Tracking Issue (take 2) for more_float_constants

Feature gate: #![feature(more_float_constants)]

This is a tracking issue for additional constants in the {f16, f32, f64, f128}::consts modules. Most of these match C++, with the exception of 1/sqrt(2π) https://en.cppreference.com/w/cpp/numeric/constants.html.

The original tracking issue was #103883 but we lost the ability to edit the top post.

Public API

// in core::{f16, f32, f64, f128}::consts

/// 1/sqrt(π)
pub const FRAC_1_SQRT_PI: f64 = 0.564189583547756286948079451560772586_f64;

/// 1/sqrt(2π)
pub const FRAC_1_SQRT_2PI: f64 = 0.398942280401432677939946059934381868_f64;

/// sqrt(3)
pub const SQRT_3: f64 = 1.732050807568877293527446341505872367_f64;

/// 1/sqrt(3)
pub const FRAC_1_SQRT_3: f64 = 0.577350269189625764509148780501957456_f64;

/* Already stabilized as euler_gamma_golden_ratio */

/// The golden ratio (φ)
pub const GOLDEN_RATIO: f64 = 1.618033988749894848204586834365638118_f64;

/// The Euler-Mascheroni constant (γ)
pub const EULER_GAMMA: f64 = 0.577215664901532860606512090082402431_f64;

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

ACP: ACP: Add a constant for the golden ratio (phi) to the floating point types libs-team#119
ACP for FRAC_1_SQRT_2PI Add FRAC_1_SQRT_2PI constant libs-team#383
Implementation: Add additional float constants #103836
Add FRAC_1_SQRT_2PI Add FRAC_1_SQRT_2PI constant to f16/f32/f64/f128 #125253
Add constants for f16 and f128 Add constants for f16 and f128 #123850
Final comment period (FCP)¹
Stabilization PR

Unresolved Questions

Naming of the Euler-Mascheroni constant (gamma).
FRAC_1_SQRT_2PI or FRAC_1_SQRT_TAU to match the existing TAU
Any additional constants?
- ln(3)
- SQRT_5 Tracking Issue for more_float_constants #103883 (comment)
- FRAC_1_SQRT_5 Tracking Issue for more_float_constants #103883 (comment)
How do we decide whether or not a constant makes the cutoff?

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

@rustbot

Since there is a proposal for more constants at rust-lang/libs-team#661, seems worth discussing whether there is interest in stabilizing the ones we already have. They have been unstably around since 2023, and most of the values have been stable for a few thousand years. There are a few unresolved questions on the issue.

@rustbot label +I-libs-api-nominated

Names for PHI and EGAMMA have come up a few times. Survey of other languages:

C++: same (phi, egamma) https://en.cppreference.com/w/cpp/numeric/constants.html
Julia: eulergamma and γ, golden and φ https://docs.julialang.org/en/v1/base/numbers/#Base.MathConstants.eulergamma
Matlab: eulergamma https://www.mathworks.com/help/symbolic/eulergamma.html, no phi
Numpy/Scipy: euler_gamma, golden and golden_ratio https://numpy.org/devdocs/reference/constants.html#numpy.euler_gamma https://docs.scipy.org/doc/scipy/reference/constants.html

@joshuarayton

@joshuarayton replying from #103883 (comment):

I also think that EGAMMA should be renamed to GAMMA. Most of the time if the full name is stated, it is either "Euler's constant" or the "Euler-Mascheroni constant". When mentioning it briefly it is just referred to as "gamma". All the other constants named after Greek letters are also only named after their respective Greek letter: pi, tau, phi. So for consistency it should simply be named GAMMA.

Note that this is a soft conflict with the gamma function (tgamma in math.h) which is represented by an uppercase gamma.

My personal preference would be to closer match Julia and Numpy by calling these EULER_GAMMA and GOLDEN_RATIO (don't feel very strongly about phi though).

Note that this is a soft conflict with the gamma function (tgamma in math.h) which is represented by an uppercase gamma.

My personal preference would be to closer match Julia and Numpy by calling these EULER_GAMMA and GOLDEN_RATIO (don't feel very strongly about phi though).

My personal preference would be GAMMA for the Euler-Mascheroni constant and PHI for the golden ratio. When these constants can be used multiple times in a single line of code, it's easier to have shorter names to prevent line wrapping.

I don't think GAMMA (constant) would conflict with gamma (function) because it is typical to have upper-case constant definitions. But I do accept this could be a reason against.

i'm fine with GAMMA/EGAMMA/EULER_GAMMA. but if you need a shorter name you could always rename the import yourself.

use std::f64::consts::EULER_GAMMA as γ;

@kennytm

My personal preference would be to closer match Julia and Numpy by calling these EULER_GAMMA and GOLDEN_RATIO (don't feel very strongly about phi though).

This seems right to me as well. “Phi” and “gamma” can mean many things, it’s better to use unambiguous names. As @kennytm mentions, you can and should rename the import if needed; that gives the benefit of brevity without losing any clarity.

Rollup merge of rust-lang#147076 - joshuarayton:more-float-constants-…

The current convention is to use the name of the symbol rather than of the constant itself (E, PI, and TAU being the stable examples).

Searching for "gamma constant" on Wikipedia yields both Euler's constant and the Fransén-Robinson constant. Yet, only the former is denoted with a gamma letter (γ vs. F). Some find GAMMA to be too ambiguous for it, but I haven't seen any actual example (yet) of how it is more so than E or TAU – or, more importantly, what it would be confused with in any practical case.

The Golden Ratio is almost always synonymous with merely "phi." I can't find any other constant that is widely denoted by a phi letter. I would argue that this is similar to how "pi" is exclusive to Archimedes' constant π.

PI is by far the most common and well-known name for the constant it references; there wasn’t really any choice there. The proponents of TAU want it to become as common as PI. As for E, personally I think it should have been EULER_E, especially as mathematics generally denotes it with a lowercase “e”, and “E” is used for a different constant.

Rollup merge of rust-lang#147076 - joshuarayton:more-float-constants-…

Yes someone needs to make a PR to rename the two constants EGAMMA, PHI to EULER_GAMMA, GOLDEN_RATIO and stabilize them (only for f32 and f64), as per the FCP decision above.

Renaming and stabilisation PR: #151164

Rollup merge of rust-lang#151164 - euler-gamma-golden-ratio, r=jhpratt

Unrolled build for #151164

I'd like to see "golden ratio" stabilized. Is there any way to stabilize some of them now?

I'd like to see "golden ratio" stabilized. Is there any way to stabilize some of them now?

🪄 abracadabra!

Rollup merge of rust-lang#151825 - joshuarayton:more-float-constants,…

Unrolled build for #151825

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Tracking Issue (take 2) for more_float_constants #146939

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tgross35 commented on Sep 26, 2025

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Tracking Issue (take 2) for `more_float_constants` #146939