Controlling function definition visibility in clients #2974
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| * Incremental compilation (typical of debug builds) allows the compiler to avoid emitting symbols for `fileprivate` and `private` functions if they aren't needed elsewhere in the file, for example because all of their uses have been inlined (or there were no uses). | ||
| * Whole-module optimization (WMO) allows the definitions of `internal` , `fileprivate`, and `private` functions to be available to other source files in the same module. The compiler may choose not to emit symbols for `internal`, `fileprivate`, or `private` entities at all if they aren't needed. (For example, because they've been inlined into all callers) | ||
| * Cross-module optimization (CMO) allows the definitions of functions to be made available to clients in other modules. The "conservative" form of CMO, which has been enabled by the Swift Package Manager since Swift 5.8, does this primarily for `public` functions. A more aggressive form of cross-module optimization can also make the definitions of `internal`, `fileprivate`, or `private` entities available to clients (for the compiler's use only!). | ||
| * [Embedded Swift](https://github.com/swiftlang/swift-evolution/blob/main/visions/embedded-swift.md) relies on WMO and the aggressive CMO described above. It will also avoid emitting symbols to binaries unless they appear to be needed, which helps reduce code size. It is also necessary, because Embedded Swift cannot create symbols for certain |
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