swift-mirror/userdocs/diagnostics/variable-never-mutated.md

Variable never mutated (VariableNeverMutated)

Warnings that identify local var bindings which are never mutated after initialization and could instead be declared with let.

Overview

When a variable is introduced with var but never reassigned or otherwise mutated, it can be declared with let instead to explicitly indicate that the variable is constant.

Example

func swap(_ a: inout Int, _ b: inout Int) {
  var temp = a // warning: variable 'temp' was never mutated; consider changing to 'let' constant
  a = b
  b = temp
}

The fix is to change var to let:

func swap(_ a: inout Int, _ b: inout Int) {
  let temp = a
  a = b
  b = temp
}

Sub-groups

The VariableNeverMutated group contains the following sub-group, which can also be enabled or controlled independently:

• doc:weak-mutability: Warns when a weak var binding is never mutated and could be a weak let instead.