Stylus Upgrades

Contents

• Stylus Upgrade Model
• Proxy Patterns
• Access Control
• Upgrade Safety

Stylus Upgrade Model

Stylus contracts run on Arbitrum as WebAssembly (WASM) programs alongside the EVM. They share the same state trie, storage model, and account system as Solidity contracts. Because of this, EVM proxy patterns work identically for Stylus — a Solidity proxy can delegate to a Stylus implementation and vice versa.

Existing Solidity contracts can upgrade to a Stylus (Rust) implementation via proxy patterns. The #[storage] macro lays out fields in the EVM state trie identically to Solidity, so storage slots line up when type definitions match.

Proxy Patterns

OpenZeppelin Contracts for Stylus provides three proxy patterns:

UUPS

The implementation contract composes UUPSUpgradeable in its #[storage] struct alongside access control (e.g., Ownable). Integration requires:

1. Add UUPSUpgradeable (and access control) as fields in the #[storage] struct
2. Call self.uups.constructor() and initialize access control in the constructor
3. Expose initialize calling self.uups.set_version() — invoked via proxy after deployment
4. Implement IUUPSUpgradeable — upgrade_to_and_call guarded by access control, upgrade_interface_version delegating to self.uups
5. Implement IErc1822Proxiable — proxiable_uuid delegating to self.uups

The proxy contract is a thin Erc1967Proxy with a constructor that takes the implementation address and initialization data, and a #[fallback] handler that delegates all calls.

Deploy the proxy with set_version as the initialization call data. Use cargo stylus deploy or a deployer contract. The initialization data is the ABI-encoded setVersion call:

let data = MyContractAbi::setVersionCall {}.abi_encode();
// Pass `data` as the proxy constructor's second argument at deployment time.

Beacon

Multiple BeaconProxy contracts point to a single UpgradeableBeacon that stores the current implementation address. Updating the beacon upgrades all proxies in one transaction.

Context detection (Stylus-specific)

Stylus does not support the immutable keyword. Instead of storing __self = address(this), UUPSUpgradeable uses a logic_flag boolean in a unique storage slot:

• The implementation's constructor sets logic_flag = true in its own storage.
• When code runs via a proxy (delegatecall), the proxy's storage does not contain this flag, so it reads as false.
• only_proxy() checks this flag to ensure upgrade functions can only be called through the proxy, not directly on the implementation.

only_proxy() also verifies that the ERC-1967 implementation slot is non-zero and that the proxy-stored version matches the implementation's VERSION_NUMBER.

Examples: See the examples/ directory of the rust-contracts-stylus repository for full working integration examples of UUPS, Beacon, and related patterns.

Access Control

Upgrade functions must be guarded with access control. OpenZeppelin's Stylus contracts do not embed access control into the upgrade logic itself — you must add it in upgrade_to_and_call:

fn upgrade_to_and_call(&mut self, new_implementation: Address, data: Bytes) -> Result<(), Vec<u8>> {
    self.ownable.only_owner()?; // or any access control check
    self.uups.upgrade_to_and_call(new_implementation, data)?;
    Ok(())
}

Common options:

• Ownable — single owner, simplest pattern
• AccessControl / RBAC — role-based, finer granularity
• Multisig or governance — for production contracts managing significant value

Upgrade Safety

Storage compatibility

Stylus #[storage] fields are laid out in the EVM state trie identically to Solidity. The same storage layout rules apply when upgrading:

• Never reorder, remove, or change the type of existing storage fields
• Never insert new fields before existing ones
• Only append new fields at the end of the struct
• ERC-1967 proxy storage slots are in high, standardized locations — they will not collide with implementation storage

One difference from Solidity: nested structs in Stylus #[storage] (e.g., composing Erc20, Ownable, UUPSUpgradeable as fields) are laid out with each nested struct starting at its own deterministic slot. This is consistent with regular struct nesting in Solidity, but not with Solidity's inheritance-based flat layout where all inherited variables share a single sequential slot range.

Initialization safety

• The implementation constructor sets logic_flag and any implementation-only state. It runs once at implementation deployment.
• set_version() must be called via the proxy (during deployment or via upgrade_to_and_call) to write the VERSION_NUMBER into the proxy's storage.
• If additional initialization is needed (ownership, token supply), expose a protected initialization function and include set_version() in it.
• Failing to initialize properly can result in orphaned contracts with no owner, uninitialized state, or denied future upgrades.

UUPS upgrade checks

The UUPS implementation enforces three safety checks:

1. Access control — restrict upgrade_to_and_call (e.g., self.ownable.only_owner())
2. Proxy context enforcement — only_proxy() reverts if the call is not via delegatecall
3. Proxiable UUID validation — proxiable_uuid() must return the ERC-1967 implementation slot, confirming UUPS compatibility

Reactivation

Stylus WASM contracts must be reactivated once per year (365 days) or after any Stylus protocol upgrade. Reactivation can be done using cargo-stylus or the ArbWasm precompile. If a contract is not reactivated, it becomes uncallable. This is orthogonal to proxy upgrades but must be factored into maintenance planning.

Testing upgrade paths

Before upgrading a production contract:

• Deploy V1 implementation and proxy on a local Arbitrum devnet
• Write state with V1, upgrade to V2 via upgrade_to_and_call, and verify that all existing state reads correctly
• Verify new functionality works as expected after the upgrade
• Confirm access control — only authorized callers can invoke upgrade_to_and_call
• Check storage layout — ensure no reordering, removal, or type changes to existing fields
• Verify VERSION_NUMBER is incremented in the new implementation
• Test reactivation — ensure the upgraded contract can be reactivated
• Manual review — there is no automated storage layout validation for Stylus Rust contracts; rely on struct comparison and devnet testing