[Pitch] `With` functions in the standard library

[cal (Cal Stephens)]

@ksluder

One problem I have is that your example does in fact look to me like it’s mutating the reference type. And that is actually a useful operation, especially when trying to mutate something stored behind a long path of accessors.

jsonDict["jwtHeader"].with {
  $0["alg"] = "none"
  $0["typ"] = "jwt"
}

looks like a perfectly cromulent in-place mutation!

To be fair, in this case the compiler would emit a “result of call to with is unused” warning.