Explictly capture variables in blocks

[allevato (Tony Allevato)]

@DevAndArtist

I personally wouldn‘t mind a compiler/ide flag to disable implicit captures and require them explicitly or error out, similar to how we can treat warnings as errors already.

This kind of feature is something that the Swift team explicitly tries to avoid as a design philosophy:

@brentdax

We try very hard not to create "dialects" of Swift where seemingly valid code can be rejected based on compiler flags or other information that isn't visible in the source file. We unfortunately don't always succeed—resilience and language version both come from compiler flags—but we only do that when one of the language's goals demands it.