Gamma function

[ghost]

It would be nice if tgamma and lgamma_r were exposed via std::num::Float. The foreign function definitions for both are present in src/libstd/num/f{32,64}.rs, but they're not exposed in the standard library.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#864

[blevin]

If anyone needs gamma in the meantime, it turns out to be pleasingly easy to invoke the C math library one:

#[link(name = "m")]
extern {
    fn tgamma(x: f64) -> f64;
}
fn gamma(x: f64) -> f64 {
    unsafe { tgamma(x) }
}

[x-ji]

Meanwhile some external libraries also provide this functionality: https://docs.rs/statrs/0.9.0/statrs/function/gamma/fn.gamma.html