Implement OneOrMany for std::collections::HashSet

Author: tenuous-guidance

serde_with/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Added
+
+* Support `OneOrMany` with `HashSet`
+
 ## [3.17.0] - 2026-02-24
 
 ### Added

serde_with/src/de/impls.rs

@@ -1771,6 +1771,45 @@ where
     }
 }
 
+#[cfg(all(feature = "alloc", feature = "std"))]
+impl<'de, T, TAs, FORMAT, S> DeserializeAs<'de, std::collections::HashSet<T, S>>
+    for OneOrMany<TAs, FORMAT>
+where
+    T: Eq + std::hash::Hash,
+    S: BuildHasher + Default,
+    TAs: DeserializeAs<'de, T>,
+    FORMAT: Format,
+{
+    fn deserialize_as<D>(deserializer: D) -> Result<std::collections::HashSet<T, S>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let is_hr = deserializer.is_human_readable();
+        let content: content::de::Content<'de> = Deserialize::deserialize(deserializer)?;
+
+        let one_err: D::Error = match <DeserializeAsWrap<T, TAs>>::deserialize(
+            content::de::ContentRefDeserializer::new(&content, is_hr),
+        ) {
+            Ok(one) => {
+                let mut hashset = std::collections::HashSet::default();
+                hashset.insert(one.into_inner());
+                return Ok(hashset);
+            }
+            Err(err) => err,
+        };
+        let many_err: D::Error =
+            match <DeserializeAsWrap<HashSet<T, S>, HashSet<TAs, S>>>::deserialize(
+                content::de::ContentDeserializer::new(content, is_hr),
+            ) {
+                Ok(many) => return Ok(many.into_inner()),
+                Err(err) => err,
+            };
+        Err(DeError::custom(format_args!(
+            "OneOrMany could not deserialize any variant:\n  One: {one_err}\n  Many: {many_err}"
+        )))
+    }
+}
+
 #[cfg(feature = "alloc")]
 impl<'de, T, TAs1> DeserializeAs<'de, T> for PickFirst<(TAs1,)>
 where

serde_with/src/ser/impls.rs

@@ -945,7 +945,7 @@ where
         S: Serializer,
     {
         match source.len() {
-            1 => SerializeAsWrap::<T, U>::new(source.iter().next().expect("Cannot be empty"))
+            1 => SerializeAsWrap::<T, U>::new(source.first().expect("Cannot be empty"))
                 .serialize(serializer),
             _ => SerializeAsWrap::<Vec<T>, Vec<U>>::new(source).serialize(serializer),
         }
@@ -976,7 +976,7 @@ where
         S: Serializer,
     {
         match source.len() {
-            1 => SerializeAsWrap::<T, TAs>::new(source.iter().next().expect("Cannot be empty"))
+            1 => SerializeAsWrap::<T, TAs>::new(source.first().expect("Cannot be empty"))
                 .serialize(serializer),
             _ => SerializeAsWrap::<&[T], &[TAs]>::new(&source.as_slice()).serialize(serializer),
         }
@@ -997,6 +997,46 @@ where
     }
 }
 
+#[cfg(all(feature = "alloc", feature = "std"))]
+impl<T, U, S> SerializeAs<std::collections::HashSet<T, S>> for OneOrMany<U, formats::PreferOne>
+where
+    T: Clone + Eq + core::hash::Hash,
+    U: SerializeAs<T>,
+{
+    fn serialize_as<Ser>(
+        source: &std::collections::HashSet<T, S>,
+        serializer: Ser,
+    ) -> Result<Ser::Ok, Ser::Error>
+    where
+        Ser: Serializer,
+    {
+        match source.len() {
+            1 => SerializeAsWrap::<T, U>::new(source.iter().next().expect("Cannot be empty")).serialize(serializer),
+            _ => SerializeAsWrap::<std::collections::HashSet<T, S>, std::collections::HashSet<U, S>>::new(source).serialize(serializer),
+        }
+    }
+}
+
+#[cfg(all(feature = "alloc", feature = "std"))]
+impl<T, U, S> SerializeAs<std::collections::HashSet<T, S>> for OneOrMany<U, formats::PreferMany>
+where
+    T: Eq + core::hash::Hash,
+    U: SerializeAs<T>,
+{
+    fn serialize_as<Ser>(
+        source: &std::collections::HashSet<T, S>,
+        serializer: Ser,
+    ) -> Result<Ser::Ok, Ser::Error>
+    where
+        Ser: Serializer,
+    {
+        SerializeAsWrap::<std::collections::HashSet<T, S>, std::collections::HashSet<U, S>>::new(
+            source,
+        )
+        .serialize(serializer)
+    }
+}
+
 #[cfg(feature = "alloc")]
 impl<T, TAs1> SerializeAs<T> for PickFirst<(TAs1,)>
 where

