bevy_reflect: Feature-gate function reflection (#14174)

# Objective

Function reflection requires a lot of macro code generation in the form
of several `all_tuples!` invocations, as well as impls generated in the
`Reflect` derive macro.

Seeing as function reflection is currently a bit more niche, it makes
sense to gate it all behind a feature.

## Solution

Add a `functions` feature to `bevy_reflect`, which can be enabled in
Bevy using the `reflect_functions` feature.

## Testing

You can test locally by running:

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

That should ensure that everything still works with the feature
disabled.

To test with the feature on, you can run:

```
cargo test --package bevy_reflect --features functions
```

---

## Changelog

- Moved function reflection behind a Cargo feature
(`bevy/reflect_functions` and `bevy_reflect/functions`)
- Add `IntoFunction` export in `bevy_reflect::prelude`

## Internal Migration Guide

> [!important]
> Function reflection was introduced as part of the 0.15 dev cycle. This
migration guide was written for developers relying on `main` during this
cycle, and is not a breaking change coming from 0.14.

Function reflection is now gated behind a feature. To use function
reflection, enable the feature:
- If using `bevy_reflect` directly, enable the `functions` feature
- If using `bevy`, enable the `reflect_functions` feature

Author: MrGVSV

Cargo.toml

@@ -348,6 +348,9 @@ ios_simulator = ["bevy_internal/ios_simulator"]
 # Enable built in global state machines
 bevy_state = ["bevy_internal/bevy_state"]
 
+# Enable function reflection
+reflect_functions = ["bevy_internal/reflect_functions"]
+
 [dependencies]
 bevy_internal = { path = "crates/bevy_internal", version = "0.15.0-dev", default-features = false }
 
@@ -2158,6 +2161,7 @@ wasm = false
 name = "function_reflection"
 path = "examples/reflection/function_reflection.rs"
 doc-scrape-examples = true
+required-features = ["reflect_functions"]
 
 [package.metadata.example.function_reflection]
 name = "Function Reflection"

crates/bevy_internal/Cargo.toml

@@ -195,6 +195,9 @@ ios_simulator = ["bevy_pbr?/ios_simulator", "bevy_render?/ios_simulator"]
 # Enable built in global state machines
 bevy_state = ["dep:bevy_state"]
 
+# Enable function reflection
+reflect_functions = ["bevy_reflect/functions"]
+
 [dependencies]
 # bevy
 bevy_a11y = { path = "../bevy_a11y", version = "0.15.0-dev" }

crates/bevy_reflect/Cargo.toml

@@ -19,6 +19,8 @@ smallvec = ["dep:smallvec"]
 uuid = ["dep:uuid"]
 # When enabled, allows documentation comments to be accessed via reflection
 documentation = ["bevy_reflect_derive/documentation"]
+# Enables function reflection
+functions = ["bevy_reflect_derive/functions"]
 
 [dependencies]
 # bevy

crates/bevy_reflect/compile_fail/Cargo.toml

@@ -8,7 +8,7 @@ license = "MIT OR Apache-2.0"
 publish = false
 
 [dependencies]
-bevy_reflect = { path = "../" }
+bevy_reflect = { path = "../", features = ["functions"] }
 
 [dev-dependencies]
 compile_fail_utils = { path = "../../../tools/compile_fail_utils" }

crates/bevy_reflect/derive/Cargo.toml

@@ -15,6 +15,8 @@ proc-macro = true
 default = []
 # When enabled, allows documentation comments to be processed by the reflection macros
 documentation = []
+# Enables macro logic related to function reflection
+functions = []
 
 [dependencies]
 bevy_macro_utils = { path = "../../bevy_macro_utils", version = "0.15.0-dev" }

crates/bevy_reflect/derive/src/impls/enums.rs

@@ -1,6 +1,6 @@
 use crate::derive_data::{EnumVariantFields, ReflectEnum, StructField};
 use crate::enum_utility::{EnumVariantOutputData, TryApplyVariantBuilder, VariantBuilder};
-use crate::impls::{impl_function_traits, impl_type_path, impl_typed};
+use crate::impls::{impl_type_path, impl_typed};
 use bevy_macro_utils::fq_std::{FQAny, FQBox, FQOption, FQResult};
 use proc_macro2::{Ident, Span};
 use quote::quote;
@@ -65,7 +65,11 @@ pub(crate) fn impl_enum(reflect_enum: &ReflectEnum) -> proc_macro2::TokenStream
 
     let type_path_impl = impl_type_path(reflect_enum.meta());
 
