variable can be declared as immutable

[allwin199]

Description

In contracts/proxies/SafeProxy.sol

address internal singleton;

constructor(address _singleton) {
        require(_singleton != address(0), "Invalid singleton address provided");
        singleton = _singleton;
 }

Can singleton be converted to immutable?

address internal immutable singleton;

[allwin199]

singleton value is mutated only in the constructor, so it can be declared as immutable.

If this change is necessary, I can raise a PR.

[mmv08]

If it's declared as immutable, upgrades would not be possible.

[allwin199]

got it!