Proposal to add a trait for EnumDiscriminants

[vpochapuis]

Hello! First of all thank you very much for this amazing crate which is very useful for lots of application.

What

I would like to make a proposal to add a Trait implementation to the structs that got derived with EnumDiscriminants in order to use them into generics and get access to the generated enum.

Currently, if my understanding is correct, a new enum is generated from the original name and appended the Discriminants suffix, however i didn't find a convenient way to reference that Type from generics.

Hence this proposal would add a new Trait, EnumDiscriminants:

/// A trait for retrieving the enum generated by [`EnumDiscriminants`] from an associated
/// Type on the original enumeration. This trait can be autoderived by `strum_macros`.
pub trait EnumDiscriminants {
    /// Enum listing the same variants as this enum but without any data fields
    /// Note: maybe we could add some trait bounds based on defaults
    type Discriminants;
}

and related implementation in the macro:

...

        impl #impl_generics #strum_module_path::EnumDiscriminants for #name #ty_generics #where_clause {
            type Discriminants = #discriminants_name;
        }
...

Why

When building applications with Generics, it can be interesting to have access to this generated enum without having to explicitly pass the type. For example, in my case, I was building a select element for enums in the web using Yew, and wanted to pass to a child that Discriminants Enum through generics, hence we could have trait bounds like so:

impl<T> yew::Component for FormSelect<T>
where
    T: FromStr + strum::EnumDiscriminants + 'static,
    <<T as strum::EnumDiscriminants>::Discriminants: FromStr
        + strum::VariantNames
        + AsRef<str>,
    <<<T as strum::EnumDiscriminants>::Discriminants as FromStr>::Err: Debug,

Etc...

This let me save the state of the enum as a lightweight enum without having the potential issue to store additional unused data of enum variants inner types.

---

If you think this proposal is on par with this crate development, please take a look at this commit from my fork, and if you would like to have a PR , I will do so. If you think this doesn't fit in this project, please let me know too!

Have a nice day

[mohe2015]

I also want this, please open a PR. Also https://github.com/Peternator7/strum/pull/176 seems to be related.

[Peternator7]

@vpochapuis thanks for the proposal. Not sure why #176 died (probably lost on my end 😟). Seems reasonable to me, let's do it. One small request, can we implement this trait instead:

/// A trait for retrieving the enum generated by [`EnumDiscriminants`] from an associated
/// Type on the original enumeration. This trait can be autoderived by `strum_macros`.
pub trait IntoDiscriminant {
    /// Enum listing the same variants as this enum but without any data fields
    /// Note: maybe we could add some trait bounds based on defaults
    type Discriminant;

    fn discriminant(&self) -> Self::Discriminant;
}

[vpochapuis]

@vpochapuis thanks for the proposal. Not sure why #176 died (probably lost on my end 😟). Seems reasonable to me, let's do it. One small request, can we implement this trait instead:

/// A trait for retrieving the enum generated by [`EnumDiscriminants`] from an associated
/// Type on the original enumeration. This trait can be autoderived by `strum_macros`.
pub trait IntoDiscriminant {
    /// Enum listing the same variants as this enum but without any data fields
    /// Note: maybe we could add some trait bounds based on defaults
    type Discriminant;

    fn discriminant(&self) -> Self::Discriminant;
}

Sure~ Updated at : https://github.com/Peternator7/strum/pull/377

thanks for taking a look at my proposal~