`Command` error handling has been simplified - Bevy

The Command trait now takes Out as an associated type rather than as a generic parameter. For function-style commands that return a Result, the code changes as follows:

rust

// 0.18
fn my_command() -> impl Command<Result> {
    move |world: &mut World| -> Result {
        // ...
    }
}

// 0.19
fn my_command() -> impl Command {
    move |world: &mut World| -> Result {
        // ...
    }
}

Implementors of the Command trait must now fill in the Out associated type:

rust

// 0.18
impl Command for Foo {
    fn apply(self, world: &mut World) {
        // ...
    }
}

// 0.19
impl Command for Foo {
    type Out = ();

    fn apply(self, world: &mut World) {
        // ...
    }
}

For commands that return Result:

rust

// 0.18
impl Command<Result> for Foo {
    fn apply(self, world: &mut World) -> Result {
        // ...
    }
}

// 0.19
impl Command for Foo {
    type Out = Result;

    fn apply(self, world: &mut World) -> Result {
        // ...
    }
}

The functionality of the HandleError and CommandWithEntity traits have been folded into Command and EntityCommand, respectively. Use the latter traits as the sole trait bound, if needed.