allow discriminants on empty enum

strum_macros/src/macros/enum_discriminants.rs

@@ -136,7 +136,12 @@ pub fn enum_discriminants_inner(ast: &DeriveInput) -> syn::Result<TokenStream> {
         })
         .collect::<Vec<_>>();
 
-    let from_fn_body = quote! { match val { #(#arms),* } };
+    let from_fn_body = if variants.is_empty() {
+        //this method on empty enum is impossible to be called. it is therefor left empty
+        quote! { unreachable!()}
+    } else {
+        quote! { match val { #(#arms),* } }
+    };
 
     let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
     let impl_from = quote! {

strum_tests/tests/enum_discriminants.rs

@@ -3,6 +3,7 @@ use std::mem::{align_of, size_of};
 use enum_variant_type::EnumVariantType;
 use strum::{
     Display, EnumDiscriminants, EnumIter, EnumMessage, EnumString, FromRepr, IntoEnumIterator,
+    VariantArray,
 };
 
 mod core {} // ensure macros call `::core`
@@ -363,3 +364,16 @@ fn with_explicit_discriminant_value() {
         WithExplicitDicriminantValueDiscriminants::Variant0 as u8
     );
 }
+
+#[allow(dead_code)]
+#[derive(Debug, Eq, PartialEq, EnumIter, EnumDiscriminants)]
+#[strum_discriminants(derive(EnumIter, VariantArray))]
+enum Empty {}
+
+#[test]
+fn empty_test() {
+    let discriminants = Empty::iter().map(|i| i.into()).collect::<Vec<_>>();
+    let expected: Vec<_> = EmptyDiscriminants::VARIANTS.to_vec();
+
+    assert_eq!(expected, discriminants);
+}