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Unused values (NoUsage)

Overview

These diagnostics are emitted when a value is computed or defined but never used, which often indicates a programming mistake such as a missing assignment or an unintentionally discarded result.

The NoUsage group covers a range of unused-value situations like calling a function and ignoring its return value or declaring a variable or constant that is never read.

Example

func sortedCopy(_ array: [Int]) -> [Int] {
    var copy = array
    copy.sort()   // ok: sort() mutates in place
    array.sorted() // warning: result of call to 'sorted()' is unused
    return copy
}

How to fix

  • Use the result, or assign it to _ to explicitly discard it:

    _ = incrementGlobalCounter() // increment the counter without using the new value

  • If ignoring the return value of a function is intentional and common for callers, then annotate the function with @discardableResult:

    private var counter = 0

@discardableResult // callers may not need to use the new value
func incrementGlobalCounter() -> Int {
	counter += 1
  return counter
}

  • Replace an unused binding with _:

    enum NetworkError: Error {
  case requestFailed(statusCode: Int, message: String)
  case timeout
}

func handle(_ error: NetworkError) {
  switch error {
  case .requestFailed(let statusCode, _):  // message not needed here
    print("Request failed with status \(statusCode)")
  case .timeout:
    print("Request timed out")
  }
}

See Also