Now working

Author: vigna

epserde-derive/src/lib.rs

@@ -292,7 +292,9 @@ pub fn epserde_derive(input: TokenStream) -> TokenStream {
 
             s.fields.iter().for_each(|field| {
                 let ty = &field.ty.to_token_stream().to_string();
-                if generics_names_raw.contains(ty) || ty.starts_with("PhantomDeserData") {
+                if ty.starts_with("PhantomDeserData") {
+                    methods.push(syn::parse_quote!(_deserialize_eps_inner_special));
+                } else if generics_names_raw.contains(ty) {
                     methods.push(syn::parse_quote!(_deserialize_eps_inner));
                 } else {
                     methods.push(syn::parse_quote!(_deserialize_full_inner));
@@ -608,7 +610,11 @@ pub fn epserde_derive(input: TokenStream) -> TokenStream {
                                     bounds: bounds_des,
                             }));
 
-                            if generics_names_raw.contains(&ty.to_token_stream().to_string()) {
+                            // TODO don't do just string comparison
+                            let ty = ty.to_token_stream().to_string();
+                            if ty.starts_with("PhantomDeserData") {
+                                methods.push(syn::parse_quote!(_deserialize_eps_inner_special));
+                            } else if generics_names_raw.contains(&ty) {
                                 methods.push(syn::parse_quote!(_deserialize_eps_inner));
                             } else {
                                 methods.push(syn::parse_quote!(_deserialize_full_inner));

epserde/src/lib.rs

@@ -11,7 +11,7 @@
 #[cfg(all(feature = "alloc", not(feature = "std")))]
 extern crate alloc;
 
-use core::{hash::Hash, marker::PhantomData};
+use core::{hash::Hash, marker::PhantomData, mem::transmute};
 
 #[cfg(feature = "derive")]
 pub use epserde_derive::{Epserde, TypeInfo};
@@ -72,6 +72,21 @@ impl<T: ?Sized> PhantomDeserData<T> {
     }
 }
 
+impl<T: ?Sized + DeserializeInner> PhantomDeserData<T> {
+    #[inline(always)]
+    pub unsafe fn _deserialize_eps_inner_special<'a>(
+        _backend: &mut SliceWithPos<'a>,
+    ) -> deser::Result<PhantomDeserData<T::DeserType<'a>>> {
+        // SAFETY: types are zero-length
+        Ok(unsafe {
+            transmute::<
+                <PhantomDeserData<T> as DeserializeInner>::DeserType<'a>,
+                PhantomDeserData<T::DeserType<'a>>,
+            >(PhantomDeserData(PhantomData))
+        })
+    }
+}
+
 impl<T: ?Sized> CopyType for PhantomDeserData<T> {
     type Copy = Zero;
 }

epserde/tests/test_phantom.rs

@@ -160,7 +160,7 @@ fn test_only_phantom() {
 struct DataWithPhantomDeserData<T> {
     data: T,
     // This will deserialize to PhantomData<T::DeserType<'a>>
-    phantom: PhantomDeserData<PhantomData<T>>,
+    phantom: PhantomDeserData<T>,
 }
 
 /// Test that PhantomDeserData works correctly with generic types that are transformed during deserialization.
@@ -193,5 +193,5 @@ fn test_deser_phantom() {
 
     // The phantom field should be PhantomData<&[i32]> (the DeserType of Vec<i32>)
     // We can't directly compare PhantomData types, but we can verify the deserialization worked
-    let _phantom_check: PhantomDeserData<PhantomData<&[i32]>> = eps.phantom;
+    let _phantom_check: PhantomDeserData<&[i32]> = eps.phantom;
 }