Unused Local Variable

ID

swift.unused_local_variable

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Dead Code

Language

Swift

Tags

CWE:563, dead-code, suspicious-code, unused-code

Description

Reports a local var or let declaration whose name is never read in its enclosing scope. Re-assigning the variable does not count as a read, so a value that is written but never observed is still flagged.

func work() {
    let x = 1              // FLAW — never read
    let y = 2
    print(y)               // OK   — `y` is read

    var z = 3
    z = 4                  // write only, no read
                           // FLAW on `z`
}

Rationale

A local that is never read is dead code: removing it cannot change the behaviour of the program, and keeping it around forces every reader to prove that for themselves. Variables that are deliberately discarded should be spelled with the wildcard _ (or a leading-underscore name) so the intent is explicit.

The rule is conservative — it ignores cases where the binding has a documentary purpose even when unused:

  • for x in seq { … } — covered by for_where / unused_index.

  • if let x = …, guard let x = … else, while let x = … — the binding asserts the optional is non-nil.

  • Pattern bindings such as case .foo(let x).

Remediation

Either use the value, or replace the binding with _:

func work() {
    let y = 2
    print(y)

    _ = sideEffectingCall()    // discard the result explicitly
}

References