New registry shape

[robertbastian]

#4721

[sffc]

Praise: this is great! Glad it works

[sffc]

Aha, I was thinking whether we should make all our marker types be empty enums in v2, but here's a use case where we need them as a value

[robertbastian]

I'm happy to redesign this, I don't like how we currently do it. The #[data_struct] macro could add a non-self fn bake_type() -> TokenStream to the marker.

[Manishearth]

LG after diffbase

[sffc]

Thought: The bake macro could generate a registry macro of the same form but containing only the keys in that one crate, and then the same cb could be used and given to different registry macros.

Of course what I want to use the registry! for is enabling easier creation of forking DataProvider impls. I was thinking that a way to do this would be if the registry created impls for all keys except a specific list. What I'm thinking is for the macro to generate code such as

#[if("MarkerA" != "MarkerB")]
#[if("MarkerA" != "MarkerC")]
impl DataProvider<MarkerA> for MyForkingProvider { ... }

#[if("MarkerB" != "MarkerB")]
#[if("MarkerB" != "MarkerC")]
impl DataProvider<MarkerB> for MyForkingProvider { ... }

#[if("MarkerC" != "MarkerB")]
#[if("MarkerC" != "MarkerC")]
impl DataProvider<MarkerC> for MyForkingProvider { ... }

#[if("MarkerD" != "MarkerB")]
#[if("MarkerD" != "MarkerC")]
impl DataProvider<MarkerD> for MyForkingProvider { ... }

With that generated code, impl DataProvider<MarkerB> and impl DataProvider<MarkerC> would be dropped, allowing them to be implemented ("specialized") in a custom way.

However, it's not immediately clear to me how to actually do this.

We could probably make a custom proc macro to help, if one doesn't already exist.

[sffc]

Got it:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=dca57af4445d1c0b44933a07c6a6c2da