bevy_reflect: implement Reflect for SmolStr (#8771)

# Objective
To upgrade winit's dependency, it's useful to reuse SmolStr, which
replaces/improves the too restrictive Key letter enums.

As Input<Key> is a resource it should implement Reflect through all its
fields.

## Solution

Add smol_str to bevy_reflect supported types, behind a feature flag.

This PR blocks winit's upgrade PR:
https://github.com/bevyengine/bevy/pull/8745.

# Current state

- I'm discovering bevy_reflect, I appreciate all feedbacks, and send me
your nitpicks!
- Lacking more tests

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Gino Valente <49806985+MrGVSV@users.noreply.github.com>

crates/bevy_reflect/Cargo.toml

@@ -11,14 +11,16 @@ readme = "README.md"
 
 [features]
 default = []
-# Provides Bevy-related reflection implementations
-bevy = ["glam", "smallvec", "bevy_math"]
+# When enabled, provides Bevy-related reflection implementations
+bevy = ["glam", "smallvec", "bevy_math", "smol_str"]
 # When enabled, allows documentation comments to be accessed via reflection
 documentation = ["bevy_reflect_derive/documentation"]
 
 [dependencies]
 # bevy
-bevy_math = { path = "../bevy_math", version = "0.11.0-dev", features = ["serialize"], optional = true }
+bevy_math = { path = "../bevy_math", version = "0.11.0-dev", features = [
+    "serialize",
+], optional = true }
 bevy_reflect_derive = { path = "bevy_reflect_derive", version = "0.11.0-dev" }
 bevy_utils = { path = "../bevy_utils", version = "0.11.0-dev" }
 bevy_ptr = { path = "../bevy_ptr", version = "0.11.0-dev" }
@@ -30,8 +32,13 @@ parking_lot = "0.12.1"
 thiserror = "1.0"
 once_cell = "1.11"
 serde = "1"
-smallvec = { version = "1.6", features = ["serde", "union", "const_generics"], optional = true }
+smallvec = { version = "1.6", features = [
+    "serde",
+    "union",
+    "const_generics",
+], optional = true }
 glam = { version = "0.24", features = ["serde"], optional = true }
+smol_str = { version = "0.2.0", optional = true }
 
 [dev-dependencies]
 ron = "0.8.0"

crates/bevy_reflect/src/impls/smol_str.rs

@@ -0,0 +1,28 @@
+use crate::std_traits::ReflectDefault;
+use crate::{self as bevy_reflect};
+use bevy_reflect_derive::{impl_from_reflect_value, impl_reflect_value};
+
+impl_reflect_value!(::smol_str::SmolStr(Debug, Hash, PartialEq, Default));
+impl_from_reflect_value!(::smol_str::SmolStr);
+
+#[cfg(test)]
+mod tests {
+    use crate::{FromReflect, Reflect};
+    use smol_str::SmolStr;
+
+    #[test]
+    fn should_partial_eq_smolstr() {
+        let a: &dyn Reflect = &SmolStr::new("A");
+        let a2: &dyn Reflect = &SmolStr::new("A");
+        let b: &dyn Reflect = &SmolStr::new("B");
+        assert_eq!(Some(true), a.reflect_partial_eq(a2));
+        assert_eq!(Some(false), a.reflect_partial_eq(b));
+    }
+
+    #[test]
+    fn smolstr_should_from_reflect() {
+        let smolstr = SmolStr::new("hello_world.rs");
+        let output = <SmolStr as FromReflect>::from_reflect(&smolstr);
+        assert_eq!(Some(smolstr), output);
+    }
+}

crates/bevy_reflect/src/lib.rs

@@ -459,6 +459,9 @@ mod impls {
     mod rect;
     #[cfg(feature = "smallvec")]
     mod smallvec;
+    #[cfg(feature = "smol_str")]
+    mod smol_str;
+
     mod std;
 
     #[cfg(feature = "glam")]