Move the info about perfect derive to shared.rs

PiperOrigin-RevId: 590702727

rust/cpp.rs

@@ -229,9 +229,6 @@ impl<'msg, T: ?Sized> RepeatedField<'msg, T> {
     }
 }
 
-// These use manual impls instead of derives to avoid unnecessary bounds on `T`.
-// This problem is referred to as "perfect derive".
-// https://smallcultfollowing.com/babysteps/blog/2022/04/12/implied-bounds-and-perfect-derive/
 impl<'msg, T: ?Sized> Copy for RepeatedField<'msg, T> {}
 impl<'msg, T: ?Sized> Clone for RepeatedField<'msg, T> {
     fn clone(&self) -> RepeatedField<'msg, T> {

rust/shared.rs

@@ -13,6 +13,11 @@
 
 use std::fmt;
 
+// There are a number of manual `Debug` and similar impls instead of using their
+// derives, in order to to avoid unnecessary bounds on a generic `T`.
+// This problem is referred to as "perfect derive".
+// https://smallcultfollowing.com/babysteps/blog/2022/04/12/implied-bounds-and-perfect-derive/
+
 /// Everything in `__public` is re-exported in `protobuf.rs`.
 /// These are the items protobuf users can access directly.
 #[doc(hidden)]