What causes "deserialize_u32 is not implemented" in tracel-ai/burn?

Change the field to i32/i64 (which the deserializer supports) and cast to u32 after deserialization.

deserialize_u32 is not implemented

Error message

deserialize_u32 is not implemented

What it means

The nested-value deserializer in burn-store panics on `deserialize_u32` because unsigned 32-bit values are not part of the supported NestedValue surface (signed ints, bools, floats, strings, etc. are). Serde routes u32 fields to this method and hits the deliberate unimplemented!().

Source

Thrown at crates/burn-store/src/nested/de.rs:214

        V: Visitor<'de>,
    {
        let val = self.extract_scalar("u8", |v| v.clone().as_u8())?;
        visitor.visit_u8(val)
    }

    fn deserialize_u16<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let val = self.extract_scalar("u16", |v| v.clone().as_u16())?;
        visitor.visit_u16(val)
    }

    fn deserialize_u32<V>(self, _visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        unimplemented!("deserialize_u32 is not implemented")
    }

    fn deserialize_u64<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let val = self.extract_scalar("u64", |v| v.clone().as_u64())?;
        visitor.visit_u64(val)
    }

    fn deserialize_f32<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    where
        V: Visitor<'de>,
    {
        let val = self.extract_scalar("f32", |v| v.clone().as_f32())?;
        visitor.visit_f32(val)
    }

View on GitHub (pinned to d16f7ba2ed)

Solutions

Change the field to i32/i64 (which the deserializer supports) and cast to u32 after deserialization.
Use #[serde(deserialize_with)] to read the value as i64 and convert with u32::try_from.
Update or regenerate the checkpoint so it matches the supported record schema.
Align Burn versions between saving and loading code.

Example fix

// before
struct Item { width: u32 } // -> deserialize_u32 panic
// after
struct Item { width: i64 }
impl Item { fn width(&self) -> u32 { self.width as u32 } }

Defensive patterns

Strategy: validation

Validate before calling

// Audit record structs for u32 fields before loading; or convert at the serde level:
fn u32_from_i64<'de, D: serde::Deserializer<'de>>(d: D) -> Result<u32, D::Error> {
    u32::try_from(i64::deserialize(d)).map_err(serde::de::Error::custom)
}

Try / catch

let record = std::panic::catch_unwind(|| from_nested_value::<MyItem>(value.clone()))
    .map_err(|_| "record contains u32: not supported by nested deserializer");

Prevention

Prefer i32/i64 over unsigned ints in record item structs.
Round-trip save/load tests after schema changes.
Align Burn versions across checkpoint producers and consumers.

When it happens

Trigger: Deserializing a Burn record/struct that contains a `u32` field (e.g., an id, count, or dimension) from nested value data via the store's recorder or from_nested_value path.

Common situations: Record item structs that added or changed a field to u32 in a newer Burn release; user-written record types mirroring numeric metadata as u32; checkpoint compatibility issues after upgrading Burn.

Related errors

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/8d95db53b9aee89d. Report an issue: GitHub.