trie panics with cannot convert a slice of length 29 to FixedBytes<32> with small data

[ufoscout]

I am trying to replace triehash::ordered_trie_root with an equivalent function based on alloy-trie. Even if it works pretty well, sometimes alloy-trie panics with a cannot convert a slice of length 29 to FixedBytes<32> error. Here is a complete reproducer for the issue:

use std::collections::BTreeMap;

use alloy_primitives::{keccak256, B256};
use alloy_rlp::Encodable;
use alloy_trie::HashBuilder;
use nybbles::Nibbles;

fn triehash_ordered_trie_root<I>(iter: I) -> B256
where
    I: IntoIterator,
    I::Item: AsRef<[u8]>,
{
    struct Keccak256Hasher;
    impl hash_db::Hasher for Keccak256Hasher {
        type Out = B256;
        type StdHasher = plain_hasher::PlainHasher;

        const LENGTH: usize = 32;

        fn hash(x: &[u8]) -> Self::Out {
            keccak256(x)
        }
    }

    triehash::ordered_trie_root::<Keccak256Hasher, _>(iter)
}

fn alloy_ordered_trie_root<I>(iter: I) -> B256
where
    I: IntoIterator,
    I::Item: AsRef<[u8]>,
{

    let hashed = iter
    .into_iter()
    .enumerate()
    .map(|(i, v)| {
        let mut buf = Vec::new();
        i.encode(&mut buf);
        (buf, v)
    })
    .collect::<BTreeMap<_, _>>();

    let mut hb = HashBuilder::default();

    hashed.iter().for_each(|(key, val)| {
        let nibbles = Nibbles::unpack(key);
        hb.add_leaf(nibbles, val.as_ref());
    });

    hb.root()
}

// This fails with error:
// cannot convert a slice of length 29 to FixedBytes<32>
#[test]
fn small_data() {
    let data = &["cake", "pie", "candy"];
    assert_eq!(triehash_ordered_trie_root(data), alloy_ordered_trie_root(data));
}

// This succeeds.
#[test]
fn big_data() {
    let data = &["a_very_big_cake", "a_very_big_pie", "a_lots_of_candies"];
    assert_eq!(triehash_ordered_trie_root(data), alloy_ordered_trie_root(data));
}

[DaniPopes]

cc @rkrasiuk @klkvr

Should we also add these helpers in this crate?