An error type can conform to one or more of these new protocols to
customize its behavior and representation. From an implementation
standpoint, the protocol conformances are used to fill in the
user-info dictionary in NSError to interoperate with the Cocoa
error-handling system.
There are a few outstanding problems with this implementation,
although it is fully functional:
* Population of the userInfo dictionary is currently eager; we
should use user info providers on platforms where they are
available.
* At present, the Swift dynamic casting machinery is unable to unbox a
_SwiftNativeNSError when trying to cast from it to (e.g.) an
existential, which makes it impossible to retrieve the
RecoverableError from the NSError. Instead, just capture the original
error---hey, they're supposed to be value types anyway!---and use that
to implement the entry points for the informal
NSErrorRecoveryAttempting protocol.
This is part (1) of the proposal solution.
The general rule here is that something needs to be SWIFT_CC(swift)
if it's just declared in Swift code using _silgen_name, as opposed to
importing something via a header.
Of course, SWIFT_CC(swift) expands to nothing by default for now, and
I haven't made an effort yet to add the indirect-result / context
parameter ABI attributes. This is just a best-effort first pass.
I also took the opportunity to shift a few files to just implement
their shims header and to demote a few things to be private stdlib
interfaces.
