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SE-0495: Clarify emitted symbols and compatibility header content for @c enums #3004

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SE-0495: Clarify emitted symbols and compatibility header content for @c enums #3004

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SE-0495: Add details on enums in the compatibility header
xymus Oct 17, 2025
5d7fa87
SE-0495: Clarify the symbols emitted and ABI compatibility
xymus Oct 17, 2025
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12 changes: 9 additions & 3 deletions proposals/0495-cdecl.md
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Expand Up @@ -65,13 +65,17 @@ A `@c` enum may declare a custom C name, and must declare an integer raw type co
```swift
@c
enum CEnum: CInt {
case a
case b
case first
case second
}
```

The attribute `@objc` is already accepted on enums. These enums qualify as an Objective-C representable type and are usable from `@objc` global function signatures but not from `@c` functions.

In the compatibility header, the `@c` enum is printed with the C name specified in the `@c` attribute or the Swift name by default. It defines a storage of the specified raw type with support for different dialects of C.

Each case is printed using a name composed of the enum name with the case name attached. The first letter of the case name is capitalized automatically. For the enum above, the generated cases for C are named `CEnumFirst` and `CEnumSecond`.

### `@c @implementation` global functions

Extend support for the `@implementation` attribute, introduced in [SE-0436](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0436-objc-implementation.md), to global functions marked with either `@c` or `@objc`. These functions are declared in an imported C or Objective-C header, while the Swift function provides their implementation. Type-checking ensures the declaration matches the implementation signature in Swift. Functions marked `@implementation` are not printed in the compatibility header.
Expand Down Expand Up @@ -165,7 +169,9 @@ Existing adopters of `@_cdecl` can replace the attribute with `@objc` to preserv

## ABI compatibility

Marking a global function with `@c` or `@objc` makes it use the C calling convention. Adding or removing these attributes on a function is an ABI breaking change. Updating existing `@_cdecl` to `@objc` or `@c` is ABI stable.
The compiler emits a single symbol for `@c` and `@objc` functions, the symbol uses the C calling convention.

Adding or removing the attributes `@c` and `@objc` on a function is an ABI breaking change. Changing between `@c` and `@objc` is ABI stable. Changing between `@_cdecl` and either `@c` or `@objc` is an ABI breaking change since `@_cdecl` emits two symbols and Swift clients of `@_cdecl` call the one with the Swift calling convention.

Adding or removing the `@c` attribute on an enum is ABI stable, but changing its raw type is not.

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