bevy_reflect: Fix binary deserialization not working for unit structs (#6722)

# Objective 

Fixes #6713

Binary deserialization is failing for unit structs as well as structs with all ignored/skipped fields.

## Solution

Add a check for the number of possible fields in a struct before deserializing. If empty, don't attempt to deserialize any fields (as there will be none).

Note: ~~This does not apply to enums as they do not properly handle skipped fields (see #6721).~~ Enums still do not properly handle skipped fields, but I decided to include the logic for it anyways to account for `#[reflect(ignore)]`'d fields in the meantime.

---

## Changelog

- Fix bug where deserializing unit structs would fail for non-self-describing formats

Author: MrGVSV

crates/bevy_reflect/src/serde/de.rs

@@ -341,6 +341,7 @@ impl<'a, 'de> DeserializeSeed<'de> for TypedReflectDeserializer<'a> {
                     struct_info.field_names(),
                     StructVisitor {
                         struct_info,
+                        registration: self.registration,
                         registry: self.registry,
                     },
                 )?;
@@ -411,6 +412,7 @@ impl<'a, 'de> DeserializeSeed<'de> for TypedReflectDeserializer<'a> {
                         enum_info.variant_names(),
                         EnumVisitor {
                             enum_info,
+                            registration: self.registration,
                             registry: self.registry,
                         },
                     )?
@@ -439,6 +441,7 @@ impl<'a, 'de> DeserializeSeed<'de> for TypedReflectDeserializer<'a> {
 
 struct StructVisitor<'a> {
     struct_info: &'static StructInfo,
+    registration: &'a TypeRegistration,
     registry: &'a TypeRegistry,
 }
 
@@ -463,6 +466,18 @@ impl<'a, 'de> Visitor<'de> for StructVisitor<'a> {
         let mut index = 0usize;
         let mut output = DynamicStruct::default();
 
+        let ignored_len = self
+            .registration
+            .data::<SerializationData>()
+            .map(|data| data.len())
+            .unwrap_or(0);
+        let field_len = self.struct_info.field_len().saturating_sub(ignored_len);
+
+        if field_len == 0 {
+            // Handle unit structs and ignored fields
+            return Ok(output);
+        }
+
         while let Some(value) = seq.next_element_seed(TypedReflectDeserializer {
             registration: self
                 .struct_info
@@ -501,6 +516,21 @@ impl<'a, 'de> Visitor<'de> for TupleStructVisitor<'a> {
         let mut index = 0usize;
         let mut tuple_struct = DynamicTupleStruct::default();
 
+        let ignored_len = self
+            .registration
+            .data::<SerializationData>()
+            .map(|data| data.len())
+            .unwrap_or(0);
+        let field_len = self
+            .tuple_struct_info
+            .field_len()
+            .saturating_sub(ignored_len);
+
+        if field_len == 0 {
+            // Handle unit structs and ignored fields
+            return Ok(tuple_struct);
+        }
+
         let get_field_registration = |index: usize| -> Result<&'a TypeRegistration, V::Error> {
             let field = self.tuple_struct_info.field_at(index).ok_or_else(|| {
                 de::Error::custom(format_args!(
@@ -675,6 +705,7 @@ impl<'a, 'de> Visitor<'de> for MapVisitor<'a> {
 
 struct EnumVisitor<'a> {
     enum_info: &'static EnumInfo,
+    registration: &'a TypeRegistration,
     registry: &'a TypeRegistry,
 }
 
@@ -701,6 +732,7 @@ impl<'a, 'de> Visitor<'de> for EnumVisitor<'a> {
                     struct_info.field_names(),
                     StructVariantVisitor {
                         struct_info,
+                        registration: self.registration,
                         registry: self.registry,
                     },
                 )?
@@ -722,6 +754,7 @@ impl<'a, 'de> Visitor<'de> for EnumVisitor<'a> {
                     tuple_info.field_len(),
                     TupleVariantVisitor {
                         tuple_info,
+                        registration: self.registration,
                         registry: self.registry,
                     },
                 )?
@@ -787,6 +820,7 @@ impl<'de> DeserializeSeed<'de> for VariantDeserializer {
 
 struct StructVariantVisitor<'a> {
     struct_info: &'static StructVariantInfo,
+    registration: &'a TypeRegistration,
     registry: &'a TypeRegistry,
 }
 
