One of the limitations of not having conditional conformance at the
moment is that the implementation of `init(from:)` and `encode(to:)` on
types which require it is that failure to cast dependent types to
`Encodable` or `Decodable` is a runtime failure. There is no way to
statically guarantee that the wrapped type is `Encodable` or
`Decodable`.
As such, in those implementations, at best we can directly call
`(element as! Encodable).encode(to: encoder)`, or similar. However, this
encodes the element directly into an encoder, without giving the encoder
a chance to intercept the type. This is problematic for `JSONEncoder`
because it cannot apply a strategy if it doesn't get to intercept the
type.
This gives a temporary workaround for JSON strategies because of
internal Foundation knowledge.
This avoids indirection by making calls directly to the C implementations which prevents potentials of mismatched intent or changes of calling convention of @_silgen. The added benefit is that all of the shims in this case are no longer visible symbols (anyone using them was not authorized out side of the Foundation overlay). Also the callout methods in the headers now all share similar naming shcemes for easier refactoring and searching in the style of __NS<class><action> style. The previous compiled C/Objective-C source files were built with MRR the new headers MUST be ARC by Swift import rules.
The one caveat is that certain functions MUST avoid the bridge case (since they are part of the bridging code-paths and that would incur a recursive potential) which have the types erased up to NSObject * via the macro NS_NON_BRIDGED.
The remaining @_silgen declarations are either swift functions exposed externally to the rest of Swift’s runtime or are included in NSNumber.gyb which the Foundation team has other plans for removing those @_silgen functions at a later date and Data.swift has one external function left with @_silgen which is blocked by a bug in the compiler which seems to improperly import that particular method as an inline c function.
Data can encapsulate it’s own sub-sequence type by housing a range of the slice in the structural type for Data. By doing this it avoids the API explosion of supporting all APIs that take Data would need overloads to take a slice of Data. This does come at a small conceptual cost: any index based iteration should always account for the startIndex and endIndex of the Data (which was an implicit requirement previously by being a Collection). Moreover this prevents the requirement of O(n) copies of Data if it is never mutated while parsing sub sequences; so more than an API amelioration this also could offer a more effecient code-path for applications to use.
* replace unused closure parameters with '_' in stdlib source
* fold some _ closure arguments into line above
* fold more _ closure arguments into line above
Apple and the Swift community has settled on this style:
https://devforums.apple.com/message/1133616#1133616
> FWIW, we've recently decided to standardize on () -> Void
> (generally, () for parameters and Void for return types) across all of our
> documentation.
