Allow #[serde(...)] attributes without De/Serialize

Author: daxpedda

CHANGELOG.md

@@ -5,7 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-<!-- ## [Unreleased] -->
+## [Unreleased]
+
+### Changed
+- `#[serde(...)]` attributes are now allowed even when not deriving
+  `De/Serialize` with `#[derive_where(...)]`.
+
 ## [1.5.0] - 2025-06-08
 
 ### Added

src/attr/item.rs

@@ -96,18 +96,6 @@ impl ItemAttr {
 			return Err(Error::none(span));
 		}
 
-		// Check for `#[serde(...)]` attributes without `De/Serialize`.
-		#[cfg(feature = "serde")]
-		if !self_.derive_wheres.iter().any(|derive_where| {
-			derive_where.contains(Trait::Deserialize) | derive_where.contains(Trait::Serialize)
-		}) {
-			for attr in attrs {
-				if attr.path().is_ident("serde") {
-					return Err(Error::serde_without_serde(attr.span()));
-				}
-			}
-		}
-
 		// Merge `DeriveWhere`s with the same bounds.
 		self_
 			.derive_wheres

src/error.rs

@@ -301,12 +301,6 @@ impl Error {
 		syn::Error::new(skip_clone, "Cannot skip `Clone` while deriving `Copy`")
 	}
 
-	/// Unsupported `serde(...)` without deriving `De/Serialize`.
-	#[cfg(feature = "serde")]
-	pub fn serde_without_serde(serde: Span) -> syn::Error {
-		syn::Error::new(serde, "Found unused `#[serde(...)]`")
-	}
-
 	/// Conflicting `serde(bound ...)` when deriving `De/Serialize`.
 	#[cfg(feature = "serde")]
 	pub fn serde_bound(serde: Span) -> syn::Error {

tests/ui/serde/serde.rs

@@ -2,10 +2,6 @@ use std::marker::PhantomData;
 
 use derive_where::derive_where;
 
-#[derive_where(Clone)]
-#[serde(crate = "serde_")]
-struct InvalidSerde<T>(PhantomData<T>);
-
 #[derive_where(Deserialize)]
 #[serde(bound = "")]
 struct ConflictingBound<T>(PhantomData<T>);

tests/ui/serde/serde.stderr

@@ -1,11 +1,5 @@
-error: Found unused `#[serde(...)]`
- --> tests/ui/serde/serde.rs:6:1
-  |
-6 | #[serde(crate = "serde_")]
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error: Found conflicting `#[serde(bound ...)]`
-  --> tests/ui/serde/serde.rs:10:9
-   |
-10 | #[serde(bound = "")]
-   |         ^^^^^^^^^^
+ --> tests/ui/serde/serde.rs:6:9
+  |
+6 | #[serde(bound = "")]
+  |         ^^^^^^^^^^