Add EnumDiscriminants Trait and related Macro impl (#377)

* Add EnumDiscriminants Trait and related Macro impl

* Apply related suggestions from Maintainer: add trait method and rename trait

---------

Co-authored-by: vchapuis <vincent.chapuis@fortune.com.tw>

strum/src/lib.rs

@@ -217,6 +217,15 @@ pub trait VariantNames {
     const VARIANTS: &'static [&'static str];
 }
 
+/// 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
+    type Discriminant;
+
+    fn discriminant(&self) -> Self::Discriminant;
+}
+
 /// A trait for retrieving a static array containing all the variants in an Enum.
 /// This trait can be autoderived by `strum_macros`. For derived usage, all the
 /// variants in the enumerator need to be unit-types, which means you can't autoderive
@@ -248,7 +257,7 @@ DocumentMacroRexports! {
     AsRefStr,
     Display,
     EnumCount,
-    EnumDiscriminants,
+    IntoDiscriminant,
     EnumIter,
     EnumMessage,
     EnumProperty,

strum_macros/src/macros/enum_discriminants.rs

@@ -22,6 +22,7 @@ pub fn enum_discriminants_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
 
     // Derives for the generated enum
     let type_properties = ast.get_type_properties()?;
+    let strum_module_path = type_properties.crate_module_path();
 
     let derives = type_properties.discriminant_derives;
 
@@ -165,6 +166,14 @@ pub fn enum_discriminants_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
             #(#discriminants),*
         }
 
+        impl #impl_generics #strum_module_path::IntoDiscriminant for #name #ty_generics #where_clause {
+            type Discriminant = #discriminants_name;
+
+            fn discriminant(&self) -> Self::Discriminant {
+                <Self::Discriminant as ::core::convert::From<&Self>>::from(self)
+            }
+        }
+
         #impl_from
         #impl_from_ref
     })