From c7e28852cb773b562a84795d382b4593756d3d94 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Tue, 1 Jul 2025 09:29:26 +1000
Subject: [PATCH] bevy_reflect: avoid duplicate type bounds in derived methods
 (#19876)

# Objective

Most of the impls derived for `#[derive(Reflect)]` have one set of type
bounds per field, like so:
```
        f32: ::bevy::reflect::FromReflect
            + ::bevy::reflect::TypePath
            + ::bevy::reflect::MaybeTyped
            + ::bevy::reflect::__macro_exports::RegisterForReflection,
```
If multiple fields have the same type, the bounds are repeated
uselessly. This can only hurt compile time and clogs up the `cargo
expand` output.

Avoiding this will help with
https://github.com/bevyengine/bevy/issues/19873.

## Solution

Use a hashset when collecting the bounds to eliminate duplicates.

## Testing

I used cargo expand to confirm the duplicate bounds are no longer
produced.

`-Zmacro-stats` outputs tells me this reduces the size of the `Reflect`
code produced for `bevy_ui` from 1_544_696 bytes to 1_467_967 bytes, a
5% drop.
---
 crates/bevy_reflect/derive/Cargo.toml         |  3 ++-
 crates/bevy_reflect/derive/src/derive_data.rs | 11 +++++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/crates/bevy_reflect/derive/Cargo.toml b/crates/bevy_reflect/derive/Cargo.toml
index 032046ae2f..b9eaa369cd 100644
--- a/crates/bevy_reflect/derive/Cargo.toml
+++ b/crates/bevy_reflect/derive/Cargo.toml
@@ -20,9 +20,10 @@ functions = []
 
 [dependencies]
 bevy_macro_utils = { path = "../../bevy_macro_utils", version = "0.17.0-dev" }
+indexmap = "2.0"
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "2.0", features = ["full"] }
+syn = { version = "2.0", features = ["full", "extra-traits"] }
 uuid = { version = "1.13.1", features = ["v4"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
diff --git a/crates/bevy_reflect/derive/src/derive_data.rs b/crates/bevy_reflect/derive/src/derive_data.rs
index a0343f619f..614f357b37 100644
--- a/crates/bevy_reflect/derive/src/derive_data.rs
+++ b/crates/bevy_reflect/derive/src/derive_data.rs
@@ -1,4 +1,5 @@
 use core::fmt;
+use indexmap::IndexSet;
 use proc_macro2::Span;
 
 use crate::{
@@ -607,9 +608,12 @@ impl<'a> ReflectStruct<'a> {
 
     /// Get a collection of types which are exposed to the reflection API
     pub fn active_types(&self) -> Vec<Type> {
+        // Collect via `IndexSet` to eliminate duplicate types.
         self.active_fields()
             .map(|field| field.reflected_type().clone())
-            .collect()
+            .collect::<IndexSet<_>>()
+            .into_iter()
+            .collect::<Vec<_>>()
     }
 
     /// Get an iterator of fields which are exposed to the reflection API.
@@ -851,9 +855,12 @@ impl<'a> ReflectEnum<'a> {
 
     /// Get a collection of types which are exposed to the reflection API
     pub fn active_types(&self) -> Vec<Type> {
+        // Collect via `IndexSet` to eliminate duplicate types.
         self.active_fields()
             .map(|field| field.reflected_type().clone())
-            .collect()
+            .collect::<IndexSet<_>>()
+            .into_iter()
+            .collect::<Vec<_>>()
     }
 
     /// Get an iterator of fields which are exposed to the reflection API