feat: impl CandidType for PhantomData (#591)

This functionality is needed in
dfinity/cdk-rs#546

Changelog.md

@@ -1,6 +1,12 @@
 
 # Changelog
 
+## 2025-01-15
+
+### Candid 0.10.12
+
+* Implement `CandidType` for `std::marker::PhantomData`.
+
 ## 2024-12-10
 
 ### Candid 0.10.11

rust/candid/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "candid"
-version = "0.10.11"
+version = "0.10.12"
 edition = "2021"
 rust-version.workspace = true
 authors = ["DFINITY Team"]

rust/candid/src/types/impls.rs

@@ -445,6 +445,22 @@ where
     }
 }
 
+impl<T> CandidType for std::marker::PhantomData<T>
+where
+    T: CandidType,
+{
+    fn _ty() -> Type {
+        T::ty()
+    }
+    fn idl_serialize<S>(&self, _: S) -> Result<(), S::Error>
+    where
+        S: Serializer,
+    {
+        use serde::ser::Error;
+        Err(S::Error::custom("`PhantomData` cannot be serialized"))
+    }
+}
+
 macro_rules! tuple_impls {
     ($($len:expr => ($($n:tt $name:ident)+))+) => {
         $(

rust/candid/tests/types.rs

@@ -96,6 +96,10 @@ fn test_primitive() {
         get_type(&[0, 1, 2, 3]),
         TypeInner::Vec(TypeInner::Int32.into()).into()
     );
+    assert_eq!(
+        get_type(&std::marker::PhantomData::<u32>),
+        TypeInner::Nat32.into()
+    );
 }
 
 #[test]