make functions `const` where possible

[rursprung]

as the title says: it'd be great if as many functions as possible were made const so that they can be used in a const environment.

currently this code will not compile because the Add implementation of Complex isn't const:

use num::complex::Complex32;

const X: Complex32 = Complex32::new(1., 0.);
const Y: Complex32 = Complex32::new(0., 1.);
const Z: Complex32 = X + Y;

you can find the same example on the playground.

error from the playground:

   Compiling playground v0.0.1 (/playground)
error[E0015]: cannot call non-const operator in constants
   --> src/main.rs:6:26
    |
6   |     const Z: Complex32 = X + Y;
    |                          ^^^^^
    |
note: impl defined here, but it is not `const`
   --> /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/num-complex-0.4.4/src/lib.rs:724:1
    |
724 | impl<T: Clone + Num> Add<Complex<T>> for Complex<T> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: calls in constants are limited to constant functions, tuple structs and tuple variants

For more information about this error, try `rustc --explain E0015`.

[cuviper]

Rust as a language doesn't have const trait implementations yet, so we don't have any way to make stuff like operator + const. Most of the inherent methods of Complex also depend on trait bounds, so they can't be const yet either -- e.g. even something simple like const fn i() would need const Zero and One.

[rursprung]

oh, sorry, i didn't even realise that!
i just looked around a bit and it seems that it'll need this nightly feature to be stabilised: rust-lang/rust#67792
once that's done it should be possible to make these implementations const

[cuviper]

For simple stuff like Add, at least it's not too bad to do it manually:

const X: Complex32 = Complex32::new(1., 0.);
const Y: Complex32 = Complex32::new(0., 1.);
const Z: Complex32 = Complex32::new(X.re + Y.re, X.im + Y.im);

But of course this would be a pain for more complex expressions. (pun intended 😄)

If you have a lot of constant values like this that you want to prepare, you could also do it in a build.rs script to write a generated source file, as described in the Cargo book:
https://doc.rust-lang.org/cargo/reference/build-script-examples.html#code-generation