rand::rand_impls::f64_rand_impls::SCALE

const SCALE: f64 = (1u64 << $mantissa_bits) as $ty;

            impl Rand for $ty {
                /// Generate a floating point number in the half-open
                /// interval `[0,1)`.
                ///
                /// See `Closed01` for the closed interval `[0,1]`,
                /// and `Open01` for the open interval `(0,1)`.
                #[inline]
                fn rand(rng: &mut R) -> $ty {
                    rng.$method_name()
                }
            }
            impl Rand for Open01<$ty> {
                #[inline]
                fn rand(rng: &mut R) -> Open01<$ty> {
                    // add a small amount (specifically 2 bits below
                    // the precision of f64/f32 at 1.0), so that small
                    // numbers are larger than 0, but large numbers
                    // aren't pushed to/above 1.
                    Open01(rng.$method_name() + 0.25 / SCALE)
                }
            }
            impl Rand for Closed01<$ty> {
                #[inline]
                fn rand(rng: &mut R) -> Closed01<$ty> {
                    // rescale so that 1.0 - epsilon becomes 1.0
                    // precisely.
                    Closed01(rng.$method_name() * SCALE / (SCALE - 1.0))
                }
            }
        }
    }
}
float_impls! { f64_rand_impls, f64

Unstable (rand)

: use rand from crates.io