Constants

In Rust there are two types of constants const and static. const are immutable values hardcoded into the program. statics are optionally mutable values that are globally available. Using mutable statics is unsafe unless wrapped in thread safe structs such as Arc and Mutex. (See Concurrency)

#![allow(unused)]
fn main() {
const VERSION: u32 = 1;
static PROGRAM_NAME: &'static str = "Example";
}

Currently const and static values must be known at compile time. To get around this you can use the lazy_static crate, it works like by lazy {} in Kotlin:

use lazy_static::lazy_static;

lazy_static! {
	static ref A_MAP = HashMap::new();
}

const values are inserted at compile time where ever they were used, and so if you make a a mutable constant every use will be it's own instance.

A method can be constant if it's all of it's internal are constant as well, for example:

#![allow(unused)]
fn main() {
const FOO: usize = 1;
const BAR: usize = 2;

const RESULT: usize = add(FOO, BAR);

const fn add(lhs: usize, rhs: usize) -> usize {
	lhs + rhs
}
}