Support use of an optional label for the first trailing closure

Evolution Pitches
24

I want to add a couple of examples to clarify the behavior of the attribute I am proposing. We'll consider this example:

extension Result {
   func fold<T>(success: (Success) -> T, failure: (Failure) -> T) -> T { ... }
}
// under the proposal, this is allowed:
let wasSuccessful = result.fold success: { _ in true } failure: { _ in false }
// under the proposal, this is also allowed:
let wasSuccessful = result.fold { _ in true } failure: { _ in false }

With the attribute I propose:

extension Result {
   // this attribute enforces the use of labels at all call sites
   @require_trailing_closure_labels
   func fold<T>(success: (Success) -> T, failure: (Failure) -> T) -> T { ... }
}
// this is now required:
let wasSuccessful = result.fold success: { _ in true } failure: { _ in false }
// this now produces a compiler error:
// "`fold` requires trailing closure labels, `success` was elided:" 
let wasSuccessful = result.fold { _ in true } failure: { _ in false }

The require_trailing_closure_labels is a strawman, if you like this idea please suggest something better.

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