fN::BITS constants for feature float_bits_const by tyhdefu · Pull Request #151109 · rust-lang/rust

tyhdefu · 2026-01-14T10:56:45Z

Also enables the feature for compiler_builtins as otherwise this causes a warning and conflicts with the Float extension trait.

Implementation for #151073

Feature flag: #![feature(float_bits_const)]

Note that this is likely to conflict with some extension traits, as it has with compiler builtins. However, assuming correct values for the constants, they are either u32, the same type, which should not cause a problem (as shown by enabling the feature for compiler_builtins), or a different type (e.g. usize), which should cause a compiler error. Either way this should never change behaviour unless the extension trait implemented an incorrect value.

Also note that it doesn't seem to be possible to put multiple unstable attributes on an item, so f128::BITS and f16::BITS are gated behind the feature flags for those primitives, rather than #![feature(float_bits_const)]

rustbot · 2026-01-14T10:56:49Z

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

cc @tgross35

rustbot · 2026-01-14T10:56:51Z

r? @scottmcm

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rustbot · 2026-01-14T19:46:56Z

The Miri subtree was changed

cc @rust-lang/miri

library/core/src/num/f128.rs

rustbot · 2026-01-15T13:16:20Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

tgross35 · 2026-01-15T13:20:48Z

Note that this is likely to conflict with some extension traits, as it has with compiler builtins. However, assuming correct values for the constants, they are either u32, the same type, which should not cause a problem (as shown by enabling the feature for compiler_builtins), or a different type (e.g. usize), which should cause a compiler error. Either way this should never change behaviour unless the extension trait implemented an incorrect value.

Either of these things are unfortunately definitely a problem; we can't expect working crates to enable a nightly feature to keep building correctly, nor do we want them to stop building if the types don't line up. Not exactly sure what our options are here but let's start a crater run to have an idea of impact.

@bors try

fN::BITS constants for feature float_bits_const

RalfJung · 2026-01-15T13:33:32Z

src/tools/miri/tests/pass/float.rs

        }

-        let fbits = <$fty>::BITS;
+        let fbits = <$fty as Float>::BITS;


This makes the BITS constant in the Float trait in this file dead, doesn't it?

The diff here I think is intentionally using BITS from the Float trait. It could go the other way and drop BITS from Flaot, just means Miri has to depend on this feature.

That's right - However when I tried to add the feature flag to the miri tests it said the feature was not allowed - I think it might be building these tests with a stable compiler that doesn't allow such features.

Regularly it would just be a warning but warnings are treated as errors, so I had to change this to get it to pass.

Miri uses the in-tree standard library built with the in-tree compiler. So it looks like something went wrong in your testing.

But anyway we still need the const in the trait since we have code generic over the trait, so it doesn't really matter.

rust-bors · 2026-01-15T15:39:07Z

☀️ Try build successful (CI)
Build commit: a3a8742 (a3a874232ae0a7586b5fbe7483c5a42e157bd62a, parent: a6acf0f07f0ed1c12e26dc0db3b9bf1d0504a0bb)

rustbot · 2026-01-15T17:11:10Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in check-cfg diagnostics

cc @Urgau

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in src/tools/opt-dist

cc @Kobzol

HIR ty lowering was modified

cc @fmease

Some changes occurred in GUI tests.

cc @GuillaumeGomez

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

craterbot · 2026-01-30T22:02:59Z

🚧 Experiment pr-151109 is now running

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

craterbot · 2026-02-01T15:06:45Z

🎉 Experiment pr-151109 is completed!
📊 5 regressed and 1 fixed (785666 total)
📊 2258 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

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-151109/retry-regressed-list.txt ↩

tgross35 · 2026-02-02T00:49:58Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-151109/retry-regressed-list.txt p=1

craterbot · 2026-02-02T00:50:04Z

👌 Experiment pr-151109-1 created and queued.
🤖 Automatically detected try build a3a8742
⚠️ Try build based on commit bb1183b, but latest commit is 3387a58. Did you forget to make a new try build?
🔍 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

craterbot · 2026-02-03T02:39:01Z

🚧 Experiment pr-151109-1 is now running

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

craterbot · 2026-02-03T07:11:04Z

🎉 Experiment pr-151109-1 is completed!
📊 0 regressed and 0 fixed (2080 total)
📊 142 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

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-151109-1/retry-regressed-list.txt ↩

tgross35 · 2026-02-03T09:00:05Z

For context, the warning looks like this:

warning: an associated constant with this name may be added to the standard library in the future
   --> libm/src/math/support/float_traits.rs:248:48
    |
248 |               const SIGN_MASK: Self::Int = 1 << (Self::BITS - 1);
    |                                                  ^^^^^^^^^^
...
335 | / float_impl!(
336 | |     f64,
337 | |     u64,
338 | |     i64,
...   |
344 | |     fmaf64
345 | | );
    | |_- in this macro invocation
    |
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = note: `#[warn(unstable_name_collisions)]` (part of `#[warn(future_incompatible)]`) on by default
    = note: this warning originates in the macro `float_impl` (in Nightly builds, run with -Z macro-backtrace for more info)
help: use the fully qualified path to the associated const
    |
248 -             const SIGN_MASK: Self::Int = 1 << (Self::BITS - 1);
248 +             const SIGN_MASK: Self::Int = 1 << (<f64 as Float>::BITS - 1);
    |
help: add `#![feature(float_bits_const)]` to the crate attributes to enable `core::f64::<impl f64>::BITS`
   --> libm/src/lib.rs:26:1
    |
 26 + #![feature(float_bits_const)]
    |

I guess crater might not actually show warnings?

