What causes "deserialize_unit_struct is not implemented" in tracel-ai/burn?

Replace the unit-struct field with an empty struct that implements Deserialize via a zero-field struct (deserialize_struct with empty fields) or remove it entirely.

tracel-ai/burn · error

deserialize_unit_struct is not implemented

Error message

deserialize_unit_struct is not implemented

What it means

The nested-value deserializer panics on `deserialize_unit_struct` because unit-like structs (e.g., `struct Marker;`) have no NestedValue representation. Any record type referencing such a struct hits this deliberate unimplemented!() during deserialization.

Source

Thrown at crates/burn-store/src/nested/de.rs:312

        }
    }

    fn deserialize_unit<V>(self, _visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        unimplemented!("deserialize_unit is not implemented")
    }

    fn deserialize_unit_struct<V>(
        self,
        _name: &'static str,
        _visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        unimplemented!("deserialize_unit_struct is not implemented")
    }

    fn deserialize_newtype_struct<V>(
        self,
        _name: &'static str,
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let value = self
            .value
            .ok_or_else(|| custom_err("expected value for newtype struct but got None"))?;
        visitor.visit_newtype_struct(Deserializer::<A>::new(
            value,
            self.default_for_missing_fields,
        ))
    }

View on GitHub (pinned to d16f7ba2ed)

Solutions

Replace the unit-struct field with an empty struct that implements Deserialize via a zero-field struct (deserialize_struct with empty fields) or remove it entirely.
Use PhantomData instead of a unit struct for type markers.
Regenerate the checkpoint to match the supported record schema.
Add #[serde(skip)] semantics by restructuring the type so the marker is not part of the serialized data.

Example fix

// before
struct Marker;
struct Item { m: Marker } // -> deserialize_unit_struct panic
// after
struct Item { _m: core::marker::PhantomData<Marker> } // not serialized

Defensive patterns

Strategy: validation

Validate before calling

// Replace unit structs with PhantomData markers:
// struct Marker;
// struct Item { _m: PhantomData<Marker> }  // not part of serialized data

Try / catch

let record = std::panic::catch_unwind(|| from_nested_value::<MyItem>(value.clone()))
    .map_err(|_| "record contains unit struct: not supported by nested deserializer");

Prevention

Avoid unit structs as fields in serializable record types.
Use PhantomData for type-state markers.
Round-trip test records after introducing new marker types.

When it happens

Trigger: Deserializing a Burn record that includes a field whose type is a unit struct (a braced-less struct with no fields), causing serde to call Deserializer::deserialize_unit_struct.

Common situations: Marker types used in module/record definitions (e.g., type-state markers); schema refactors that introduced marker fields; version drift between the code that saved the checkpoint and the code loading it.

Related errors

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/769f29b2798dac8a. Report an issue: GitHub.