borsh-incremental

Incremental Borsh deserialization with defaults for backward-compatible schema evolution.

Example

use borsh::{to_vec, BorshSerialize};
use borsh_incremental::BorshDeserializeIncremental;

#[derive(BorshSerialize, BorshDeserializeIncremental, Debug, PartialEq)]
struct ExampleV1 {
    #[incremental(default = "Unnamed".to_string())]
    name: String,
    count: u32,
}

#[derive(BorshDeserializeIncremental, Debug, PartialEq)]
struct ExampleV2 {
    #[incremental(default = "Unnamed".to_string())]
    name: String,
    count: u32,
    #[incremental(default = "active".to_string())]
    status: String, // new field added in the next version
}

fn main() {
    // Encode data using the old V1 struct (no 'status' field)
    let old = ExampleV1 { name: "Alice".into(), count: 5 };
    let old_bytes = to_vec(&old).unwrap();

    // Decode using the new V2 struct — the missing field gets its default
    let parsed_v2 = ExampleV2::try_from(&old_bytes[..]).unwrap();
    println!("Backward-compatible decode: {:?}", parsed_v2);

    // Simulate truncated input — only part of the serialized name
    let truncated = &old_bytes[..4];
    let partial = ExampleV2::try_from(truncated).unwrap();
    println!("Partial decode with defaults: {:?}", partial);
}

Attributes

Attribute	Applies To	Description
#[incremental(error = <Path>)]	struct	Sets the error type for TryFrom<&[u8]>.
#[incremental(default = <Expr>)]	field	Expression or value used if field cannot be deserialized.
#[incremental(deser_with = <fn>)]	field	Uses a custom function fn(&mut &[u8]) -> Result<T, E> to deserialize that field.

Features

• Implements TryFrom<&[u8]> for your struct
• Automatically fills missing or invalid fields with defaults
• Supports per-field custom deserializers
• Ideal for forward/backward compatibility or partial account decoding