State variable could be constant

Severity: Gas ID: could-be-constant

Flags state variables that have a compile-time-constant inline initializer and are never written anywhere — making them eligible to be declared constant.

What it does

Reports each non-constant, non-immutable state variable when all of the following hold:

• Its type is constant-compatible: any elementary type (value types, string, bytes) or a contract type.
• It has an inline initializer composed only of literals, type casts (address(...), uint160(...), contract/interface casts like IToken(...)), type(T).{min,max,interfaceId}, allowed pure builtin calls (keccak256, sha256, ripemd160, ecrecover, addmod, mulmod), and references to other constant variables.
• It is never written by the constructor body or by any other function.

Why is this bad?

constant state variables are inlined directly into the deployed bytecode rather than read from storage, eliminating SLOAD costs on every access. Declaring such variables constant also expresses intent and prevents future writes.

Example

Bad

contract C {
    uint256 LIMIT = 100;
    bytes32 SALT = keccak256("foundry");
}

Good

contract C {
    uint256 constant LIMIT = 100;
    bytes32 constant SALT = keccak256("foundry");
}

Notes

This lint requires an inline compile-time-constant initializer. Variables without an initializer (uint256 x;) are not flagged, since converting them to constant requires choosing a value, not just adding a keyword. This is a Gas-severity lint and is not applied to test or script files.