Unused error

Severity: Info ID: unused-error

Flags a custom error declaration that is never referenced anywhere in the compiled sources.

What it does

Reports each error declaration whose symbol is never referenced. Any resolved reference counts as a use: a revert Err(...) statement (including the qualified revert Lib.Err(...)), require(cond, Err(...)) (Solidity 0.8.26+), and Err.selector (including through abi.encodeWithSelector). Uses are collected across the whole compilation unit, so an error declared in a shared file and reverted elsewhere in the project is not reported.

Errors declared in interfaces and abstract contracts are not reported: they are ABI surface meant for implementers and off-chain consumers, which may live outside the compiled sources.

Why is this bad?

An unused error is dead code. It suggests a missing revert path or a leftover from a refactor.

Example

Bad

solidity

error Unauthorized(); // declared but never referenced

Good

solidity

error Unauthorized();

function withdraw() external {
    if (msg.sender != owner) revert Unauthorized();
}

Limitations

A selector hardcoded in inline assembly or built with abi.encodeWithSignature("Err(...)") does not reference the declaration and is not seen as a use.