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# Objective Fixes #5101 Alternative to #6511 ## Solution Corrected the behavior for ignored fields in `FromReflect`, which was previously using the incorrect field indexes. Similarly, fields marked with `#[reflect(skip_serializing)]` no longer break when using `FromReflect` after deserialization. This was done by modifying `SerializationData` to store a function pointer that can later be used to generate a default instance of the skipped field during deserialization. The function pointer points to a function generated by the derive macro using the behavior designated by `#[reflect(default)]` (or just `Default` if none provided). The entire output of the macro is now wrapped in an [unnamed constant](https://doc.rust-lang.org/stable/reference/items/constant-items.html#unnamed-constant) which keeps this behavior hygienic. #### Rationale The biggest downside to this approach is that it requires fields marked `#[reflect(skip_serializing)]` to provide the ability to create a default instance— either via a `Default` impl or by specifying a custom one. While this isn't great, I think it might be justified by the fact that we really need to create this value when using `FromReflect` on a deserialized object. And we need to do this _during_ deserialization because after that (at least for tuples and tuple structs) we lose information about which field is which: _"is the value at index 1 in this `DynamicTupleStruct` the actual value for index 1 or is it really the value for index 2 since index 1 is skippable...?"_ #### Alternatives An alternative would be to store `Option<Box<dyn Reflect>>` within `DynamicTuple` and `DynamicTupleStruct` instead of just `Box<dyn Reflect>`. This would allow us to insert "empty"/"missing" fields during deserialization, thus saving the positional information of the skipped fields. However, this may require changing the API of `Tuple` and `TupleStruct` such that they can account for their dynamic counterparts returning `None` for a skipped field. In practice this would probably mean exposing the `Option`-ness of the dynamics onto implementors via methods like `Tuple::drain` or `TupleStruct::field`. Personally, I think requiring `Default` would be better than muddying up the API to account for these special cases. But I'm open to trying out this other approach if the community feels that it's better. --- ## Changelog ### Public Changes #### Fixed - The behaviors of `#[reflect(ignore)]` and `#[reflect(skip_serializing)]` are no longer dependent on field order #### Changed - Fields marked with `#[reflect(skip_serializing)]` now need to either implement `Default` or specify a custom default function using `#[reflect(default = "path::to::some_func")]` - Deserializing a type with fields marked `#[reflect(skip_serializing)]` will now include that field initialized to its specified default value - `SerializationData::new` now takes the new `SkippedField` struct along with the skipped field index - Renamed `SerializationData::is_ignored_field` to `SerializationData::is_field_skipped` #### Added - Added `SkippedField` struct - Added methods `SerializationData::generate_default` and `SerializationData::iter_skipped` ### Internal Changes #### Changed - Replaced `members_to_serialization_denylist` and `BitSet<u32>` with `SerializationDataDef` - The `Reflect` derive is more hygienic as it now outputs within an [unnamed constant](https://doc.rust-lang.org/stable/reference/items/constant-items.html#unnamed-constant) - `StructField::index` has been split up into `StructField::declaration_index` and `StructField::reflection_index` #### Removed - Removed `bitset` dependency ## Migration Guide * Fields marked `#[reflect(skip_serializing)]` now must implement `Default` or specify a custom default function with `#[reflect(default = "path::to::some_func")]` ```rust #[derive(Reflect)] struct MyStruct { #[reflect(skip_serializing)] #[reflect(default = "get_foo_default")] foo: Foo, // <- `Foo` does not impl `Default` so requires a custom function #[reflect(skip_serializing)] bar: Bar, // <- `Bar` impls `Default` } #[derive(Reflect)] struct Foo(i32); #[derive(Reflect, Default)] struct Bar(i32); fn get_foo_default() -> Foo { Foo(123) } ``` * `SerializationData::new` has been changed to expect an iterator of `(usize, SkippedField)` rather than one of just `usize` ```rust // BEFORE SerializationData::new([0, 3].into_iter()); // AFTER SerializationData::new([ (0, SkippedField::new(field_0_default_fn)), (3, SkippedField::new(field_3_default_fn)), ].into_iter()); ``` * `Serialization::is_ignored_field` has been renamed to `Serialization::is_field_skipped` * Fields marked `#[reflect(skip_serializing)]` are now included in deserialization output. This may affect logic that expected those fields to be absent. |
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