Repository: paritytech/substrate
File: primitives/state-machine/src/trie_backend.rs (Rust)

Code before:
	trie_backend_essence::{RawIter, TrieBackendEssence, TrieBackendStorage},
	Backend, StorageKey, StorageValue,
};

#[cfg(not(feature = "std"))]
use core::marker::PhantomData;

use codec::Codec;
#[cfg(feature = "std")]
use hash_db::HashDB;
use hash_db::Hasher;
use sp_core::storage::{ChildInfo, StateVersion};
#[cfg(feature = "std")]
use sp_trie::{
	cache::{LocalTrieCache, TrieCache},
	recorder::Recorder,
	MemoryDB, StorageProof,
};
#[cfg(not(feature = "std"))]
use sp_trie::{Error, NodeCodec};
use trie_db::TrieCache as TrieCacheT;
#[cfg(not(feature = "std"))]
use trie_db::{node::NodeOwned, CachedValue};

/// A provider of trie caches that are compatible with [`trie_db::TrieDB`].
pub trait MergeableTrieCacheProvider<H: Hasher> {
	/// Cache type that implements [`trie_db::TrieCache`].
	type Cache<'a>: TrieCacheT<sp_trie::NodeCodec<H>> + 'a
	where
		Self: 'a;

	/// Return a [`trie_db::TrieDB`] compatible cache. The `storage_root`
	/// parameter should be the storage root of the used trie.
	fn as_trie_db_cache(&self, storage_root: H::Out) -> Self::Cache<'_>;

	/// Return a [`trie_db::TrieDBMut`] compatible cache. The `storage_root`
	/// parameter should be the storage root of the used trie.
	fn as_trie_db_mut_cache(&self) -> Self::Cache<'_>;

	/// Merge a cache back into this cache provider.
	fn merge<'a>(&'a self, other: Self::Cache<'a>, new_root: H::Out);
}

#[cfg(feature = "std")]
impl<H: Hasher> MergeableTrieCacheProvider<H> for LocalTrieCache<H> {
	type Cache<'a> = TrieCache<'a, H> where H: 'a;

	fn as_trie_db_cache(&self, storage_root: H::Out) -> Self::Cache<'_> {
		self.as_trie_db_cache(storage_root)
	}


Review comment (performance):
```suggestion
/// A provider of trie caches that are compatible with [`trie_db::TrieDB`].
pub trait TrieCacheProvider<H: Hasher> {
```

Code after:
	trie_backend_essence::{RawIter, TrieBackendEssence, TrieBackendStorage},
	Backend, StorageKey, StorageValue,
};

use codec::Codec;
#[cfg(feature = "std")]
use hash_db::HashDB;
use hash_db::Hasher;
use sp_core::storage::{ChildInfo, StateVersion};
#[cfg(feature = "std")]
use sp_trie::{
	cache::{LocalTrieCache, TrieCache},
	recorder::Recorder,
	MemoryDB, StorageProof,
};
#[cfg(not(feature = "std"))]
use sp_trie::{Error, NodeCodec};
use trie_db::TrieCache as TrieCacheT;
#[cfg(not(feature = "std"))]
use trie_db::{node::NodeOwned, CachedValue};

/// A provider of trie caches that are compatible with [`trie_db::TrieDB`].
pub trait TrieCacheProvider<H: Hasher> {
	/// Cache type that implements [`trie_db::TrieCache`].
	type Cache<'a>: TrieCacheT<sp_trie::NodeCodec<H>> + 'a
	where
		Self: 'a;

	/// Return a [`trie_db::TrieDB`] compatible cache.
	///
	/// The `storage_root` parameter should be the storage root of the used trie.
	fn as_trie_db_cache(&self, storage_root: H::Out) -> Self::Cache<'_>;

	/// Returns a cache that can be used with a [`trie_db::TrieDBMut`].
	///
	/// When finished with the operation on the trie, it is required to call [`Self::merge`] to
	/// merge the cached items for the correct `storage_root`.
	fn as_trie_db_mut_cache(&self) -> Self::Cache<'_>;

	/// Merge the cached data in `other` into the provider using the given `new_root`.
	///
	/// This must be used for the cache returned by [`Self::as_trie_db_mut_cache`] as otherwise the
	/// cached data is just thrown away.
	fn merge<'a>(&'a self, other: Self::Cache<'a>, new_root: H::Out);
}

#[cfg(feature = "std")]
impl<H: Hasher> TrieCacheProvider<H> for LocalTrieCache<H> {
	type Cache<'a> = TrieCache<'a, H> where H: 'a;

	fn as_trie_db_cache(&self, storage_root: H::Out) -> Self::Cache<'_> {