Add an interface for `State` to easily define and enforce legal state transitions

What problem does this solve or what need does it fill?

Currently, you can define a finite state machine like the following:

#[derive(States, Clone, Copy, PartialEq, Eq, Hash, Debug, Default)]
enum NewFSM {
    #[default]
    Idle,
    Walk,
    Walk2Swim,
    Swim,
    Swim2Walk,
    Drowning
}

When using this new State resource, transitions to all states defined are legal by default.

Below is a simple diagram of our new FSM. (excluding idle):

The arrows in the diagram represent the legal transitions between states. In this example, you would not want to ever transition from the Walk state to the Drowning state, or vice versa.

Currently, to enforce these transition rules, you have to implement this logic manually at all states' enter/exit systems. This leads to the developer writing more of an "all over the place" solution to enforce these transition rules.

What solution would you like?

I'm actually still getting my feet wet in Rust, so I do not have a clear solution in mind.

Although, when thinking of a solution, a certain feature from strum does come to mind. The following sample shows how you can add custom properties to enum variants using strum:

use strum::EnumProperty;

#[derive(PartialEq, Eq, Debug, EnumProperty)]
enum Class {
    #[strum(props(Teacher="Ms.Frizzle", Room="201"))]
    History,
    #[strum(props(Teacher="Mr.Smith"))]
    #[strum(props(Room="103"))]
    Mathematics,
    #[strum(props(Time="2:30"))]
    Science,
}

Possibly could have a similar solution for declaring which enum variant a certain state can transition to. Then, when requesting to transition states, Bevy can enforce these rules by checking if the transition is legal based on the rules defined by the developer.

What alternative(s) have you considered?

As mentioned above, the current alternative solution is to manually write the necessary logic to enforce transition rules at all states' enter/exit systems.

bevyengine / bevy