[Pitch] Proposal for `safe` keyword in closures to simplify `weak` capture pattern

[cal (Cal Stephens)]

I proposed this back in 2021: `guard` capture specifier for closure capture lists

The core team shared this feedback at the time:

@John_McCall

The core team talked about this, and we're reluctant to add features that introduce new sources of control flow unless they significantly improve expressive power. This proposal is a simple sugaring of a very specific pattern, but it would not work in a lot of superficially similar situations, like when the function needs to return anything other than Void. The amount of convenience offered also seems fairly low.

@cal, thank you for your suggestion, it was certainly worth considering. However, at this time, we don't think this proposal is likely to be accepted, and so it's not worth taking through the Evolution process. We can reconsider this if someone makes a strong argument to do so in the future.

Since then, SE-0345 (if let self instead of if let self = self) and SE-0365 (support for implicit self after self has been unwrapped) both improved the boilerplate situation enough that I don't really think any other sugar is necessary. guard let self else { return } is pretty reasonably lightweight