Rollup merge of rust-lang#66769 - fusion-engineering-forks:tau-constant, r=dtolnay

Add core::{f32,f64}::consts::TAU.

### **`τ`**

Author: tmandry

src/libcore/num/f32.rs

@@ -75,6 +75,12 @@ pub mod consts {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const PI: f32 = 3.14159265358979323846264338327950288_f32;
 
+    /// The full circle constant (τ)
+    ///
+    /// Equal to 2π.
+    #[unstable(feature = "tau_constant", issue = "66770")]
+    pub const TAU: f32 = 6.28318530717958647692528676655900577_f32;
+
     /// π/2
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const FRAC_PI_2: f32 = 1.57079632679489661923132169163975144_f32;

src/libcore/num/f64.rs

@@ -75,6 +75,12 @@ pub mod consts {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const PI: f64 = 3.14159265358979323846264338327950288_f64;
 
+    /// The full circle constant (τ)
+    ///
+    /// Equal to 2π.
+    #[unstable(feature = "tau_constant", issue = "66770")]
+    pub const TAU: f64 = 6.28318530717958647692528676655900577_f64;
+
     /// π/2
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const FRAC_PI_2: f64 = 1.57079632679489661923132169163975144_f64;