Add `const` support for float rounding methods #141521

ruancomelli · 2025-05-24T20:42:42Z

Add `const` support for float rounding methods

This PR makes the following float rounding methods const:

f64::{floor, ceil, trunc, round, round_ties_even}
and the corresponding methods for f16, f32 and f128

Tracking issue: #141555

Procedure

I followed c09ed3e as closely as I could in making float methods const, and also received great guidance from https://internals.rust-lang.org/t/const-rounding-methods-in-float-types/22957/3?u=ruancomelli.

Note

This is my first code contribution to the Rust project, so please let me know if I missed anything - I'd be more than happy to revise and learn more. Thank you for taking the time to review it!

rustbot · 2025-05-24T20:42:46Z

r? @ibraheemdev

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rustbot · 2025-05-24T20:42:48Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

RalfJung · 2025-05-24T20:58:56Z

However, I could have implemented a single one taking a rustc_apfloat::Round parameter:

Yes, that's a lot better I think. :)

RalfJung · 2025-05-24T21:17:46Z

Note that your branch has conflicts with the master branch. Not sure how that happened given that the PR is fresh, but it means you'll have to rebase and resolve those conflicts.

ruancomelli · 2025-05-24T23:23:40Z

Strange, CI is failing with intrinsic is not supported at compile-time for each one of the new methods.
I'm looking into that.

ruancomelli · 2025-05-25T00:24:36Z

As far as I can see, the main difference between the newly const methods (e.g. floor) and the ones that already work (e.g. abs) is the attributes. So my current hypothesis is that I got them wrong.

CAD97

AIUI, the reason you're hitting "intrinsic not supported at compile time" is that std currently gets built with the bootstrap compiler, which is then used when building the new compiler. So you need to have a #[cfg(bootstrap)] version of the function/intrinsic which isn't const and a #[cfg(not(bootstrap))] which is.

IIUC, we're in the process of flipping this so that the compiler gets built with the bootstrap std, and std always gets built with the new compiler. Once that switch happens, this won't be necessary anymore.

library/core/src/intrinsics/mod.rs

library/core/src/num/f32.rs

library/std/src/f64.rs

RalfJung · 2025-05-25T07:18:14Z

AIUI, the reason you're hitting "intrinsic not supported at compile time" is that std currently gets built with the bootstrap compiler, which is then used when building the new compiler. So you need to have a #[cfg(bootstrap)] version of the function/intrinsic which isn't const and a #[cfg(not(bootstrap))] which is.

The problem occurs when building the tests. So you don't need to do any cfg in libcore itself, but the new tests in library/coretests need a #[cfg(not(bootstrap))].

library/coretests/tests/num/mod.rs

tests/ui/consts/const-eval/float_methods.rs

RalfJung · 2025-05-25T14:32:39Z

Your functions are not meant to be stable, right? So do the part under "help: if the function is not (yet) meant to be exposed to stable". The other part explicitly says "as a last resort" and "requires team approval"... I don't know how much more scary we have to make that message to stop people from using the attribute. :( Maybe we should just stop mentioning it entirely.

ruancomelli · 2025-05-25T17:23:35Z

Hey @RalfJung and @CAD97, I've addressed or replied to all of your review comments.

I've also created a tracking issue for this feature here: #141555.

I have two main open questions now.

First one is: should the rounding methods on f16 and f128 be gated behind #[rustc_const_unstable(feature = {"f16", "f128"}, issue = "116909")] (from #116909) or #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]? I think it should be the former one, but I'm not sure.

The second question: I added #![feature(const_float_round_methods)] to library/std/src/lib.rs, which was suggested by the compiler itself. Without it, ./x check results in the following errors:

error: `core::f32::math::floor` is not yet stable as a const fn
  --> library/std/src/f32.rs:50:9
   |
50 |         core::f32::math::floor(self)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: add `#![feature(const_float_round_methods)]` to the crate attributes to enable
  --> library/std/src/lib.rs:433:1
   |
433+ #![feature(const_float_round_methods)]
   |

... and similar for the other rounding methods

error: could not compile `std` (lib) due to 12 previous errors

This is similar to what was done in the reference PR, even though they changed library/core/src/lib.rs (notice core and not std).
However, I'm not sure if this is the correct way to proceed. I read the dev guide book, but that question remained unanswered.

library/coretests/tests/num/mod.rs

RalfJung · 2025-05-25T20:45:40Z

First one is: should the rounding methods on f16 and f128 be gated behind #[rustc_const_unstable(feature = {"f16", "f128"}, issue = "116909")] (from #116909) or #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]? I think it should be the former one, but I'm not sure.

