Borrowing nested withUnsafePointer: cannot be captured by an escaping closure since it is a borrowed parameter

saulpalv · July 19, 2025, 4:48pm

The point was to show the compiler error

here is the playground code without if let/var and borrowing, it compiles only for ~Copyable structs, the issue is that even though this compiles, the real code does not work because structs are generated without ~Copyable on swift swift's C Import

struct NC: ~Copyable {}

func drawTexture(
    _ srcrect: borrowing NC?,
    _ dstrect: borrowing NC
) {
    withUnsafePointer(to: dstrect) { ptrDstRect in
      if srcrect != nil {
        withUnsafePointer(to: srcrect!) { ptrSrcRect in
            foo(ptrSrcRect, ptrDstRect)
        }
     } else {
        foo(nil, ptrDstRect)
     }
    }
}

func foo(_ ptr1: UnsafePointer<NC>!, _ ptr2: UnsafePointer<NC>!) {}

If this is changed the Swift, the final error appears

- struct NC: ~Copyable {}
+ struct NC {}

So in summary C Imports on swift are mutable, do not play well with borrowing and ~Copyable structs, there is no way to customize the generated c bindings, and may be not that bad to use inout for C interop