Unable to serialize after adding elements to array in struct about borsh-rs

[matklad]

Compiling the code gives the following warning:

warning: unused `Result` that must be used
  --> src/main.rs:25:5
   |
25 |     decoded_a.serialize(&mut &mut encoded_a.borrow_mut()[..]);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: this `Result` may be an `Err` variant, which should be handled

If you handle that result, you'll see that serialization actually errors. That is because you are trying to serialize to &mut &mut [u8], rather than &mut Vec<u8>, which doesn't have enough space.

Here's a fixed example which works.

use borsh::{BorshDeserialize, BorshSerialize};

fn main() {
    #[derive(BorshSerialize, BorshDeserialize, PartialEq, Debug)]
    struct A {
        x: u8,
        y: Vec<String>,
    }

    let a = A {
        x: 1,
        y: vec!["cat".to_string()],
    };

    let encoded_a = a.try_to_vec().unwrap();
    println!("Before: {:?}", encoded_a);

    let mut decoded_a = A::try_from_slice(&encoded_a).unwrap();

    println!("Decoded Before:  {:?}", decoded_a);
    decoded_a.x = 2;
    decoded_a.y.push("dog".to_string());

    let mut encoded_a = Vec::new();
    decoded_a.serialize(&mut encoded_a).unwrap();

    println!("After:  {:?}", encoded_a);
    let decoded2_a = A::try_from_slice(&encoded_a).unwrap();
    println!("Decoded After:  {:?}", decoded2_a);
}

Closing, as this is not a borsh bug, but rather a programming error which the compiler warns about.