feat: remove hashbrown dep

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Added validation to `PartialMerkleTree::with_leaves()` to reject internal nodes ([#684](https://github.com/0xMiden/crypto/pull/684)).
 - [BREAKING] Moved `LargeSmt` root ownership from storage to in-memory layer ([#694](https://github.com/0xMiden/crypto/pull/694)). 
 - Remove use of `transmute()` in blake3 implementation ([#704](https://github.com/0xMiden/crypto/pull/704)).
+- [BREAKING] Removed the direct `hashbrown` dependency and made the `hashmaps` feature use `std::collections::{HashMap, HashSet}` ([#696](https://github.com/0xMiden/crypto/issues/696)).
 
 ## 0.19.2 (2025-12-04)
 

README.md

@@ -138,7 +138,7 @@ This crate can be compiled with the following features:
 - `concurrent`- enabled by default; enables multi-threaded implementation of `Smt::with_entries()` which significantly improves performance on multi-core CPUs.
 - `std` - enabled by default and relies on the Rust standard library.
 - `no_std` does not rely on the Rust standard library and enables compilation to WebAssembly.
-- `hashmaps` - uses hashbrown hashmaps in SMT and Merkle Store implementation which significantly improves performance of updates. Keys ordering in iterators is not guaranteed when this feature is enabled.
+- `hashmaps` - uses standard library hash maps in SMT and Merkle Store implementation which significantly improves performance of updates (requires the `std` feature). Keys ordering in iterators is not guaranteed when this feature is enabled.
 - `rocksdb` - enables the RocksDB-backed storage for `LargeSmt` and related utilities. Implies `concurrent`.
 All of these features imply the use of [alloc](https://doc.rust-lang.org/alloc/) to support heap-allocated collections.
 

miden-crypto/Cargo.toml

@@ -62,11 +62,11 @@ harness = false
 name    = "rand"
 
 [features]
-concurrent = ["dep:rayon", "hashbrown?/rayon"]
+concurrent = ["dep:rayon"]
 default = ["concurrent", "std"]
 executable = ["dep:clap", "dep:rand-utils", "std"]
 fuzzing = []
-hashmaps = ["dep:hashbrown"]
+hashmaps = ["std"]
 internal = []
 rocksdb = ["concurrent", "dep:rocksdb"]
 serde = ["dep:serde", "serde?/alloc", "winter-math/serde"]
@@ -87,7 +87,6 @@ clap                          = { features = ["derive"], optional = true, versio
 curve25519-dalek              = { default-features = false, version = "4" }
 ed25519-dalek                 = { features = ["zeroize"], version = "2" }
 flume                         = { version = "0.11" }
-hashbrown                     = { features = ["serde"], optional = true, version = "0.16" }
 hkdf                          = { default-features = false, version = "0.12" }
 k256                          = { features = ["ecdh", "ecdsa"], version = "0.13" }
 miden-crypto-derive.workspace = true

miden-crypto/benches/README.md

@@ -80,7 +80,7 @@ For each algorithm, we benchmark three core operations:
 
 ### Sparse Merkle Tree
 
-We build cryptographic data structures incorporating these hash functions. What follows are benchmarks of operations on sparse Merkle trees (SMTs) which use the above `RPO_256` hash function. We perform a batched modification of 1,000 values in a tree with 1,000,000 leaves (with the `hashmaps` feature to use the `hashbrown` crate).
+We build cryptographic data structures incorporating these hash functions. What follows are benchmarks of operations on sparse Merkle trees (SMTs) which use the above `RPO_256` hash function. We perform a batched modification of 1,000 values in a tree with 1,000,000 leaves (with the `hashmaps` feature enabled to leverage the standard library `HashMap`).
 
 ### Scenario 1: SMT Construction (1M pairs)
 

miden-crypto/src/lib.rs

@@ -5,6 +5,9 @@ extern crate alloc;
 #[cfg(feature = "std")]
 extern crate std;
 
+#[cfg(all(feature = "hashmaps", not(feature = "std")))]
+compile_error!("`hashmaps` feature requires `std` to provide HashMap/HashSet support");
+
 pub mod aead;
 pub mod dsa;
 pub mod ecdh;
@@ -31,35 +34,35 @@ pub use word::{Word, WordError};
 /// An alias for a key-value map.
 ///
 /// By default, this is an alias for the [`alloc::collections::BTreeMap`], however, when the
-/// `hashmaps` feature is enabled, this is an alias for the `hashbrown`'s `HashMap`.
-#[cfg(feature = "hashmaps")]
-pub type Map<K, V> = hashbrown::HashMap<K, V>;
+/// `hashmaps` feature is enabled, this is an alias for the `std::collections::HashMap`.
+#[cfg(all(feature = "hashmaps", feature = "std"))]
+pub type Map<K, V> = std::collections::HashMap<K, V>;
 
-#[cfg(feature = "hashmaps")]
-pub use hashbrown::hash_map::Entry as MapEntry;
+#[cfg(all(feature = "hashmaps", feature = "std"))]
+pub use std::collections::hash_map::Entry as MapEntry;
 
 /// An alias for a key-value map.
 ///
 /// By default, this is an alias for the [`alloc::collections::BTreeMap`], however, when the
-/// `hashmaps` feature is enabled, this is an alias for the `hashbrown`'s `HashMap`.
-#[cfg(not(feature = "hashmaps"))]
+/// `hashmaps` feature is enabled, this is an alias for the `std::collections::HashMap`.
+#[cfg(not(all(feature = "hashmaps", feature = "std")))]
 pub type Map<K, V> = alloc::collections::BTreeMap<K, V>;
 
-#[cfg(not(feature = "hashmaps"))]
+#[cfg(not(all(feature = "hashmaps", feature = "std")))]
 pub use alloc::collections::btree_map::Entry as MapEntry;
 
 /// An alias for a simple set.
 ///
 /// By default, this is an alias for the [`alloc::collections::BTreeSet`]. However, when the
-/// `hashmaps` feature is enabled, this becomes an alias for hashbrown's HashSet.
-#[cfg(feature = "hashmaps")]
-pub type Set<V> = hashbrown::HashSet<V>;
+/// `hashmaps` feature is enabled, this becomes an alias for `std::collections::HashSet`.
+#[cfg(all(feature = "hashmaps", feature = "std"))]
+pub type Set<V> = std::collections::HashSet<V>;
 
 /// An alias for a simple set.
 ///
 /// By default, this is an alias for the [`alloc::collections::BTreeSet`]. However, when the
-/// `hashmaps` feature is enabled, this becomes an alias for hashbrown's HashSet.
-#[cfg(not(feature = "hashmaps"))]
+/// `hashmaps` feature is enabled, this becomes an alias for `std::collections::HashSet`.
+#[cfg(not(all(feature = "hashmaps", feature = "std")))]
 pub type Set<V> = alloc::collections::BTreeSet<V>;
 
 // CONSTANTS

miden-crypto/src/merkle/smt/forest/store.rs

@@ -169,9 +169,9 @@ impl SmtStore {
         #[allow(unused_mut)]
         let mut sorted_leaf_indices = leaves_by_index.keys().cloned().collect::<Vec<_>>();
 
-        #[cfg(feature = "hashmaps")]
+        #[cfg(all(feature = "hashmaps", feature = "std"))]
         // Sort leaves by NodeIndex to easily detect when leaves share a parent (only neighboring
-        // leaves can share a parent). Hashbrown::HashMap doesn't maintain key ordering, so
+        // leaves can share a parent). `std::collections::HashMap` doesn't maintain key ordering, so
         // we need to sort the indices.
         sorted_leaf_indices.sort();