Use immutable for OpenZeppelin AccessControl's Roles Declarations

[pcaversaccio]

Access roles marked as constant results in computing the keccak256 operation each time the variable is used because assigned operations for constant variables are re-evaluated every time. Changing the variables to immutable results in computing the hash only once on deployment, leading to gas savings. Some background information can be found here and here.

[frangio]

Your observation is correct about constant variables in general, but keccak256 of string literals is treated specially and the hash is evaluated at compile time. It's not even necessary to have optimizations enabled. You can see this in the changelog items for 0.6.12, and I've just confirmed that this is the way it works currently.

immutable may result in cheaper deployment costs because the hash doesn't have to be included in the bytecode... But this is something we'd have to confirm through benchmarks.

[pcaversaccio]

Yeah, you are right - thanks for the clarification. For the sake of completeness, there is however a general issue that is still open: ethereum/solidity#3157.

[pcaversaccio]

@frangio your _PERMIT_TYPEHASH variable is however declared as immutable here. Is there a specific reason? Otherwise, I would propose to make this constant as well due to the sake of consistency (happy to make a PR).