bevy_reflect: Generic parameter info (bevyengine#15475)

# Objective

Currently, reflecting a generic type provides no information about the
generic parameters. This means that you can't get access to the type of
`T` in `Foo<T>` without creating custom type data (we do this for
[`ReflectHandle`](https://docs.rs/bevy/0.14.2/bevy/asset/struct.ReflectHandle.html#method.asset_type_id)).

## Solution

This PR makes it so that generic type parameters and generic const
parameters are tracked in a `Generics` struct stored on the `TypeInfo`
for a type.

For example, `struct Foo<T, const N: usize>` will store `T` and `N` as a
`TypeParamInfo` and `ConstParamInfo`, respectively.

The stored information includes:

- The name of the generic parameter (i.e. `T`, `N`, etc.)
- The type of the generic parameter (remember that we're dealing with
monomorphized types, so this will actually be a concrete type)
- The default type/value, if any (e.g. `f32` in `T = f32` or `10` in
`const N: usize = 10`)

### Caveats

The only requirement for this to work is that the user does not opt-out
of the automatic `TypePath` derive with `#[reflect(type_path = false)]`.

Doing so prevents the macro code from 100% knowing that the generic type
implements `TypePath`. This in turn means the generated `Typed` impl
can't add generics to the type.

There are two solutions for this—both of which I think we should explore
in a future PR:

1. We could just not use `TypePath`. This would mean that we can't store
the `Type` of the generic, but we can at least store the `TypeId`.
2. We could provide a way to opt out of the automatic `Typed` derive
with a `#[reflect(typed = false)]` attribute. This would allow users to
manually implement `Typed` to add whatever generic information they need
(e.g. skipping a parameter that can't implement `TypePath` while the
rest can).

I originally thought about making `Generics` an enum with `Generic`,
`NonGeneric`, and `Unavailable` variants to signify whether there are
generics, no generics, or generics that cannot be added due to opting
out of `TypePath`. I ultimately decided against this as I think it adds
a bit too much complexity for such an uncommon problem.

Additionally, user's don't necessarily _have_ to know the generics of a
type, so just skipping them should generally be fine for now.

## Testing

You can test locally by running:

```
cargo test --package bevy_reflect
```

---

## Showcase

You can now access generic parameters via `TypeInfo`!

```rust
#[derive(Reflect)]
struct MyStruct<T, const N: usize>([T; N]);

let generics = MyStruct::<f32, 10>::type_info().generics();

// Get by index:
let t = generics.get(0).unwrap();
assert_eq!(t.name(), "T");
assert!(t.ty().is::<f32>());
assert!(!t.is_const());

// Or by name:
let n = generics.get_named("N").unwrap();
assert_eq!(n.name(), "N");
assert!(n.ty().is::<usize>());
assert!(n.is_const());
```

You can even access parameter defaults:

```rust
#[derive(Reflect)]
struct MyStruct<T = String, const N: usize = 10>([T; N]);

let generics = MyStruct::<f32, 5>::type_info().generics();

let GenericInfo::Type(info) = generics.get_named("T").unwrap() else {
    panic!("expected a type parameter");
};

let default = info.default().unwrap();

assert!(default.is::<String>());

let GenericInfo::Const(info) = generics.get_named("N").unwrap() else {
    panic!("expected a const parameter");
};

let default = info.default().unwrap();

assert_eq!(default.downcast_ref::<usize>().unwrap(), &10);
```

crates/bevy_reflect/derive/src/derive_data.rs

@@ -15,6 +15,7 @@ use crate::{
 use quote::{quote, ToTokens};
 use syn::token::Comma;
 
+use crate::generics::generate_generics;
 use syn::{
     parse_str, punctuated::Punctuated, spanned::Spanned, Data, DeriveInput, Field, Fields,
     GenericParam, Generics, Ident, LitStr, Meta, Path, PathSegment, Type, TypeParam, Variant,
@@ -627,20 +628,19 @@ impl<'a> ReflectStruct<'a> {
             .custom_attributes()
             .to_tokens(bevy_reflect_path);
 
-        #[cfg_attr(
-            not(feature = "documentation"),
-            expect(
-                unused_mut,
-                reason = "Needs to be mutable if `documentation` feature is enabled.",
-            )
-        )]
         let mut info = quote! {
             #bevy_reflect_path::#info_struct::new::<Self>(&[
                 #(#field_infos),*
             ])
             .with_custom_attributes(#custom_attributes)
         };
 
+        if let Some(generics) = generate_generics(self.meta()) {
+            info.extend(quote! {
+                .with_generics(#generics)
+            });
+        }
+
         #[cfg(feature = "documentation")]
         {
             let docs = self.meta().doc();
@@ -730,20 +730,19 @@ impl<'a> ReflectEnum<'a> {
             .custom_attributes()
             .to_tokens(bevy_reflect_path);
 
-        #[cfg_attr(
-            not(feature = "documentation"),
-            expect(
-                unused_mut,
-                reason = "Needs to be mutable if `documentation` feature is enabled.",
-            )
-        )]
         let mut info = quote! {
             #bevy_reflect_path::EnumInfo::new::<Self>(&[
                 #(#variants),*
             ])
             .with_custom_attributes(#custom_attributes)
         };
 
