serde usage example for https://www.reddit.com/r/rust/comments/cw6jau/how_do_i_implement_storing_a_vecvecu8_in_multiple/

main.rs

// bincode is a backend for serde. bincode can serialize and deserialize type that implement
// serde::Serialize and serde::Deserialize
use bincode;

// Bincode serializes into a binary format, but other backends exist to serialize into other
// formats, like json.

// make sure to add the following to your Cargo.toml
//
// [dependencies]
// bincode = "1"
// serde = { version = "1", features = ["derive"] }

fn main() {
    using_vec();
    custom_struct();
}

fn using_vec() {
    let blocks: Vec<Vec<u8>> = vec![
        vec![1, 2, 3],
        vec![1, 2, 3, 4, 5],
        vec![1, 2, b'c', 4],
        vec![1, 2, 3, b'a'],
    ];

    // write
    let mut file = std::fs::File::create("blocks").unwrap();
    bincode::serialize_into(&mut file, &blocks).unwrap();
    drop(file); // close the file

    // read
    let file = std::fs::File::open("blocks").unwrap();
    let read_blocks: Vec<Vec<u8>> = bincode::deserialize_from(&file).unwrap();

    assert_eq!(blocks, read_blocks);
}

fn custom_struct() {
    #[derive(PartialEq, Debug, serde::Serialize, serde::Deserialize)] // automatically implements serde for the type
    struct Blocks {
        blocks: Vec<Vec<u8>>,
        metadata: String,
    }

    let blocks = Blocks {
        blocks: vec![
            vec![1, 2, 3],
            vec![1, 2, 3, 4, 5],
            vec![1, 2, b'c', 4],
            vec![1, 2, 3, b'a'],
        ],
        metadata: "you can use any type for metadata as long as it implements serde".to_string(),
    };

    // write
    let mut file = std::fs::File::create("blocks").unwrap();
    bincode::serialize_into(&mut file, &blocks).unwrap();
    drop(file); // close the file

    // read
    let file = std::fs::File::open("blocks").unwrap();
    let read_blocks: Blocks = bincode::deserialize_from(&file).unwrap();

    assert_eq!(blocks, read_blocks);
}