Easy strongified weak `self` in closures

Evolution Pitches
8

First, for anyone who doesn't know, guard self = self else {} is now (finally) implemented to be valid.
I think this is sufficient for circumstances where you still want the function to run when self == nil, but in my experience that's mostly not the case when I use [weak self].

Been thinking about this some and got some ideas from reading the discussion for Optional iteration. I don't like autoweak and partially like [guard self], but I don't think it's necessary to add/adapt keywords when we already have syntax around optionals.

#1

let myClosure = { [weak self]? in
    //self is already unwrapped
}

This would probably the simplest syntactic change and incredibly easy to add for end users, while also remaining syntactically familiar.

#2
Another option would be to put it on the right side of in:

let myClosure = { in [weak self]? 
    //self is already unwrapped
}
//or since it's on the right side, in could no longer be required
let myClosure = { [weak self]? //self is already unwrapped }

This semantically puts it on the wrong side of the expression, but it would make more sense in cases where we also add a default return value:

let myClosure = { in [weak self]? else *defaultValue* // Or "[weak self] ?? *defaultValue*" ?
    //self is already unwrapped
}

#3
If in? was added as syntax, it could be a good candidate here:

let myClosure = { [weak self] in?
    //self is already unwrapped
}
// with default return value
let myClosure = { [weak self] in? else *defaultValue*
    //self is already unwrapped
}

To make it simple, these could compile to:

let myClosure = { [weak self] in
    guard let self = self else {
        return *defaultValueIfNecessary*
    }
}