Good question! The typical thing we do is to gate it behind the f16/f128 feature, but then also have a second commented out more specific feature gate:

#[rustc_const_unstable(feature = "f16", issue = "116909")]
// #[rustc_const_unstable(feature = "const_float_round_methods", issue = "141555")]
pub const fn trunc(self) -> self { ... }

The second question: I added #![feature(const_float_round_methods)] to library/std/src/lib.rs, which was suggested by the compiler itself.

Yes, that's fine.
Thanks for asking for confirmation. :)

RalfJung · 2025-05-25T20:46:15Z

@rustbot author

rustbot · 2025-05-25T20:46:18Z

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

RalfJung · 2025-05-29T16:05:57Z

This PR still has 20 commits -- if you squashed something, you didn't actually push it?

ruancomelli · 2025-05-29T17:05:10Z

Hey @RalfJung!

Actually, I hadn't seen your comment #141521 (comment) when I posted #141521 (comment). I just meant to let you know that I had addressed all other comments and had the CI passing.

I have now squashed all commits into a single one as well. I didn't see value in keeping more than one commit in this PR. I again followed #130568 to make this decision.

Please let me know if you would like me to try and split the final commit into multiple ones.

And thank you so much for your patience and constant feedback in this PR! It really made the process much easier, and I am very happy with the end result.
Also, sorry for the friction... this is my first code contribution to the Rust project 😀

@rustbot ready

tgross35 · 2025-05-29T17:08:35Z

Fwiw usually "squash" actually means "fixup", it's not too useful to keep in the messages that you had to do things like "adjust after rebase" and "fix float tests" at some point :) Not that it's much of a problem of course.

RalfJung · 2025-05-29T18:03:20Z

And thank you so much for your patience and constant feedback in this PR! It really made the process much easier, and I am very happy with the end result.
Also, sorry for the friction... this is my first code contribution to the Rust project

No worries! It's all good. Thanks for your first PR, and congrats on pushing it through to the end. :)

@bors r+

bors · 2025-05-29T18:03:23Z

📌 Commit a1d9cc4 has been approved by RalfJung

It is now in the queue for this repository.

bors · 2025-05-30T03:13:45Z

☔ The latest upstream changes (presumably #141739) made this pull request unmergeable. Please resolve the merge conflicts.

Add const support for the float rounding methods floor, ceil, trunc, fract, round and round_ties_even. This works by moving the calculation logic from src/tools/miri/src/intrinsics/mod.rs into compiler/rustc_const_eval/src/interpret/intrinsics.rs. All relevant method definitions were adjusted to include the `const` keyword for all supported float types: f16, f32, f64 and f128. The constness is hidden behind the feature gate feature(const_float_round_methods) which is tracked in rust-lang#141555 This commit is a squash of the following commits: - test: add tests that we expect to pass when float rounding becomes const - feat: make float rounding methods `const` - fix: replace `rustc_allow_const_fn_unstable(core_intrinsics)` attribute with `#[rustc_const_unstable(feature = "f128", issue = "116909")]` in `library/core/src/num/f128.rs` - revert: undo update to `library/stdarch` - refactor: replace multiple `float_<mode>_intrinsic` rounding methods with a single, parametrized one - fix: add `#[cfg(not(bootstrap))]` to new const method tests - test: add extra sign tests to check `+0.0` and `-0.0` - revert: undo accidental changes to `round` docs - fix: gate `const` float round method behind `const_float_round_methods` - fix: remove unnecessary `#![feature(const_float_methods)]` - fix: remove unnecessary `#![feature(const_float_methods)]` [2] - revert: undo changes to `tests/ui/consts/const-eval/float_methods.rs` - fix: adjust after rebase - test: fix float tests - test: add tests for `fract` - chore: add commented-out `const_float_round_methods` feature gates to `f16` and `f128` - fix: adjust NaN when rounding floats - chore: add FIXME comment for de-duplicating float tests - test: remove unnecessary test file `tests/ui/consts/const-eval/float_methods.rs` - test: fix tests after upstream simplification of how float tests are run

RalfJung · 2025-05-31T20:59:28Z

@bors r+

bors · 2025-05-31T20:59:32Z

📌 Commit f8e97ba has been approved by RalfJung

It is now in the queue for this repository.