@@ -811,6 +845,18 @@ impl<'a, 'de> Visitor<'de> for StructVariantVisitor<'a> {
         let mut index = 0usize;
         let mut output = DynamicStruct::default();
 
+        let ignored_len = self
+            .registration
+            .data::<SerializationData>()
+            .map(|data| data.len())
+            .unwrap_or(0);
+        let field_len = self.struct_info.field_len().saturating_sub(ignored_len);
+
+        if field_len == 0 {
+            // Handle all fields being ignored
+            return Ok(output);
+        }
+
         while let Some(value) = seq.next_element_seed(TypedReflectDeserializer {
             registration: self
                 .struct_info
@@ -831,6 +877,7 @@ impl<'a, 'de> Visitor<'de> for StructVariantVisitor<'a> {
 
 struct TupleVariantVisitor<'a> {
     tuple_info: &'static TupleVariantInfo,
+    registration: &'a TypeRegistration,
     registry: &'a TypeRegistry,
 }
 
@@ -845,6 +892,18 @@ impl<'a, 'de> Visitor<'de> for TupleVariantVisitor<'a> {
     where
         V: SeqAccess<'de>,
     {
+        let ignored_len = self
+            .registration
+            .data::<SerializationData>()
+            .map(|data| data.len())
+            .unwrap_or(0);
+        let field_len = self.tuple_info.field_len().saturating_sub(ignored_len);
+
+        if field_len == 0 {
+            // Handle all fields being ignored
+            return Ok(DynamicTuple::default());
+        }
+
         visit_tuple(&mut seq, self.tuple_info, self.registry)
     }
 }
@@ -1011,10 +1070,15 @@ mod tests {
         map_value: HashMap<u8, usize>,
         struct_value: SomeStruct,
         tuple_struct_value: SomeTupleStruct,
+        unit_struct: SomeUnitStruct,
         unit_enum: SomeEnum,
         newtype_enum: SomeEnum,
         tuple_enum: SomeEnum,
         struct_enum: SomeEnum,
+        ignored_struct: SomeIgnoredStruct,
+        ignored_tuple_struct: SomeIgnoredTupleStruct,
+        ignored_struct_variant: SomeIgnoredEnum,
+        ignored_tuple_variant: SomeIgnoredEnum,
         custom_deserialize: CustomDeserialize,
     }
 
@@ -1026,6 +1090,18 @@ mod tests {
     #[derive(Reflect, FromReflect, Debug, PartialEq)]
     struct SomeTupleStruct(String);
 
+    #[derive(Reflect, FromReflect, Debug, PartialEq)]
+    struct SomeUnitStruct;
+
+    #[derive(Reflect, FromReflect, Debug, PartialEq)]
+    struct SomeIgnoredStruct {
+        #[reflect(ignore)]
+        ignored: i32,
+    }
+
+    #[derive(Reflect, FromReflect, Debug, PartialEq)]
+    struct SomeIgnoredTupleStruct(#[reflect(ignore)] i32);
+
     #[derive(Reflect, FromReflect, Debug, PartialEq, Deserialize)]
     struct SomeDeserializableStruct {
         foo: i64,
@@ -1050,14 +1126,27 @@ mod tests {
         Struct { foo: String },
     }
 
