Local with required config as a const generic parameter

[mockersf]

Objective

• Alternative to Add Required system param #2440, but with a const generic parameter on Local instead of a new type

Solution

Either:

• parameter should use the default value or a FromWorld impl, use Local<T, true>
• parameter must be configured using config, use Local<T, false>

If the parameter is false, the bound for T on FromWorld is lifted.

#[derive(Default)]
struct DefaultableStruct(u32);

struct NotDefaultableStruct(u32);

fn my_system(a: Local<DefaultableStruct, true>, b: Local<NotDefaultableStruct, false>) {}

Ideally, I would want the Local type to be:

enum LocalType {
    FromWorld,
    NeedConfig
}

pub struct Local<'a, T: Component, const LOCAL_TYPE: LocalType = LocalType::FromWorld>(&'a mut T);

Then my system would be:

fn my_system(a: Local<DefaultableStruct>, b: Local<NotDefaultableStruct, LocalType::NeedConfig>) {}

But this is not possible in Rust for now 😞 :

• we can't have our own type as a const generic: https://blog.rust-lang.org/2021/02/26/const-generics-mvp-beta.html#const-generics-for-custom-types
• we can't have a default value for const generics: https://blog.rust-lang.org/2021/02/26/const-generics-mvp-beta.html#const-generics-and-default-arguments

Those are the two top issues in "what's next", so hopefully they should come soonish...

[alice-i-cecile]

I prefer this solution in general, but would like to hold off for a prettier version. I suppose the churn isn't too bad; this is not a particularly prominent feature and the refactor would be easy.

Overall, making sure that the configured and defaulted versions of Local are the same type makes communicating about the API much simpler and reduces code duplication.

[mockersf]

once we can use an enum, we could have more control on how it's initialized:

enum LocalType {
    FromWorld,
    Default, // now separate from FromWorld
    NeedConfig
}

[cart]

Ooh this is a clever take / a pattern that wasn't on my radar. I'd also prefer to hold off until const generic defaults are an option (and ideally, enums).

[mockersf]

it seems feature const_generics_defaults is on a good way to stabilisation 🎉

[TheRawMeatball]

We could already use something like this design, if we substitute a module with 2 unit structs instead of an enum:

mod local_mode {
    struct FromWorld;
    struct NeedsConfig;
}

[NathanSWard]

it seems feature const_generics_defaults is on a good way to stabilisation 🎉

Just upgrade the compiler we use to nightly and all our problems are solved 😉

[DJMcNab]

As of #3633, this no longer makes sense.