serde_with/tests/serde_as/lib.rs

@@ -35,7 +35,7 @@ use serde_with::{
     NoneAsEmptyString, OneOrMany, Same, Seq, StringWithSeparator,
 };
 use std::{
-    collections::HashMap,
+    collections::{HashMap, HashSet},
     sync::{Mutex, RwLock},
 };
 
@@ -1183,11 +1183,11 @@ fn test_one_or_many_prefer_one() {
     is_equal(
         S1Vec(vec![1, 2, 3]),
         expect![[r#"
-            [
-              1,
-              2,
-              3
-            ]"#]],
+        [
+          1,
+          2,
+          3
+        ]"#]],
     );
     check_deserialization(S1Vec(vec![1]), r#"1"#);
     check_deserialization(S1Vec(vec![1]), r#"[1]"#);
@@ -1323,6 +1323,50 @@ fn test_one_or_many_prefer_many() {
     );
 }
 
+#[test]
+fn test_one_or_many_hashset_prefer_one() {
+    #[serde_as]
+    #[derive(Debug, Serialize, Deserialize, PartialEq)]
+    struct S1(#[serde_as(as = "OneOrMany<_>")] HashSet<u32>);
+
+    is_equal(S1(HashSet::new()), expect![[r#"[]"#]]);
+    is_equal(S1(HashSet::from([1])), expect![[r#"1"#]]);
+    check_deserialization(S1(HashSet::from([1])), r#"1"#);
+    check_deserialization(S1(HashSet::from([1])), r#"[1]"#);
+    check_deserialization(S1(HashSet::from([1, 2, 3])), r#"[1, 2, 3]"#);
+    check_deserialization(S1(HashSet::from([1, 2, 3])), r#"[3, 2, 1]"#);
+
+    #[serde_as]
+    #[derive(Debug, Serialize, Deserialize, PartialEq)]
+    struct S2(#[serde_as(as = "OneOrMany<DisplayFromStr>")] HashSet<u32>);
+
+    is_equal(S2(HashSet::from([1])), expect![[r#""1""#]]);
+    check_deserialization(S2(HashSet::from([1])), r#""1""#);
+    check_deserialization(S2(HashSet::from([1])), r#"["1"]"#);
+    check_deserialization(S2(HashSet::from([1, 2, 3])), r#"["1", "2", "3"]"#);
+}
+
+#[test]
+fn test_one_or_many_hashset_prefer_many() {
+    use serde_with::formats::PreferMany;
+
+    #[serde_as]
+    #[derive(Debug, Serialize, Deserialize, PartialEq)]
+    struct S1(#[serde_as(as = "OneOrMany<_, PreferMany>")] HashSet<u32>);
+
+    is_equal(S1(HashSet::new()), expect![[r#"[]"#]]);
+    is_equal(
+        S1(HashSet::from([1])),
+        expect![[r#"
+            [
+              1
+            ]"#]],
+    );
+    check_deserialization(S1(HashSet::from([1])), r#"1"#);
+    check_deserialization(S1(HashSet::from([1])), r#"[1]"#);
+    check_deserialization(S1(HashSet::from([1, 2, 3])), r#"[1, 2, 3]"#);
+}
+
 /// Test that Cow borrows from the input
 #[test]
 fn test_borrow_cow_str() {