Document RustcEncodable/RustcDecodable

src/rust-2024/prelude.md

@@ -7,6 +7,7 @@ More information may be found in the tracking issue at <https://github.com/rust-
 
 - The [`Future`] and [`IntoFuture`] traits are now part of the prelude.
 - This might make calls to trait methods ambiguous which could make some code fail to compile.
+- `RustcEncodable` and `RustcDecodable` have been removed from the prelude.
 
 [`Future`]: ../../std/future/trait.Future.html
 [`IntoFuture`]: ../../std/future/trait.IntoFuture.html
@@ -26,10 +27,24 @@ However, adding a _trait_ to the prelude can break existing code in a subtle way
 For example, a call to `x.poll()` which comes from a `MyPoller` trait might fail to compile if `std`'s `Future` is also imported, because the call to `poll` is now ambiguous and could come from either trait.
 
 As a solution, Rust 2024 will use a new prelude.
-It's identical to the current one, except for two new additions:
+It's identical to the current one, except for the following changes:
 
-- [`std::future::Future`][`Future`]
-- [`std::future::IntoFuture`][`IntoFuture`]
+- Added:
+    - [`std::future::Future`][`Future`]
+    - [`std::future::IntoFuture`][`IntoFuture`]
+- Removed:
+    - `RustcEncodable`
+    - `RustcDecodable`
+
+### `RustcEncodable` and `RustcDecodable` removal
+
+`RustcEncodable` and `RustcDecodable` are two undocumented derive macros that have been removed from the prelude.
+These were deprecated before Rust 1.0, but remained within the standard library prelude.
+The 2024 Edition has removed these from the prelude since they are not expected to be used.
+
+If in the unlikely case there is a project still using these, it is recommended to switch to a serialization library, such as those found on [crates.io].
+
+[crates.io]: https://crates.io/categories/encoding
 
 ## Migration
 
@@ -67,3 +82,13 @@ fn main() {
     <_ as MyPoller>::poll(&core::pin::pin!(async {}));
 }
 ```
+
+#### `RustcEncodable` and `RustcDecodable`
+
+It is strongly recommended that you migrate to a different serialization library if you are still using these.
+However, these derive macros are still available in the standard library, they are just required to be imported from the older prelude now:
+
+```rust,edition2021
+#[allow(soft_unstable)]
+use core::prelude::v1::{RustcDecodable, RustcEncodable};
+```