Code shared from the Rust Playground

playground.rs

#![cfg_attr(not(feature = "std"), no_std)]

use ink_lang as ink;

#[ink::contract]
mod test {
    use ink_storage::traits::{SpreadAllocate, SpreadLayout};
    use ink_storage::Mapping;

    #[derive(SpreadAllocate, SpreadLayout, Debug)]
    #[cfg_attr(feature = "std", derive(scale_info::TypeInfo, ink_storage::traits::StorageLayout))]
    pub struct MerkleTree<const LEAVES: u32> {
        nodes: Mapping<u32, Hash>,
        next_free_leaf: u32,
    }

    /// The only way of constructing `MerkleTree`.
    impl<const LEAVES: u32> Default for MerkleTree<LEAVES> {
        fn default() -> Self {
            if !LEAVES.is_power_of_two() {
                panic!("Please have 2^n leaves")
            }

            Self {
                nodes: Mapping::default(),
                next_free_leaf: LEAVES,
            }
        }
    }

    #[ink(storage)]
    #[derive(SpreadAllocate)]
    pub struct Contract {
        tree: MerkleTree<1024>,
    }

    impl Contract {
        #[ink(constructor)]
        pub fn new() -> Self {
            // ink_lang::utils::initialize_contract(|_: &mut Test| {})
            Self { tree: Default::default() }
        }
        
        #[ink(message)]
        pub fn get(&self)  {}
    }
}

// #[cfg(test)]
// mod tests {
//     use crate::test::Contract;

//     use super::*;

//     #[ink::test]
//     fn default_works() {
//         let t = Contract::new();
//         // assert_eq!(t.get(), 100)
//     }

// }