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This is mostly just cleanup: 1. Removes `diagnostic-descriptions.md` since it isn't used any more 2. Adds the group name to all the old notes files 3. Removes trailing whitespace 4. Adds "See Also" sections for notes that have links
30 lines
1.6 KiB
Markdown
30 lines
1.6 KiB
Markdown
# Deprecated implementation-only imports (ImplementationOnlyDeprecated)
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Warnings that identify `import` declarations with the `@_implementationOnly` attribute.
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## Overview
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When applied to `import` declarations, the compiler-internal attribute `@_implementationOnly` attempts prevents declarations from the imported module from being exposed in the ABI or public interface of the dependent module. This attribute became deprecated when support for access levels on `import` declarations was introduced with [SE-0409].
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One reason `@_implementationOnly import` is deprecated is that it is unsafe when used in modules that are built _without_ [library evolution][library-evolution] enabled. For example, suppose the following code were part of a library named `Foo`:
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```swift
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// Library `Foo`
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@_implementationOnly import ImplementationDetail
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public struct Bar {
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internal var x: Baz // defined in ImplementationDetail
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}
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```
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If `Foo` is not compiled with library evolution, then the memory layout of values of `Bar` must be known at compile time in clients of `Foo`. However, the `@_implementationOnly import` of `ImplementationDetail` prevents clients from being able to look up `Baz` which is a type that contributes to the layout of `Foo`. As a result, the layout of `Foo` will be miscalculated resulting in undefined behavior.
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## See Also
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- [SE-0409: Access level on imports][SE-0409]
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- [Library evolution][library-evolution]
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[SE-0409]: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0409-access-level-on-imports.md
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[library-evolution]: https://www.swift.org/blog/library-evolution/
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