…r=RalfJung Add `const` support for float rounding methods # Add `const` support for float rounding methods This PR makes the following float rounding methods `const`: - `f64::{floor, ceil, trunc, round, round_ties_even}` - and the corresponding methods for `f16`, `f32` and `f128` Tracking issue: rust-lang#141555 ## Procedure I followed rust-lang@c09ed3e as closely as I could in making float methods `const`, and also received great guidance from https://internals.rust-lang.org/t/const-rounding-methods-in-float-types/22957/3?u=ruancomelli. ## Note This is my first code contribution to the Rust project, so please let me know if I missed anything - I'd be more than happy to revise and learn more. Thank you for taking the time to review it!

Rollup of 6 pull requests Successful merges: - #141072 (Stabilize feature `result_flattening`) - #141215 (std: clarify Clone trait documentation about duplication semantics) - #141277 (Miri CI: test aarch64-apple-darwin in PRs instead of the x86_64 target) - #141521 (Add `const` support for float rounding methods) - #141812 (Fix "consider borrowing" for else-if) - #141832 (library: explain TOCTOU races in `fs::remove_dir_all`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #141521 - ruancomelli:const-float-rounding, r=RalfJung Add `const` support for float rounding methods # Add `const` support for float rounding methods This PR makes the following float rounding methods `const`: - `f64::{floor, ceil, trunc, round, round_ties_even}` - and the corresponding methods for `f16`, `f32` and `f128` Tracking issue: #141555 ## Procedure I followed c09ed3e as closely as I could in making float methods `const`, and also received great guidance from https://internals.rust-lang.org/t/const-rounding-methods-in-float-types/22957/3?u=ruancomelli. ## Note This is my first code contribution to the Rust project, so please let me know if I missed anything - I'd be more than happy to revise and learn more. Thank you for taking the time to review it!

Rollup of 6 pull requests Successful merges: - rust-lang/rust#141072 (Stabilize feature `result_flattening`) - rust-lang/rust#141215 (std: clarify Clone trait documentation about duplication semantics) - rust-lang/rust#141277 (Miri CI: test aarch64-apple-darwin in PRs instead of the x86_64 target) - rust-lang/rust#141521 (Add `const` support for float rounding methods) - rust-lang/rust#141812 (Fix "consider borrowing" for else-if) - rust-lang/rust#141832 (library: explain TOCTOU races in `fs::remove_dir_all`) r? `@ghost` `@rustbot` modify labels: rollup

…r=RalfJung Add `const` support for float rounding methods # Add `const` support for float rounding methods This PR makes the following float rounding methods `const`: - `f64::{floor, ceil, trunc, round, round_ties_even}` - and the corresponding methods for `f16`, `f32` and `f128` Tracking issue: rust-lang#141555 ## Procedure I followed rust-lang@c09ed3e as closely as I could in making float methods `const`, and also received great guidance from https://internals.rust-lang.org/t/const-rounding-methods-in-float-types/22957/3?u=ruancomelli. ## Note This is my first code contribution to the Rust project, so please let me know if I missed anything - I'd be more than happy to revise and learn more. Thank you for taking the time to review it!

Rollup of 6 pull requests Successful merges: - rust-lang#141072 (Stabilize feature `result_flattening`) - rust-lang#141215 (std: clarify Clone trait documentation about duplication semantics) - rust-lang#141277 (Miri CI: test aarch64-apple-darwin in PRs instead of the x86_64 target) - rust-lang#141521 (Add `const` support for float rounding methods) - rust-lang#141812 (Fix "consider borrowing" for else-if) - rust-lang#141832 (library: explain TOCTOU races in `fs::remove_dir_all`) r? `@ghost` `@rustbot` modify labels: rollup

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Tracking Issue for const_float_euclidean_division #141572

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constify float::{div,rem}_euclid #141755

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Rollup of 7 pull requests #141841

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Add const support for float rounding methods #141521

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