Stabilize fN::BITS

[Apersoma]

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Tracking issue: #151073
Closes: #151073

[rustbot]

r? @scottmcm

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[rustbot]

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

[tgross35]

This has only been around since January but it seems pretty unobjectionable to have. However, this does conflict with a constant in use in the ecosystem (showed up in the implementation PR) so we need to take a look at crater.

@bors try

@rustbot label +I-libs-api-nominated

[rust-bors]

☀️ Try build successful (CI)
Build commit: 0abe69e (0abe69e6c16114311cc2700e972492e64d69c28f, parent: 1f7f8ea0721a3b1eb73e6c6d25cccb371434b320)

[tgross35]

Doing a run with tests in case somebody has a different value for BITS for some reason.

@craterbot run mode=build-and-test

[craterbot]

👌 Experiment pr-154065 created and queued.
🤖 Automatically detected try build 0abe69e
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[Amanieu]

FCP started in #151073

[tgross35]

@Apersoma please squash the commits and give it a reasonable message, the first commit just says "cmplt" (we don't have squash merging on this repo).

[craterbot]

🚧 Experiment pr-154065 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-154065 is completed!
📊 297 regressed and 292 fixed (857798 total)
📊 132042 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-154065/retry-regressed-list.txt ↩

[tgross35]

@craterbot run mode=build-and-test p=1 crates=https://crater-reports.s3.amazonaws.com/pr-154065/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-154065-1 created and queued.
🤖 Automatically detected try build 0abe69e
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[tgross35]

Crater isn't too bad, but there are a couple of crates that I think we could resolve reasonably easy:

• [common_traits]: this one has a few depenents. It seems pretty actively developed so I opened f*::BITS will break if float_bits_const stabilizes zommiommy/common_traits#7
• float_math: Doesn't seem to have any dependents and hasn't been published in a while. Author is @Lokathor, any chance you could publish a version that disambiguates?

Crates that depend on common_traits might run into some an issue too, we'll have to rerun the regressed list once a new version is published.

Let's give some time for fixes here.

@rustbot blocked
@SnoozeThis wait 2 months

[SnoozeThis]

(https://snoozeth.is/wG5NpmKaHZw) I will wait until Thu, 28 May 2026 19:27:23 UTC and then add a comment.

@rustbot claim.

[Apersoma]

Crater isn't too bad, but there are a couple of crates that I think we could resolve reasonably easy:

• [common_traits]: this one has a few depenents. It seems pretty actively developed so I opened f*::BITS will break if float_bits_const stabilizes zommiommy/common_traits#7
• float_math: Doesn't seem to have any dependents and hasn't been published in a while. Author is @Lokathor, any chance you could publish a version that disambiguates?

Crates that depend on common_traits might run into some an issue too, we'll have to rerun the regressed list once a new version is published.

Let's give some time for fixes here.

@rustbot blocked @SnoozeThis wait 2 months

How do you filter for what's actually useful in crater?

[tgross35]

Look for "regressed: dependencies" (some crate in the dep tree failed) and "regressed: root results" (the crate itself failed) in the summary. We retry the spurious results.

You can also go to the downloads tab to grab the results and do something like rg '::BITS' which can be a bit more clear.

[Apersoma]

This probably going to need a second crater run for errors causes by users of common_traits after the new version gets published, right?

[tgross35]

This probably going to need a second crater run for errors causes by users of common_traits after the new version gets published, right?

I think so ^^ 🙂

Crates that depend on common_traits might run into some an issue too, we'll have to rerun the regressed list once a new version is published.

[zommiommy]

done, sorry!

[tgross35]

Not a problem, there’s no way of knowing until this happens :) thank you for the quick action!

[craterbot]

🚧 Experiment pr-154065-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-154065-1 is completed!
📊 114 regressed and 120 fixed (132208 total)
📊 1656 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-154065-1/retry-regressed-list.txt ↩