Tracking issue for core::{f32,f64}::consts::TAU

[m-ou-se]

Tracking issue for the TAU constant added in #66769.

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Before stabilization:

• Confirm that the numeric values we have are accurate.
• Decide whether we are comfortable using the name TAU or instead a longer but more widely accepted name like TWO_PI could be better.
• Wait for RFC that moves math constants from std::{f32,f64}::consts::.. to {f32,f64}::.. ?

[steveklabnik]

We only open tracking issues after an RFC is accepted. Thanks!

[m-ou-se]

@kennytm asked me to open this tracking issue in #66769 (comment)

[steveklabnik]

I don't think that's how the process is supposed to work at all but I'll defer to him (and I saw this before I saw that)

[jonas-schievink]

We've never required RFCs for simple libstd additions like this, so opening this tracking issue seems fine

[tesuji]

I don't think we should land this constant.
Acording to wikipedia:

It has also been used in at least one mathematical research article,[33] authored by the τ-promoter Peter Harremoës.[34]

However, none of these proposals have received widespread acceptance by the mathematical and scientific communities.[35

[qnighy]

Naming aside, such a constant would be definitely useful. 2π couples tightly in the mathematical notation where 2.0 * PI doesn't, like x / (2.0 * PI).

[m-ou-se]

• Confirm that the numeric values we have are accurate.

Sources:

• https://oeis.org/A019692
• https://www.wolframalpha.com/input/?i=2+*+pi
• https://tauday.com/tau-digits

Can someone other than me verify that I copied the number properly?

Edit: Done by dtolnay.

[m-ou-se]

• Decide whether we are comfortable using the name TAU or instead a longer but more widely accepted name like TWO_PI could be better.

Unfortunately, the constant TWO_PI or 2_PI has been used in several programming languages for 2/π instead of 2π:

• https://www.gnu.org/software/libc/manual/html_node/Mathematical-Constants.html
• https://ziglang.org/documentation/master/std/#std;math.two_pi
• https://dlang.org/phobos/std_math.html#.M_2_PI

Although I agree that having TWO_PI or 2_PI refer to anything other than 2π makes absolutely no sense, I do think it might cause confusion because of the existing practice.