Make portable implementation const

.github/workflows/ci.yml

@@ -66,6 +66,9 @@ jobs:
     - run: cargo test --features= --release
     - run: cargo test --features=prefer_intrinsics --release
     - run: cargo test --features=pure --release
+    # const fn
+    - run: cargo test --features=const
+    - run: cargo test --features=const --release
     # No AVX-512.
     - run: cargo test --features=no_avx512
     - run: cargo test --features=no_avx512,prefer_intrinsics

Cargo.toml

@@ -31,6 +31,9 @@ wasm32_simd = []
 # entire build, with e.g. RUSTFLAGS="-C target-cpu=native".
 std = []
 
+# This feature enables `const fn` variants of functions, but requires Rust 1.85.0+
+const = []
+
 # The `rayon` feature (disabled by default, but enabled for docs.rs) adds the
 # `update_rayon` and (in combination with `mmap` below) `update_mmap_rayon`
 # methods, for multithreaded hashing. However, even if this feature is enabled,
@@ -114,7 +117,7 @@ arrayref = "0.3.5"
 arrayvec = { version = "0.7.4", default-features = false }
 constant_time_eq = { version = "0.3.1", default-features = false }
 cfg-if = "1.0.0"
-digest = { version = "0.10.1", features = [ "mac" ], optional = true }
+digest = { version = "0.10.1", features = ["mac"], optional = true }
 memmap2 = { version = "0.9", optional = true }
 rayon-core = { version = "1.12.1", optional = true }
 serde = { version = "1.0", default-features = false, features = ["derive"], optional = true }

src/hazmat.rs

@@ -368,7 +368,7 @@ fn test_max_subtree_len() {
 /// # }
 /// ```
 #[inline(always)]
-pub fn left_subtree_len(input_len: u64) -> u64 {
+pub const fn left_subtree_len(input_len: u64) -> u64 {
     debug_assert!(input_len > CHUNK_LEN as u64);
     // Note that .next_power_of_two() is greater than *or equal*.
     ((input_len + 1) / 2).next_power_of_two()
@@ -555,6 +555,15 @@ pub fn hash_derive_key_context(context: &str) -> ContextKey {
     .0
 }
 
+/// Hash a [`derive_key`](crate::derive_key) context like [`hash_derive_key_context`], but
+/// `const fn`.
+#[cfg(feature = "const")]
+pub const fn const_hash_derive_key_context(context: &str) -> ContextKey {
+    crate::const_hash_all_at_once(context.as_bytes(), IV, crate::DERIVE_KEY_CONTEXT)
+        .root_hash()
+        .0
+}
+
 #[cfg(test)]
 mod test {
     use super::*;