When mutating methods from a protocol are called on a variable conforming to that protocol, the compiler misses the mutation and emits a warning. Casting the variable to a type that implements the protocol will silence the warning.

bug.swift

protocol Increaser {
    var value: Int { get set }
    mutating func increase()
}
 
func doIncrease(var myIncreaser: Increaser) { // Parameter 'myIncreaser' was never mutated; consider
    myIncreaser.increase()                    // changing to 'let' constant
    myIncreaser.value
}
 
struct Foo: Increaser {
    var value: Int
    init() {
        value = 0
    }
    mutating func increase() {
        value++
    }
}
 
func increaseAFoo(var myFoo: Foo) { // No warning emitted.
    myFoo.increase()
    myFoo.value
}
 
doIncrease(Foo())