Crate const_init_macros 

When deriving ConstInit a struct will have a const_init() function which allows an initialization at build time.

The fields are initialized with a global variable matching the field name in uppercase by default. You can also specify which constant value you want to initialize your field with, using the attribute #[const_init(value = {constant value/variable})].

This is basically the Default derive macros but with constant values. The goal is to initialize the struct with values that are known at compile time.

It is meant to be used with the crate const_init_build which helps you create constant variables at build time from a configuration file.

Examples

use const_init_macros::ConstInit;

#[derive(ConstInit)]
// This attribute is used to import constant variables from another module.
// Here it would import the variables
#[const_init(import = generated::settings)]
struct FooBar {
    // With attribute to specify a constant variable
    #[const_init(value = FOO)]
    foo: bool,
    // Without attribute, looking for matching uppercase field name "BAR"
    bar: usize,
    #[const_init(value = 3.14)]
    toto: f64,
}

Derived code

impl FooBar {
    use generated::settings::*;
    pub const fn const_init() -> Self {
        FooBar {
            foo: FOO,
            bar: BAR,
            toto: 3.14,
        }
    }
}

Notes: we don’t implement a trait ConstInit because const function in traits are not allowed as of now

Use case

fn main() {
    // Get constant variable for better compiler optimizations
    const FOO_BAR: FooBar = FooBar::const_init()
    ...
}

Derive Macros

ConstInit