allow Deserialize derive to handle generic unit structs

serde_derive/src/de.rs

@@ -410,16 +410,29 @@ fn deserialize_unit_struct(params: &Parameters, cattrs: &attr::Container) -> Fra
     let this_type = &params.this_type;
     let this_value = &params.this_value;
     let type_name = cattrs.name().deserialize_name();
+    let (de_impl_generics, de_ty_generics, ty_generics, where_clause) =
+        split_with_de_lifetime(params);
+    let delife = params.borrowed.de_lifetime();
 
     let expecting = format!("unit struct {}", params.type_name());
     let expecting = cattrs.expecting().unwrap_or(&expecting);
 
+    let visitor_expr = quote! {
+        __Visitor {
+            marker: _serde::__private::PhantomData::<#this_type #ty_generics>,
+            lifetime: _serde::__private::PhantomData,
+        }
+    };
+
     quote_block! {
         #[doc(hidden)]
-        struct __Visitor;
+        struct __Visitor #de_impl_generics #where_clause {
+            marker: _serde::__private::PhantomData<#this_type #ty_generics>,
+            lifetime: _serde::__private::PhantomData<&#delife ()>,
+        }
 
-        impl<'de> _serde::de::Visitor<'de> for __Visitor {
+        impl #de_impl_generics _serde::de::Visitor<#delife> for __Visitor #de_ty_generics #where_clause {
-            type Value = #this_type;
+            type Value = #this_type #ty_generics;
 
             fn expecting(&self, __formatter: &mut _serde::__private::Formatter) -> _serde::__private::fmt::Result {
                 _serde::__private::Formatter::write_str(__formatter, #expecting)
@@ -434,7 +447,7 @@ fn deserialize_unit_struct(params: &Parameters, cattrs: &attr::Container) -> Fra
             }
         }
 
-        _serde::Deserializer::deserialize_unit_struct(__deserializer, #type_name, __Visitor)
+        _serde::Deserializer::deserialize_unit_struct(__deserializer, #type_name, #visitor_expr)
     }
 }
 

test_suite/tests/test_de.rs

@@ -46,6 +46,9 @@ mod macros;
 #[derive(Copy, Clone, PartialEq, Debug, Deserialize)]
 struct UnitStruct;
 
+#[derive(Copy, Clone, PartialEq, Debug, Deserialize)]
+struct GenericUnitStruct<const N: u8>;
+
 #[derive(PartialEq, Debug, Deserialize)]
 struct NewtypeStruct(i32);
 
@@ -883,6 +886,17 @@ fn test_unit_struct() {
     test(UnitStruct, &[Token::UnitStruct { name: "UnitStruct" }]);
 }
 
+#[test]
+fn test_generic_unit_struct() {
+    test(GenericUnitStruct::<8>, &[Token::Unit]);
+    test(
+        GenericUnitStruct::<8>,
+        &[Token::UnitStruct {
+            name: "GenericUnitStruct",
+        }],
+    );
+}
+
 #[test]
 fn test_newtype_struct() {
     test(