Literal macros for extprim.

This crate provides a syntex extension (on stable) so that the extprim types can be constructed at compile-time using the i128!() and u128!() macros.

Since built-in u128/i128 types are provided on nightly (Rust 1.16), the i128!() and u128!() macros will simply cast the input from these built-in types.

Setup as syntex extension (stable)

Supply a build.rs, and add syntex and extprim_literals to the build-dependencies in Cargo.toml:

[package]
build = "build.rs"

[build-dependencies]
extprim_literals = "1.2.0"
syntex = "0.48.0"

Register extprim_literals to syntex in build.rs:

extern crate syntex;
extern crate extprim_literals;

use syntex::Registry;
use std::env;
use std::path::Path;

fn main() {
    let mut registry = Registry::new();
    extprim_literals::register(&mut registry);

    let src = Path::new("src/consts.rs.in");
    let dst = Path::new(&env::var("OUT_DIR").unwrap()).join("consts.rs");

    registry.expand("extprim_literals", &src, &dst).unwrap();
}

Use the macros in src/consts.rs.in:

#[macro_use] extern crate extprim_literals;
use extprim::u128::u128;

const TEN: u128 = u128!(10);

Include the expanded file in src/consts.rs:

include!(concat!(env!("OUT_DIR"), "/consts.rs"));