Array Initializer with Access to Uninitialized Buffer

[Nevin]

@jrose

The problem with "juggling the error" is that you can't use it to build rethrows functions, because the compiler can't prove that you'll only rethrow whatever the inner closure throws.

I’m fairly certain that rethrows does not impose that requirement. This same concept of “juggling the error” came up last month and last winter, and DispatchQueue.sync does exactly that in the standard library by calling _syncHelper.