+    #[derive(Reflect, FromReflect, Debug, PartialEq)]
+    enum SomeIgnoredEnum {
+        Tuple(#[reflect(ignore)] f32, #[reflect(ignore)] f32),
+        Struct {
+            #[reflect(ignore)]
+            foo: String,
+        },
+    }
+
     fn get_registry() -> TypeRegistry {
         let mut registry = TypeRegistry::default();
         registry.register::<MyStruct>();
         registry.register::<SomeStruct>();
         registry.register::<SomeTupleStruct>();
+        registry.register::<SomeUnitStruct>();
+        registry.register::<SomeIgnoredStruct>();
+        registry.register::<SomeIgnoredTupleStruct>();
         registry.register::<CustomDeserialize>();
         registry.register::<SomeDeserializableStruct>();
         registry.register::<SomeEnum>();
+        registry.register::<SomeIgnoredEnum>();
         registry.register::<i8>();
         registry.register::<String>();
         registry.register::<i64>();
@@ -1090,12 +1179,19 @@ mod tests {
             map_value: map,
             struct_value: SomeStruct { foo: 999999999 },
             tuple_struct_value: SomeTupleStruct(String::from("Tuple Struct")),
+            unit_struct: SomeUnitStruct,
             unit_enum: SomeEnum::Unit,
             newtype_enum: SomeEnum::NewType(123),
             tuple_enum: SomeEnum::Tuple(1.23, 3.21),
             struct_enum: SomeEnum::Struct {
                 foo: String::from("Struct variant value"),
             },
+            ignored_struct: SomeIgnoredStruct { ignored: 0 },
+            ignored_tuple_struct: SomeIgnoredTupleStruct(0),
+            ignored_struct_variant: SomeIgnoredEnum::Struct {
+                foo: String::default(),
+            },
+            ignored_tuple_variant: SomeIgnoredEnum::Tuple(0.0, 0.0),
             custom_deserialize: CustomDeserialize {
                 value: 100,
                 inner_struct: SomeDeserializableStruct { foo: 101 },
@@ -1125,12 +1221,17 @@ mod tests {
                     foo: 999999999,
                 ),
                 tuple_struct_value: ("Tuple Struct"),
+                unit_struct: (),
                 unit_enum: Unit,
                 newtype_enum: NewType(123),
                 tuple_enum: Tuple(1.23, 3.21),
                 struct_enum: Struct(
                     foo: "Struct variant value",
                 ),
+                ignored_struct: (),
+                ignored_tuple_struct: (),
+                ignored_struct_variant: Struct(),
+                ignored_tuple_variant: Tuple(),
                 custom_deserialize: (
                     value: 100,
                     renamed: (
@@ -1337,12 +1438,19 @@ mod tests {
             map_value: map,
             struct_value: SomeStruct { foo: 999999999 },
             tuple_struct_value: SomeTupleStruct(String::from("Tuple Struct")),
+            unit_struct: SomeUnitStruct,
             unit_enum: SomeEnum::Unit,
             newtype_enum: SomeEnum::NewType(123),
             tuple_enum: SomeEnum::Tuple(1.23, 3.21),
             struct_enum: SomeEnum::Struct {
                 foo: String::from("Struct variant value"),
             },
+            ignored_struct: SomeIgnoredStruct { ignored: 0 },
+            ignored_tuple_struct: SomeIgnoredTupleStruct(0),
+            ignored_struct_variant: SomeIgnoredEnum::Struct {
+                foo: String::default(),
+            },
+            ignored_tuple_variant: SomeIgnoredEnum::Tuple(0.0, 0.0),
             custom_deserialize: CustomDeserialize {
                 value: 100,
                 inner_struct: SomeDeserializableStruct { foo: 101 },
@@ -1363,7 +1471,8 @@ mod tests {
             108, 101, 32, 83, 116, 114, 117, 99, 116, 0, 0, 0, 0, 1, 0, 0, 0, 123, 0, 0, 0, 0, 0,
             0, 0, 2, 0, 0, 0, 164, 112, 157, 63, 164, 112, 77, 64, 3, 0, 0, 0, 20, 0, 0, 0, 0, 0,
             0, 0, 83, 116, 114, 117, 99, 116, 32, 118, 97, 114, 105, 97, 110, 116, 32, 118, 97,
-            108, 117, 101, 100, 0, 0, 0, 0, 0, 0, 0, 101, 0, 0, 0, 0, 0, 0, 0,
+            108, 117, 101, 1, 0, 0, 0, 0, 0, 0, 0, 100, 0, 0, 0, 0, 0, 0, 0, 101, 0, 0, 0, 0, 0, 0,
+            0,
         ];
 
