Good evening all,
Attached is the proposal text that came out of a short discussion on Twitter
<https://twitter.com/dgregor79/status/844760063663259651> about squashing one
more case of implicit conversions in the language. This proposal seeks to
remove the implicit nature of toll-free bridging in favor of an explicit
as-cast. The proposal can also be read as a gist
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f>, and the most
up-to-date copy may be found on swift-evolution itself
<https://github.com/apple/swift-evolution/pull/655>.
Cheers,
~Robert Widmann
Explicit Toll-Free Conversions
Proposal: SE-NNNN <https://gist.github.com/CodaFi/NNNN-filename.md>
Authors: Robert Widmann <https://github.com/codafi>
Review Manager: TBD
Status: Awaiting review
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#introduction>Introduction
To continue removing further implicit conversions from the language, implicit
toll-free bridging should be deprecated in Swift 3 and removed in Swift 4.0.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#motivation>Motivation
Swift currently allows implicit conversions between CF types and their
toll-free-bridged Objective-C counterparts. This was meant to maintain parity
with the feature in Objective-C. However, the presence of implicit conversions
is no longer in-line with the expectations of Swift's type system. In addition,
advancements in the Clang importer have made interaction with imported CF and
Foundation types nearly seamless - obviating the need for this feature in
general. For the sake of safety, and in the spirit of continuing the work of
SE-0072
<https://github.com/apple/swift-evolution/blob/master/proposals/0072-eliminate-implicit-bridging-conversions.md>
and facilitating SE-0083
<https://github.com/apple/swift-evolution/blob/master/proposals/0083-remove-bridging-from-dynamic-casts.md>,
we propose the deprecation and removal of this implicit conversion in favor of
an explicit as cast.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#proposed-solution>Proposed
solution
The implicit conversion between toll-free-bridged types shall be removed. In
its place, the user should use explicit ascasts to convert bewteen bridged CF
and Objective-C types.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#detailed-design>Detailed
Design
When in Swift 3 mode, the compiler shall warn when it encounters the need to
perform an implicit conversion. At the warning site, it can offer the explicit
as cast to the user as a fix-it.
func nsToCF(_ ns: NSString) -> CFString {
return ns // warning: 'NSString' is not implicitly convertible to 'CFString';
did you mean to use 'as' to explicitly convert?
}
func cfToNS(_ cf: CFString) -> NSString {
return cf // warning: 'CFString' is not implicitly convertible to 'NSString';
did you mean to use 'as' to explicitly convert?
}
When in Swift 4 mode, the compiler shall no longer perform the implicit
conversion and relying on this behavior will be a hard error.
func nsToCF(_ ns: NSString) -> CFString {
return ns // error: cannot convert return expression of type 'NSString' to
return type 'CFString'
}
func cfToNS(_ cf: CFString) -> NSString {
return cf // error: cannot convert return expression of type 'CFString' to
return type 'NSString'
}
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#source-compatibility>Source
compatibility
This introduces a source-breaking change that affects consumers of APIs that
are explicitly taking or vending instances of bridged CF or Objective-C types.
However, recent importer advances have made APIs like this far less numerous.
In practice, we expect the source breakage to eludicate places where implicit
conversions were accidentally being performed behind the user's back.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#effect-on-abi-stability>Effect
on ABI stability
This proposal has no effect on the Swift ABI.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#effect-on-api-resilience>Effect
on API resilience
This proposal has no effect on API resilience.
<https://gist.github.com/CodaFi/fc6fa3aac64c4b1deffd1434a37e295f#alternatives-considered>Alternatives
considered
Do nothing and continue to accept this implicit conversion._______________________________________________
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