partition_to_path panics for identity/truncate fields whose source column was dropped #3381

Apache Iceberg Rust version

v0.10.0

Describe the bug

Verified on main at 49b91d1 (the version field only lists releases).

PartitionSpec::partition_to_path panics when a partition field's source column is no longer in the schema and the field is identity or truncate over a non-string type.

#2773 made partition_type fall back to a source-independent result type for dropped source columns (Unknown for identity/truncate), so the partition_type(..).unwrap() succeeds. The panic happens later: to_human_string falls back to Display for Datum, which has no arm for an Unknown-typed int/long and hits unreachable!() (datum.rs).

Java's PartitionSpec.partitionToPath handles the same case: resultType uses UnknownType for a dropped source and toHumanString falls back to value.toString().

Today this is reachable through the public partition_to_path. #3253 also starts summarizing deleted files under their historical spec, which would put it on the commit path (see the discussion there).

To Reproduce

use iceberg::spec::{Literal, NestedField, PartitionSpec, PrimitiveType, Schema, Struct, Transform, Type};

let old_schema = Schema::builder()
    .with_fields(vec![
        NestedField::required(1, "x", Type::Primitive(PrimitiveType::Int)).into(),
        NestedField::required(2, "y", Type::Primitive(PrimitiveType::Int)).into(),
    ])
    .build()
    .unwrap();
let spec = PartitionSpec::builder(old_schema)
    .add_partition_field("y", "p", Transform::Identity)
    .unwrap()
    .build()
    .unwrap();

// `y` has been dropped from the current schema.
let current = Schema::builder()
    .with_fields(vec![NestedField::required(1, "x", Type::Primitive(PrimitiveType::Int)).into()])
    .build()
    .unwrap();

// Panics: internal error: entered unreachable code (datum.rs)
spec.partition_to_path(&Struct::from_iter([Some(Literal::int(1))]), current.into());

Expected behavior

No panic. Format the value as Java does for an unknown source type (p=1, p=10).

Willingness to contribute

I can contribute a fix for this bug independently