ACP: Add a constant for the golden ratio (phi) to the floating point types

[ghost]

Proposal

Problem statement

The std::f64::consts module (and its friend std::f32::consts) currently contain constants for e (euler's number) and pi, along with some others (ln(2), 1/sqrt(2), etc). These are a subset of the ones in C++, although C++ also includes phi (φ), the golden ratio.

I am proposing to add a PHI constant to the standard library, in similar fashion to the existing floating point constants.

Motivation, use-cases

From a math expression parsing library I'm working on:

use std::f64;

enum MathConst {
    E, // euler's number
    Pi, // pi
    Phi, // golden ratio
}

impl From<MathConst> for f64 {
    fn from(value: MathConst) -> f64 {
        match value {
            MathConst::E => f64::consts::E,
            MathConst::Pi => f64::consts::PI,
            MathConst::Phi => f64::consts::PHI, // not a thing :(
        }
    }
}

Solution sketches

In core/src/num/f64.rs:

pub mod consts {
    // ...    

    /// The golden ratio (φ)
    pub const PHI: f64 = 1.618033988749894848204586834365638118_f64;
    
    // ...
}

In core/src/num/f32.rs:

pub mod consts {
    // ...    

    /// The golden ratio (φ)
    pub const PHI: f64 = 1.618033988749894848204586834365638118_f32;
    
    // ...
}

Links and related work

• An RFC discussion on the addition of the TAU constant (with some mentions of phi)
• cppreference.com <numbers> header
• libstdc++ inline constexpr double phi

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[cuviper]

These are a subset of the ones in C++, although C++ also includes phi (φ), the golden ratio.

Maybe we should go ahead and add all that we're missing?

[ghost]

My statement about a "subset" of C++ was incorrect, as can be seen from the comparison table below.

Of the total 24 constants, Rust has 19 and C++ has 13.
8 of the constants are in both languages. Rust has 11 unique and C++ has 5 unique.

It may be worth adding the 5 constants that are in C++ but not in Rust (phi, gamma, 1/sqrt(pi), sqrt(3), and 1/sqrt(3)).

Click To See Table

Constant	Rust	C++
e	✅	✅
pi	✅	✅
phi	❌	✅
tau	✅	❌
gamma	❌	✅
1/pi	✅	✅
2/pi	✅	❌
1/sqrt(pi)	❌	✅
2/sqrt(pi)	✅	❌
sqrt(2)	✅	✅
sqrt(3)	❌	✅
1/sqrt(2)	✅	❌
1/sqrt(3)	❌	✅
pi/2	✅	❌
pi/3	✅	❌
pi/4	✅	❌
pi/6	✅	❌
pi/8	✅	❌
ln(2)	✅	✅
ln(10)	✅	✅
log2(10)	✅	❌
log2(e)	✅	✅
log10(2)	✅	❌
log10(e)	✅	✅

[joshtriplett]

We reviewed this in today's @rust-lang/libs-api meeting, and we're fine with adding all five of the additional constants C++ has in that table (the five added by rust-lang/rust#103883).