bevy_reflect: avoid useless with_{custom_attributes,docs} calls (#19875)

# Objective

`#[derive(Reflect)]` derives `Typed` impls whose `type_info` methods
contain useless calls to `with_custom_attributes` and `with_docs`, e.g.:
```
::bevy::reflect::NamedField:🆕:<f32>("x")
    .with_custom_attributes(
        ::bevy::reflect::attributes::CustomAttributes::default()
    )
    .with_docs(::core::option::Option::None),
```
This hurts compile times and makes the `cargo expand` output harder to
read. It might also hurt runtime speed, depending on whether the
compiler can optimize away the no-op methods.

Avoiding this will help with #19873.

## Solution

Check if the attributes/docs are empty before appending the method
calls.

## Testing

I used `cargo expand` to confirm the useless calls are no longer
produced.

`-Zmacro-stats` outputs tells me this reduces the size of the `Reflect`
impls produced for `bevy_ui` from 1_544_696 bytes to 1_511_214 bytes, a
2.2% drop. Only a small improvement, but it's a start.

crates/bevy_reflect/derive/src/custom_attributes.rs

@@ -28,6 +28,11 @@ impl CustomAttributes {
         Ok(())
     }
 
+    /// Is the collection empty?
+    pub fn is_empty(&self) -> bool {
+        self.attributes.is_empty()
+    }
+
     /// Parse `@` (custom attribute) attribute.
     ///
     /// Examples:

crates/bevy_reflect/derive/src/derive_data.rs

@@ -514,25 +514,27 @@ impl<'a> StructField<'a> {
         };
 
         let ty = self.reflected_type();
-        let custom_attributes = self.attrs.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! {
-            #field_info::new::<#ty>(#name).with_custom_attributes(#custom_attributes)
+            #field_info::new::<#ty>(#name)
         };
 
+        let custom_attributes = &self.attrs.custom_attributes;
+        if !custom_attributes.is_empty() {
+            let custom_attributes = custom_attributes.to_tokens(bevy_reflect_path);
+            info.extend(quote! {
+                .with_custom_attributes(#custom_attributes)
+            });
+        }
+
         #[cfg(feature = "documentation")]
         {
             let docs = &self.doc;
-            info.extend(quote! {
+            if !docs.is_empty() {
-                .with_docs(#docs)
+                info.extend(quote! {
-            });
+                    .with_docs(#docs)
+                });
+            }
         }
 
         info
@@ -653,19 +655,20 @@ impl<'a> ReflectStruct<'a> {
             .active_fields()
             .map(|field| field.to_info_tokens(bevy_reflect_path));
 
-        let custom_attributes = self
-            .meta
-            .attrs
-            .custom_attributes()
-            .to_tokens(bevy_reflect_path);
-
         let mut info = quote! {
             #bevy_reflect_path::#info_struct::new::<Self>(&[
                 #(#field_infos),*
             ])
-            .with_custom_attributes(#custom_attributes)
         };
 
+        let custom_attributes = self.meta.attrs.custom_attributes();
+        if !custom_attributes.is_empty() {
+            let custom_attributes = custom_attributes.to_tokens(bevy_reflect_path);
+            info.extend(quote! {
+                .with_custom_attributes(#custom_attributes)
+            });
+        }
+
         if let Some(generics) = generate_generics(self.meta()) {
             info.extend(quote! {
                 .with_generics(#generics)
@@ -675,9 +678,11 @@ impl<'a> ReflectStruct<'a> {
         #[cfg(feature = "documentation")]
         {
             let docs = self.meta().doc();
-            info.extend(quote! {
+            if !docs.is_empty() {
-                .with_docs(#docs)
+                info.extend(quote! {
-            });
+                    .with_docs(#docs)
+                });
+            }
         }
 
         quote! {
@@ -884,19 +889,20 @@ impl<'a> ReflectEnum<'a> {
             .iter()
             .map(|variant| variant.to_info_tokens(bevy_reflect_path));
 
-        let custom_attributes = self
-            .meta
-            .attrs
-            .custom_attributes()
-            .to_tokens(bevy_reflect_path);
-
         let mut info = quote! {
             #bevy_reflect_path::EnumInfo::new::<Self>(&[
                 #(#variants),*
             ])
-            .with_custom_attributes(#custom_attributes)
         };
 
+        let custom_attributes = self.meta.attrs.custom_attributes();
+        if !custom_attributes.is_empty() {
+            let custom_attributes = custom_attributes.to_tokens(bevy_reflect_path);
+            info.extend(quote! {
+                .with_custom_attributes(#custom_attributes)
+            });
+        }
+
         if let Some(generics) = generate_generics(self.meta()) {
             info.extend(quote! {
                 .with_generics(#generics)
@@ -906,9 +912,11 @@ impl<'a> ReflectEnum<'a> {
         #[cfg(feature = "documentation")]
         {
             let docs = self.meta().doc();
-            info.extend(quote! {
+            if !docs.is_empty() {
-                .with_docs(#docs)
+                info.extend(quote! {
-            });
+                    .with_docs(#docs)
+                });
+            }
         }
 
         quote! {
@@ -1008,26 +1016,26 @@ impl<'a> EnumVariant<'a> {
             }
         };
 
-        let custom_attributes = self.attrs.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(#args)
-                .with_custom_attributes(#custom_attributes)
         };
 
+        let custom_attributes = &self.attrs.custom_attributes;
+        if !custom_attributes.is_empty() {
+            let custom_attributes = custom_attributes.to_tokens(bevy_reflect_path);
+            info.extend(quote! {
+                .with_custom_attributes(#custom_attributes)
+            });
+        }
+
         #[cfg(feature = "documentation")]
         {
             let docs = &self.doc;
-            info.extend(quote! {
+            if !docs.is_empty() {
-                .with_docs(#docs)
+                info.extend(quote! {
-            });
+                    .with_docs(#docs)
+                });
+            }
         }
 
         quote! {

crates/bevy_reflect/derive/src/documentation.rs

@@ -61,6 +61,11 @@ impl Documentation {
         )
     }
 
+    /// Is the collection empty?
+    pub fn is_empty(&self) -> bool {
+        self.docs.is_empty()
+    }
+
     /// Push a new docstring to the collection
     pub fn push(&mut self, doc: String) {
         self.docs.push(doc);