resources

This crate provides the Resources struct: a container that stores at most one value of each specific type, and allows safely and concurrently accessing any of them with interior mutability, without violating borrow rules.

It's intended to be used as an implementation of storage for data that is not associated with any specific entity in an ECS (entity component system), but still needs concurrent access by systems.

use resources::*;

struct SomeNumber(usize);

struct SomeString(&'static str);

fn main() {
    let mut resources = Resources::new();
    resources.insert(SomeNumber(4));
    resources.insert(SomeString("Hello!"));
    let resources = resources;  // This shadows the mutable binding with an immutable one.

    {
        let mut some_number = resources.get_mut::<SomeNumber>().unwrap();
        let mut some_string = resources.get_mut::<SomeNumber>().unwrap();

        // Immutably borrowing a resource that's already borrowed mutably is not allowed.
        assert!(resources.get::<SomeNumber>().is_err());

        some_number.0 = 2;
        some_string.0 = "Bye!";
    }

    // The mutable borrows now went out of scope, so it's okay to borrow again however needed.
    assert_eq!(resources.get::<SomeNumber>().0, 2);

    // Multiple immutable borrows are okay.
    let some_string1 = resources.get::<SomeString>().unwrap();
    let some_string2 = resources.get::<SomeString>().unwrap();
    assert_eq!(some_string1.0, some_string2.0);
}