From 8909fc0c603949ad60ee503d1d49a440b7b6f414 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sat, 13 Sep 2025 09:39:58 -0700
Subject: [PATCH] Enforce derive cannot be used against serde_core

Even for a simple data structure that would ordinarily expand to an impl
that only refers to the public Serde traits without serde::__private, we
still disallow a (renamed) serde_core dependency. This leaves the
liberty to new usage of private helpers in such impls over time. For
example, similar to the optimization of the standard library's
derive(Debug) that introduced debug_struct_field1_finish to improve
compile time of derived impls.
---
 serde/src/lib.rs          |  6 ++++++
 serde_core/src/lib.rs     | 10 ++++++++++
 serde_derive/src/dummy.rs |  3 +++
 3 files changed, 19 insertions(+)

diff --git a/serde/src/lib.rs b/serde/src/lib.rs
index 54b0d408..6f53ed11 100644
--- a/serde/src/lib.rs
+++ b/serde/src/lib.rs
@@ -256,3 +256,9 @@ extern crate serde_derive;
 #[cfg(feature = "serde_derive")]
 #[cfg_attr(docsrs, doc(cfg(feature = "derive")))]
 pub use serde_derive::{Deserialize, Serialize};
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! __require_serde_not_serde_core {
+    () => {};
+}
diff --git a/serde_core/src/lib.rs b/serde_core/src/lib.rs
index a409d286..e566bb50 100644
--- a/serde_core/src/lib.rs
+++ b/serde_core/src/lib.rs
@@ -266,3 +266,13 @@ mod seed;
 
 #[cfg(all(not(feature = "std"), no_core_error))]
 mod std_error;
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! __require_serde_not_serde_core {
+    () => {
+        ::core::compile_error!(
+            "Serde derive requires a dependency on the serde crate, not serde_core"
+        );
+    };
+}
diff --git a/serde_derive/src/dummy.rs b/serde_derive/src/dummy.rs
index 8fd5494b..e0bca647 100644
--- a/serde_derive/src/dummy.rs
+++ b/serde_derive/src/dummy.rs
@@ -22,6 +22,9 @@ pub fn wrap_in_const(serde_path: Option<&syn::Path>, code: TokenStream) -> Token
         )]
         const _: () = {
             #use_serde
+
+            _serde::__require_serde_not_serde_core!();
+
             #code
         };
     }