Merge pull request #2767 from weiznich/feature/diagnostic_on_unimplemented

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

Author: dtolnay

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.
     ///