`@_cdecl` functions should be declared public

[SirVer]

I ran into a subtle bug that took me a while to track down. I presume the fix is not too hard, but I have not looked deep.

I have the setup where I call from Swift into Rust, but I want to hand a callback into Swift to rust. The pattern I choose for this looks like this:

#[swift_bridge::bridge]
mod ffi {
    extern "Swift" {
        type Observer;
        fn on_update(&self, path: &str);
    }

    extern "Rust" {
        type Actor;
        fn do_some_stuff(&mut self, observer: Observer);
    }
}

// some rust code implementing Actor and calling `on_update`

I create a swift package after code gen using swift-bridge-cli create-package; I then drop a tiny .swift file into the Sources dir that contains a dummy implementation of Observer. My real code depends on this Swift Package as a local dependency.

This worked wonderfully for Debug builds, however for Release builds I got linker errors, __swift_bridge__$Observer$on_update and __swift_bridge__$Observer$free could not be found. I tracked down that toggling ENABLE_TESTABILITY=YES made the code compile again. After some ChatGPTing I came to the understanding that without this, the symbols are stripped from the compiler. The solution was pretty simple: just go over the generated code and make all functions tagged with @_cdecl public.

As a workaround I am currently using this python script.

#!/usr/bin/env python3

"""
add_public_to_swift_bridge.py  <swift‑bridge file>

For every line that starts with  “@_cdecl”   the script inserts
“public ” at the beginning of the very next line, preserving whatever
indentation was already there.  It edits the file in‑place.
"""

import re
import sys
from pathlib import Path


def main(path: Path) -> None:
    text = path.read_text()
    lines = text.splitlines(keepends=True)
    for i, line in enumerate(lines[:-1]):
        if re.match(r"^@_cdecl", line):
            if not lines[i + 1].lstrip().startswith("public "):
                lines[i + 1] = f"public " + lines[i + 1]
    path.write_text("".join(lines))


if __name__ == "__main__":
    if len(sys.argv) != 2:
        sys.exit(
            "usage: add_public_to_swift_bridge.py  path/to/taskpaper_bridge.swift"
        )
    main(Path(sys.argv[1]))

Thank you so much for this software - it is a lot of fun to use and very useful indeed!

[chinedufn]

Thank you for the detailed report.

I’ll close this in favor of #166

There’s an abandoned PR that fixes this issue:
#262