Rust References and Lifetimes Guide is not the place for tau advocacy

[joshtriplett]

Quoting the Rust References and Lifetimes Guide:

    Circle(_, radius) => 0.5 * tau * radius * radius,

...

(Like any up-to-date engineer, we use the tau circle constant and not that dreadfully outdated notion of pi).

Without arguing over the merits of pi versus tau, a Rust guide is not the place for introducing a less common mathematical notation that most people will not have seen. Doubly so because this is one of the specific cases where tau is less clear than pi; the equivalent code with pi would not need the extra factor of 0.5. And triply so because this should be using a constant from the standard library, like std::f64::consts::PI.

All three make this an unnecessary distraction in the middle of a Rust tutorial guide.

[steveklabnik]

This guide is being re-written, and I can assure you, there will be no Tau advocacy in it. That said, in the meantime I would accept a patch to fix this.=