The Swift 4 release was designed around two primary goals: to provide source stability for Swift 3 code and to provide ABI stability for the Swift standard library. To that end, the Swift 4 release was divided into two stages.
Stage 1 focused on the essentials required for source and ABI stability. Features that don't fundamentally change the ABI of existing language features or imply an ABI-breaking change to the standard library were not considered in this stage.
The high-priority features supporting Stage 1's source and ABI stability goals were:
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Source stability features: The Swift language will need some accommodations to support code bases that target different language versions, to help Swift deliver on its source-compatibility goals while still enabling rapid progress.
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Resilience: Resilience provides a way for public APIs to evolve over time, while maintaining a stable ABI. For example, resilience eliminates the fragile base class problem that occurs in some object-oriented languages (e.g., C++) by describing the types of API changes that can be made without breaking ABI (e.g., "a new stored property or method can be added to a class").
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Stabilizing the ABI: There are a ton of small details that need to be audited and improved in the code generation model, including interaction with the Swift runtime. While not specifically user-facing, the decisions here affect performance and (in some rare cases) the future evolution of Swift.
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Generics improvements needed by the standard library: The standard library has a number of workarounds for language deficiencies, many of which manifest as extraneous underscored protocols and workarounds. If the underlying language deficiencies remain, they become a permanent part of the stable ABI. Conditional conformances, recursive protocol requirements, and where clauses for associated types are known to be in this category, but it's plausible that other features will be in scope if they would be used in the standard library.
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String re-evaluation: String is one of the most important fundamental types in the language. Swift 4 seeks to make strings more powerful and easier-to-use, while retaining Unicode correctness by default.
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Memory ownership model: An (opt-in) Cyclone/Rust-inspired memory ownership model is highly desired by systems programmers and for other high-performance applications that want predictable and deterministic performance. This feature will fundamentally shape the ABI, from low-level language concerns such as "inout" and low-level "addressors" to its impact on the standard library. While a full memory ownership model is likely too large for Swift 4 stage 1, we need a comprehensive design to understand how it will change the ABI.
Swift 4 stage 2 built on the goals of stage 1. It differed in that stage 2 proposals could include some additive changes and changes to existing features that don't affect the ABI. There were a few focus areas for Swift 4 stage 2:
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Stage 1 proposals: Any proposal that would have been eligible for stage 1 is a priority for stage 2.
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Source-breaking changes: The Swift 4 compiler will provide a source-compatibility mode to allow existing Swift 3 sources to compile, but source-breaking changes can manifest in "Swift 4" mode. That said, changes to fundamental parts of Swift's syntax or standard library APIs that break source code are better front-loaded into Swift 4 than delayed until later releases. Relative to Swift 3, the bar for such changes is significantly higher:
- The existing syntax/API being changed must be actively harmful.
- The new syntax/API must clearly be better and not conflict with existing Swift syntax.
- There must be a reasonably automatable migration path for existing code.
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Improvements to existing standard library facilities: Additive changes that improve existing standard library facilities can be considered. With standard library additions in particular, proposals that provide corresponding implementations are preferred. Potential focus areas for improvement include collections (e.g., new collection algorithms) and improvements to the ergonomics of
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Foundation improvements: We anticipate proposing some targeted improvements to Foundation API to continue the goal of making the Cocoa SDK work seamlessly in Swift.