         let deserializer = UntypedReflectDeserializer::new(&registry);
@@ -1392,12 +1501,19 @@ mod tests {
             map_value: map,
             struct_value: SomeStruct { foo: 999999999 },
             tuple_struct_value: SomeTupleStruct(String::from("Tuple Struct")),
+            unit_struct: SomeUnitStruct,
             unit_enum: SomeEnum::Unit,
             newtype_enum: SomeEnum::NewType(123),
             tuple_enum: SomeEnum::Tuple(1.23, 3.21),
             struct_enum: SomeEnum::Struct {
                 foo: String::from("Struct variant value"),
             },
+            ignored_struct: SomeIgnoredStruct { ignored: 0 },
+            ignored_tuple_struct: SomeIgnoredTupleStruct(0),
+            ignored_struct_variant: SomeIgnoredEnum::Struct {
+                foo: String::default(),
+            },
+            ignored_tuple_variant: SomeIgnoredEnum::Tuple(0.0, 0.0),
             custom_deserialize: CustomDeserialize {
                 value: 100,
                 inner_struct: SomeDeserializableStruct { foo: 101 },
@@ -1409,14 +1525,15 @@ mod tests {
         let input = vec![
             129, 217, 40, 98, 101, 118, 121, 95, 114, 101, 102, 108, 101, 99, 116, 58, 58, 115,
             101, 114, 100, 101, 58, 58, 100, 101, 58, 58, 116, 101, 115, 116, 115, 58, 58, 77, 121,
-            83, 116, 114, 117, 99, 116, 158, 123, 172, 72, 101, 108, 108, 111, 32, 119, 111, 114,
-            108, 100, 33, 145, 123, 146, 202, 64, 73, 15, 219, 205, 5, 57, 149, 254, 255, 0, 1, 2,
-            149, 254, 255, 0, 1, 2, 129, 64, 32, 145, 206, 59, 154, 201, 255, 145, 172, 84, 117,
-            112, 108, 101, 32, 83, 116, 114, 117, 99, 116, 164, 85, 110, 105, 116, 129, 167, 78,
-            101, 119, 84, 121, 112, 101, 123, 129, 165, 84, 117, 112, 108, 101, 146, 202, 63, 157,
-            112, 164, 202, 64, 77, 112, 164, 129, 166, 83, 116, 114, 117, 99, 116, 145, 180, 83,
-            116, 114, 117, 99, 116, 32, 118, 97, 114, 105, 97, 110, 116, 32, 118, 97, 108, 117,
-            101, 146, 100, 145, 101,
+            83, 116, 114, 117, 99, 116, 220, 0, 19, 123, 172, 72, 101, 108, 108, 111, 32, 119, 111,
+            114, 108, 100, 33, 145, 123, 146, 202, 64, 73, 15, 219, 205, 5, 57, 149, 254, 255, 0,
+            1, 2, 149, 254, 255, 0, 1, 2, 129, 64, 32, 145, 206, 59, 154, 201, 255, 145, 172, 84,
+            117, 112, 108, 101, 32, 83, 116, 114, 117, 99, 116, 144, 164, 85, 110, 105, 116, 129,
+            167, 78, 101, 119, 84, 121, 112, 101, 123, 129, 165, 84, 117, 112, 108, 101, 146, 202,
+            63, 157, 112, 164, 202, 64, 77, 112, 164, 129, 166, 83, 116, 114, 117, 99, 116, 145,
+            180, 83, 116, 114, 117, 99, 116, 32, 118, 97, 114, 105, 97, 110, 116, 32, 118, 97, 108,
+            117, 101, 144, 144, 129, 166, 83, 116, 114, 117, 99, 116, 144, 129, 165, 84, 117, 112,
+            108, 101, 144, 146, 100, 145, 101,
         ];
 
         let deserializer = UntypedReflectDeserializer::new(&registry);