+        if let Some(generics) = generate_generics(self.meta()) {
+            info.extend(quote! {
+                .with_generics(#generics)
+            });
+        }
+
         #[cfg(feature = "documentation")]
         {
             let docs = self.meta().doc();

crates/bevy_reflect/derive/src/generics.rs

@@ -0,0 +1,72 @@
+use crate::derive_data::ReflectMeta;
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::punctuated::Punctuated;
+use syn::{GenericParam, Token};
+
+/// Creates a `TokenStream` for generating an expression that creates a `Generics` instance.
+///
+/// Returns `None` if `Generics` cannot or should not be generated.
+pub(crate) fn generate_generics(meta: &ReflectMeta) -> Option<TokenStream> {
+    if !meta.attrs().type_path_attrs().should_auto_derive() {
+        // Cannot verify that all generic parameters implement `TypePath`
+        return None;
+    }
+
+    let bevy_reflect_path = meta.bevy_reflect_path();
+
+    let generics = meta
+        .type_path()
+        .generics()
+        .params
+        .iter()
+        .filter_map(|param| match param {
+            GenericParam::Type(ty_param) => {
+                let ident = &ty_param.ident;
+                let name = ident.to_string();
+                let with_default = ty_param
+                    .default
+                    .as_ref()
+                    .map(|default_ty| quote!(.with_default::<#default_ty>()));
+
+                Some(quote! {
+                    #bevy_reflect_path::GenericInfo::Type(
+                        #bevy_reflect_path::TypeParamInfo::new::<#ident>(
+                            ::std::borrow::Cow::Borrowed(#name),
+                        )
+                        #with_default
+                    )
+                })
+            }
+            GenericParam::Const(const_param) => {
+                let ty = &const_param.ty;
+                let name = const_param.ident.to_string();
+                let with_default = const_param.default.as_ref().map(|default| {
+                    // We add the `as #ty` to ensure that the correct type is inferred.
+                    quote!(.with_default(#default as #ty))
+                });
+
+                Some(quote! {
+                    #[allow(
+                        clippy::unnecessary_cast,
+                        reason = "reflection requires an explicit type hint for const generics"
+                    )]
+                    #bevy_reflect_path::GenericInfo::Const(
+                        #bevy_reflect_path::ConstParamInfo::new::<#ty>(
+                            ::std::borrow::Cow::Borrowed(#name),
+                        )
+                        #with_default
+                    )
+                })
+            }
+            GenericParam::Lifetime(_) => None,
+        })
+        .collect::<Punctuated<_, Token![,]>>();
+
+    if generics.is_empty() {
+        // No generics to generate
+        return None;
+    }
+
+    Some(quote!(#bevy_reflect_path::Generics::from_iter([ #generics ])))
+}

crates/bevy_reflect/derive/src/lib.rs

@@ -25,6 +25,7 @@ mod documentation;
 mod enum_utility;
 mod field_attributes;
 mod from_reflect;
+mod generics;
 mod ident;
 mod impls;
 mod meta;

crates/bevy_reflect/src/array.rs

@@ -1,7 +1,8 @@
+use crate::generics::impl_generic_info_methods;
 use crate::{
     self as bevy_reflect, type_info::impl_type_methods, utility::reflect_hasher, ApplyError,
-    MaybeTyped, PartialReflect, Reflect, ReflectKind, ReflectMut, ReflectOwned, ReflectRef, Type,
-    TypeInfo, TypePath,
+    Generics, MaybeTyped, PartialReflect, Reflect, ReflectKind, ReflectMut, ReflectOwned,
+    ReflectRef, Type, TypeInfo, TypePath,
 };
 use bevy_reflect_derive::impl_type_path;
 use core::{
@@ -79,6 +80,7 @@ pub trait Array: PartialReflect {
 #[derive(Clone, Debug)]
 pub struct ArrayInfo {
     ty: Type,
+    generics: Generics,
     item_info: fn() -> Option<&'static TypeInfo>,
     item_ty: Type,
     capacity: usize,
@@ -97,6 +99,7 @@ impl ArrayInfo {
     ) -> Self {
         Self {
             ty: Type::of::<TArray>(),
+            generics: Generics::new(),
             item_info: TItem::maybe_type_info,
             item_ty: Type::of::<TItem>(),
             capacity,
@@ -138,6 +141,8 @@ impl ArrayInfo {
     pub fn docs(&self) -> Option<&'static str> {
         self.docs
     }
+
+    impl_generic_info_methods!(generics);
 }
 
 /// A fixed-size list of reflected values.

crates/bevy_reflect/src/enums/enum_trait.rs

@@ -1,7 +1,8 @@
+use crate::generics::impl_generic_info_methods;
 use crate::{
     attributes::{impl_custom_attribute_methods, CustomAttributes},
     type_info::impl_type_methods,
-    DynamicEnum, PartialReflect, Type, TypePath, VariantInfo, VariantType,
+    DynamicEnum, Generics, PartialReflect, Type, TypePath, VariantInfo, VariantType,
 };
 use alloc::sync::Arc;
 use bevy_utils::HashMap;
@@ -138,6 +139,7 @@ pub trait Enum: PartialReflect {
 #[derive(Clone, Debug)]
 pub struct EnumInfo {
     ty: Type,
+    generics: Generics,
     variants: Box<[VariantInfo]>,
     variant_names: Box<[&'static str]>,
     variant_indices: HashMap<&'static str, usize>,
@@ -163,6 +165,7 @@ impl EnumInfo {
 
         Self {
             ty: Type::of::<TEnum>(),
+            generics: Generics::new(),
             variants: variants.to_vec().into_boxed_slice(),
             variant_names,
             variant_indices,
@@ -239,6 +242,8 @@ impl EnumInfo {
     }
 
     impl_custom_attribute_methods!(self.custom_attributes, "enum");
+
+    impl_generic_info_methods!(generics);
 }
 
 /// An iterator over the fields in the current enum variant.