`if let` shorthand

[Saklad5 (Jeremy Saklad)]

There is a superior way to implement that concept: generalize the recent proposals for advanced memory management.

@Joe_Groff

As above, we really want to make a local variable, that asserts exclusive access to the value being modified for the scope of the variable, allowing us to mutate it in-place without repeating the access sequence to get to it:

inout greatAunt = &mother.father.sister
greatAunt.name = "Grace"
mother.father.sister = otherGreatAunt // error, can't access `mother.father.sister` while exclusively borrowed by `greatAunt`
greatAunt.age = 115

There are other places where inout bindings for in-place mutation are desirable, but aren’t currently available, and we can extend inout bindings to be available in those places as well. For instance, when switch -ing an enum , we would like to be able to bind to its payload, and update it in place:

enum ZeroOneOrMany<T> {
    case zero
    case one(T)
    case many([T])

    mutating func append(_ value: consuming T) {
        switch &self {
        case .zero:
            self = .one(move(value))
        case .one(let oldValue):
            self = .many([move(oldValue), move(value)])
        case .many(inout oldValues):
            oldValues.append(move(value))
        }
    }
}