JuniorTheoryCommon

What happens when this program is compiled and run?

SWIFT

struct Counter {
    var count = 0

    mutating func increment() {
        count += 1
    }
}

let c = Counter()
c.increment()
print(c.count)

It does not compile

Expecting 1: thinking let only stops c from being reassigned. An instance in a let cannot change at all, so the call to a mutating method does not compile.
Expecting 0: thinking Swift changes a hidden copy of c. It makes no copy; calling a mutating method on a let is a compile error.

JuniorTheoryCommon

SWIFT

struct Counter {
    var count = 0

    mutating func increment() {
        count += 1
    }
}

let c = Counter()
c.increment()
print(c.count)

It does not compile

Expecting 1: thinking let only stops c from being reassigned. An instance in a let cannot change at all, so the call to a mutating method does not compile.
Expecting 0: thinking Swift changes a hidden copy of c. It makes no copy; calling a mutating method on a let is a compile error.