fix: make salt dependency optional to support no_std compilation

The salt crate enables serde/std by default, which breaks no_std
compilation for riscv64imac-unknown-none-elf target. This commit
makes salt optional and only enables it with the std feature.

Changes:
- Make salt dependency optional in Cargo.toml
- Enable salt only when std feature is active
- Add conditional compilation for salt::state::hasher usage
- In no_std environments, bucket_id_for_account and bucket_id_for_slot
  panic if not overridden, preventing incorrect bucket ID calculations
- Mark ADDRESS_KEY_LEN, STORAGE_SLOT_LEN, STORAGE_KEY_LEN as std-only

The panic behavior ensures that no_std users must provide their own
bucket ID implementation, preventing silent correctness issues from
using a different hashing algorithm than the std version.

Verified:
- cargo check --target riscv64imac-unknown-none-elf --no-default-features ✓
- cargo check -p mega-evm ✓
- cargo test ✓

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: WonderLawrence

crates/mega-evm/Cargo.toml

@@ -30,7 +30,7 @@ revm = { workspace = true, features = ["dev", "serde", "kzg-rs"] }
 
 # megaeth
 mega-system-contracts = { workspace = true, default-features = false }
-salt = { workspace = true, features = [] }
+salt = { workspace = true, features = [], optional = true }
 
 # misc
 auto_impl.workspace = true
@@ -53,7 +53,13 @@ sha2.workspace = true
 
 [features]
 default = ["std", "revm/default", "op-revm/default"]
-std = ["revm/std", "op-revm/std", "revm/alloydb", "mega-system-contracts/std"]
+std = [
+    "revm/std",
+    "op-revm/std",
+    "revm/alloydb",
+    "mega-system-contracts/std",
+    "dep:salt",
+]
 test-utils = []
 
 [[bench]]

crates/mega-evm/src/external/salt.rs

@@ -7,12 +7,19 @@ use core::{
     fmt::{Debug, Display},
 };
 
-use alloy_primitives::{Address, B256, U256};
+#[cfg(feature = "std")]
+use alloy_primitives::B256;
+use alloy_primitives::{Address, U256};
 use auto_impl::auto_impl;
+
+#[cfg(feature = "std")]
 use salt::state::hasher;
 
+#[cfg(feature = "std")]
 const ADDRESS_KEY_LEN: usize = Address::len_bytes();
+#[cfg(feature = "std")]
 const STORAGE_SLOT_LEN: usize = B256::len_bytes();
+#[cfg(feature = "std")]
 const STORAGE_KEY_LEN: usize = ADDRESS_KEY_LEN + STORAGE_SLOT_LEN;
 
 /// SALT bucket identifier. Accounts and storage slots are mapped to buckets, which have
@@ -78,8 +85,23 @@ pub trait SaltEnv: Debug + Unpin {
     /// # Arguments
     ///
     /// * `account` - The account address to map
+    ///
+    /// # Panics
+    ///
+    /// Panics in `no_std` environments if not overridden, as the default implementation
+    /// requires the `salt` crate which is only available with the `std` feature.
     fn bucket_id_for_account(account: Address) -> BucketId {
-        hasher::bucket_id(account.as_slice())
+        #[cfg(feature = "std")]
+        {
+            hasher::bucket_id(account.as_slice())
+        }
+        #[cfg(not(feature = "std"))]
+        {
+            unimplemented!(
+                "bucket_id_for_account({:?}) requires std feature or must be overridden in no_std environments",
+                account
+            )
+        }
     }
 
     /// Maps a storage slot to its bucket ID.
@@ -91,10 +113,26 @@ pub trait SaltEnv: Debug + Unpin {
     ///
     /// * `address` - The contract address owning the storage
     /// * `key` - The storage slot key
+    ///
+    /// # Panics
+    ///
+    /// Panics in `no_std` environments if not overridden, as the default implementation
+    /// requires the `salt` crate which is only available with the `std` feature.
     fn bucket_id_for_slot(address: Address, key: U256) -> BucketId {
-        hasher::bucket_id(
-            address.concat_const::<STORAGE_SLOT_LEN, STORAGE_KEY_LEN>(key.into()).as_slice(),
-        )
+        #[cfg(feature = "std")]
+        {
+            hasher::bucket_id(
+                address.concat_const::<STORAGE_SLOT_LEN, STORAGE_KEY_LEN>(key.into()).as_slice(),
+            )
+        }
+        #[cfg(not(feature = "std"))]
+        {
+            unimplemented!(
+                "bucket_id_for_slot({:?}, {:?}) requires std feature or must be overridden in no_std environments",
+                address,
+                key
+            )
+        }
     }
 }