-    let function_impls = impl_function_traits(reflect_enum.meta(), &where_clause_options);
+    #[cfg(not(feature = "functions"))]
+    let function_impls = None::<proc_macro2::TokenStream>;
+    #[cfg(feature = "functions")]
+    let function_impls =
+        crate::impls::impl_function_traits(reflect_enum.meta(), &where_clause_options);
 
     let get_type_registration_impl = reflect_enum.get_type_registration(&where_clause_options);
 

crates/bevy_reflect/derive/src/impls/mod.rs

@@ -1,11 +1,13 @@
 mod enums;
+#[cfg(feature = "functions")]
 mod func;
 mod structs;
 mod tuple_structs;
 mod typed;
 mod values;
 
 pub(crate) use enums::impl_enum;
+#[cfg(feature = "functions")]
 pub(crate) use func::impl_function_traits;
 pub(crate) use structs::impl_struct;
 pub(crate) use tuple_structs::impl_tuple_struct;

crates/bevy_reflect/derive/src/impls/structs.rs

@@ -1,4 +1,4 @@
-use crate::impls::{impl_function_traits, impl_type_path, impl_typed};
+use crate::impls::{impl_type_path, impl_typed};
 use crate::utility::ident_or_index;
 use crate::ReflectStruct;
 use bevy_macro_utils::fq_std::{FQAny, FQBox, FQDefault, FQOption, FQResult};
@@ -54,7 +54,11 @@ pub(crate) fn impl_struct(reflect_struct: &ReflectStruct) -> proc_macro2::TokenS
 
     let type_path_impl = impl_type_path(reflect_struct.meta());
 
-    let function_impls = impl_function_traits(reflect_struct.meta(), &where_clause_options);
+    #[cfg(not(feature = "functions"))]
+    let function_impls = None::<proc_macro2::TokenStream>;
+    #[cfg(feature = "functions")]
+    let function_impls =
+        crate::impls::impl_function_traits(reflect_struct.meta(), &where_clause_options);
 
     let get_type_registration_impl = reflect_struct.get_type_registration(&where_clause_options);
 

crates/bevy_reflect/derive/src/impls/tuple_structs.rs

@@ -1,4 +1,4 @@
-use crate::impls::{impl_function_traits, impl_type_path, impl_typed};
+use crate::impls::{impl_type_path, impl_typed};
 use crate::ReflectStruct;
 use bevy_macro_utils::fq_std::{FQAny, FQBox, FQDefault, FQOption, FQResult};
 use quote::{quote, ToTokens};
@@ -46,7 +46,11 @@ pub(crate) fn impl_tuple_struct(reflect_struct: &ReflectStruct) -> proc_macro2::
 
     let type_path_impl = impl_type_path(reflect_struct.meta());
 
-    let function_impls = impl_function_traits(reflect_struct.meta(), &where_clause_options);
+    #[cfg(not(feature = "functions"))]
+    let function_impls = None::<proc_macro2::TokenStream>;
+    #[cfg(feature = "functions")]
+    let function_impls =
+        crate::impls::impl_function_traits(reflect_struct.meta(), &where_clause_options);
 
     let (impl_generics, ty_generics, where_clause) = reflect_struct
         .meta()

crates/bevy_reflect/derive/src/impls/values.rs

@@ -1,4 +1,4 @@
-use crate::impls::{impl_function_traits, impl_type_path, impl_typed};
+use crate::impls::{impl_type_path, impl_typed};
 use crate::utility::WhereClauseOptions;
 use crate::ReflectMeta;
 use bevy_macro_utils::fq_std::{FQAny, FQBox, FQClone, FQOption, FQResult};
@@ -33,7 +33,10 @@ pub(crate) fn impl_value(meta: &ReflectMeta) -> proc_macro2::TokenStream {
 
     let type_path_impl = impl_type_path(meta);
 
-    let function_impls = impl_function_traits(meta, &where_clause_options);
+    #[cfg(not(feature = "functions"))]
+    let function_impls = None::<proc_macro2::TokenStream>;
+    #[cfg(feature = "functions")]
+    let function_impls = crate::impls::impl_function_traits(meta, &where_clause_options);
 
     let (impl_generics, ty_generics, where_clause) = type_path.generics().split_for_impl();
     let where_reflect_clause = where_clause_options.extend_where_clause(where_clause);