In any case, there isn't a problem adding them but we may need to revisit at stabilization.

@bors r+

rust-bors · 2026-02-03T09:00:09Z

📌 Commit 3387a58 has been approved by tgross35

It is now in the queue for this repository.

fN::BITS constants for feature float_bits_const Also enables the feature for compiler_builtins as otherwise this causes a warning and conflicts with the Float extension trait. --- Implementation for rust-lang#151073 Feature flag: `#![feature(float_bits_const)]` Note that this is likely to conflict with some extension traits, as it has with compiler builtins. However, assuming correct values for the constants, they are either `u32`, the same type, which should not cause a problem (as shown by enabling the feature for compiler_builtins), or a different type (e.g. `usize`), which should cause a compiler error. Either way this should never change behaviour unless the extension trait implemented an incorrect value. Also note that it doesn't seem to be possible to put multiple unstable attributes on an item, so `f128::BITS` and `f16::BITS` are gated behind the feature flags for those primitives, rather than `#![feature(float_bits_const)]`

Rollup of 6 pull requests Successful merges: - #152008 (`rust-analyzer` subtree update) - #151109 (fN::BITS constants for feature float_bits_const) - #151976 (Rename `collect_active_jobs` to several distinct names) - #151691 (compiletest: Don't assume `aux-crate` becomes a `*.so` with `no-prefer-dynamic`) - #151919 (fix: Make `--color always` always print color with `--explain`) - #152028 (Convert to inline diagnostics in `rustc_driver_impl`)

Rollup of 7 pull requests Successful merges: - #152008 (`rust-analyzer` subtree update) - #151109 (fN::BITS constants for feature float_bits_const) - #151976 (Rename `collect_active_jobs` to several distinct names) - #151691 (compiletest: Don't assume `aux-crate` becomes a `*.so` with `no-prefer-dynamic`) - #151919 (fix: Make `--color always` always print color with `--explain`) - #152017 (Remove `with_no_trimmed_paths` use in query macro) - #152028 (Convert to inline diagnostics in `rustc_driver_impl`)

Rollup merge of #151109 - tyhdefu:float_bits_const, r=tgross35 fN::BITS constants for feature float_bits_const Also enables the feature for compiler_builtins as otherwise this causes a warning and conflicts with the Float extension trait. --- Implementation for #151073 Feature flag: `#![feature(float_bits_const)]` Note that this is likely to conflict with some extension traits, as it has with compiler builtins. However, assuming correct values for the constants, they are either `u32`, the same type, which should not cause a problem (as shown by enabling the feature for compiler_builtins), or a different type (e.g. `usize`), which should cause a compiler error. Either way this should never change behaviour unless the extension trait implemented an incorrect value. Also note that it doesn't seem to be possible to put multiple unstable attributes on an item, so `f128::BITS` and `f16::BITS` are gated behind the feature flags for those primitives, rather than `#![feature(float_bits_const)]`

Rollup of 7 pull requests Successful merges: - rust-lang/rust#152008 (`rust-analyzer` subtree update) - rust-lang/rust#151109 (fN::BITS constants for feature float_bits_const) - rust-lang/rust#151976 (Rename `collect_active_jobs` to several distinct names) - rust-lang/rust#151691 (compiletest: Don't assume `aux-crate` becomes a `*.so` with `no-prefer-dynamic`) - rust-lang/rust#151919 (fix: Make `--color always` always print color with `--explain`) - rust-lang/rust#152017 (Remove `with_no_trimmed_paths` use in query macro) - rust-lang/rust#152028 (Convert to inline diagnostics in `rustc_driver_impl`)

rustbot assigned scottmcm Jan 14, 2026

tyhdefu mentioned this pull request Jan 14, 2026

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tyhdefu force-pushed the float_bits_const branch 2 times, most recently from 9a456a5 to 10e2d54 Compare January 14, 2026 16:20

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Auto merge of #151109 - tyhdefu:float_bits_const, r=<try>

a3a8742

fN::BITS constants for feature float_bits_const

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tyhdefu force-pushed the float_bits_const branch from bb1183b to a4662d0 Compare January 15, 2026 17:11

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Rollup of 6 pull requests #152032

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Rollup of 7 pull requests #152035

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