Use the #[diagnostic::on_unimplemented] attribute when possible

This change enables the `#[diagnostic::on_unimplemented]` attribute for
the `Serialize` and `Deserialize` trait to point the user to the
relevant derives and point out that they might want to check crates
features for external types by adding the relevant hints an note.

serde/build.rs

@@ -25,6 +25,7 @@ fn main() {
         println!("cargo:rustc-check-cfg=cfg(no_std_atomic64)");
         println!("cargo:rustc-check-cfg=cfg(no_systemtime_checked_add)");
         println!("cargo:rustc-check-cfg=cfg(no_target_has_atomic)");
+        println!("cargo:rustc-check-cfg=cfg(no_diagnostic_namespace)");
     }
 
     let target = env::var("TARGET").unwrap();
@@ -84,6 +85,12 @@ fn main() {
     if minor < 74 {
         println!("cargo:rustc-cfg=no_core_num_saturating");
     }
+
+    // Support for the `#[diagnostic]` tool attribute namespace
+    // https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html#diagnostic-attributes
+    if minor < 78 {
+        println!("cargo:rustc-cfg=no_diagnostic_namespace");
+    }
 }
 
 fn rustc_minor_version() -> Option<u32> {

serde/src/de/mod.rs

@@ -532,6 +532,13 @@ impl<'a> Display for Expected + 'a {
 /// deserializer lifetimes] for a more detailed explanation of these lifetimes.
 ///
 /// [Understanding deserializer lifetimes]: https://serde.rs/lifetimes.html
+#[cfg_attr(
+    not(no_diagnostic_namespace),
+    diagnostic::on_unimplemented(
+        note = "for local types consider adding `#[derive(serde::Deserialize)]` to your `{Self}` type",
+        note = "for types from other crates check whether the crate offers a `serde` feature flag",
+    )
+)]
 pub trait Deserialize<'de>: Sized {
     /// Deserialize this value from the given Serde deserializer.
     ///

serde/src/ser/mod.rs

@@ -215,6 +215,13 @@ declare_error_trait!(Error: Sized + Debug + Display);
 /// [`linked-hash-map`]: https://crates.io/crates/linked-hash-map
 /// [`serde_derive`]: https://crates.io/crates/serde_derive
 /// [derive section of the manual]: https://serde.rs/derive.html
+#[cfg_attr(
+    not(no_diagnostic_namespace),
+    diagnostic::on_unimplemented(
+        note = "for local types consider adding `#[derive(serde::Serialize)]` to your `{Self}` type",
+        note = "for types from other crates check whether the crate offers a `serde` feature flag",
+    )
+)]
 pub trait Serialize {
     /// Serialize this value into the given Serde serializer.
     ///