checks

Adds several compile time checks for const generics.

Idea was based off of this reddit post.

Examples:

#![feature(generic_const_exprs)]

struct AlphabeticTest<const C: char>
	where checks::char::Alphabetic<C>: checks::Passed;

// All letters are alphabetic, check passes.
let pass = AlphabeticTest::<'a'>; // Compiles
let pass = AlphabeticTest::<'b'>; // Compiles
let pass = AlphabeticTest::<'c'>; // Compiles

// Letters are not alphabetic, check fails.
let fail = AlphabeticTest::<'1'>; // Compile error!
let fail = AlphabeticTest::<'2'>; // Compile error!
let fail = AlphabeticTest::<'3'>; // Compile error!

Custom checks can be implemented as well:

const fn is_binary(num: u8) -> bool { num == 0 || num == 1 }
struct BinaryOnly<const N: u8> where
    Check<{ is_binary(N) }>: Passed;

let pass = BinaryOnly::<0>; // Compiles
let pass = BinaryOnly::<1>; // Compiles

let fail = BinaryOnly::<2>; // Compiler error!

Checks can also be defined with the check! macro:

check! { u8 =>
    /// Matches all binary numbers (0, 1)
    Binary(
        passes: 0, 1
        fails: 2, 3, 4
    ): |N| (N == 0 || N == 1)

}

Full list of checks can be found on the docs.rs page.