fail when introducing u65536 and i65536 which seems unlikely to ever happen.

[WaffleLapkin]

@tyilo it's not necessarily that we have to use 32 bits, and more so that u32 is the default integer type for when you don't really care about the width. you may notice that it's not only count_ones, but a lot of things like BITS, ilog, {leading,trailing}_{zeros,ones}, etc.

[Amanieu]

@rfcbot fcp merge

[rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