Support Box<FnOnce> in extern "Rust"

[daytime-em]

When you declare FnOnce on a Swift type, it doesn't work. The error output says:

 thread 'main' panicked at 'not yet implemented: Support Box<dyn FnOnce(A, B) -> C>', /Users/edixon/.cargo/registry/src/index.crates.io-6f17d22bba15001f/swift-bridge-ir-0.1.52/src/bridged_type.rs:1643:21

It looks like considerable work has been done to support FnOnce, but they're not hooked up in bridged_types. There's just TODOs there. Is there something about the implementation that needs to be fixed? If so I'd be happy to look into it, I'd just appreciate some context :)

Thanks for sharing this project btw, it's amazing 😎

[chinedufn]

When you declare FnOnce on a Swift type, it doesn't work. The error output says:

Can you comment with a minimal example bridge module that generates this error?

It looks like considerable work has been done to support FnOnce, but they're not hooked up in bridged_types

Right now we support passing an Box<dyn FnOnce> from Rust -> Swift. As in, Rust creates the callback and Swift calls the callback.

Is there something about the implementation that needs to be fixed? If so I'd be happy to look into it

Here are some relevant links that should help with adding support for your Box<dyn FnOnce> case:

https://github.com/chinedufn/swift-bridge/blob/af962ca051ba15b81794575a0837f1e2abde9984/book/src/contributing/adding-support-for-a-signature/README.md

swift-bridge/crates/swift-bridge-ir/src/codegen/codegen_tests/boxed_fnonce_codegen_tests.rs

Lines 382 to 430 in af962ca

	/// Verify that we can pass a callback with an opaque Rust arg from Rust to Swift.
	mod test_swift_takes_callback_return_opaque_rust_type {
	use super::*;
	fn bridge_module_tokens() -> TokenStream {
	quote! {
	mod ffi {
	extern "Swift" {
	fn some_function(callback: Box<dyn FnOnce() -> ARustType>);
	}
	extern "Rust" {
	type ARustType;
	}
	}
	}
	}
	fn expected_rust_tokens() -> ExpectedRustTokens {
	ExpectedRustTokens::ContainsMany(vec![
	quote! {
	pub fn some_function (callback: Box<dyn FnOnce() -> super::ARustType>) {
	unsafe {
	__swift_bridge__some_function(
	Box::into_raw(Box::new(callback)) as *mut Box<dyn FnOnce() -> super::ARustType>
	)
	}
	}
	},
	quote! {
	#[export_name = "__swift_bridge__$some_function$param0"]
	pub extern "C" fn some_function_param0(some_function_callback: *mut Box<dyn FnOnce() -> super::ARustType>) -> *mut super::ARustType {
	Box::into_raw(Box::new({
	let val: super::ARustType = unsafe { Box::from_raw(some_function_callback)() };
	val
	})) as *mut super::ARustType
	}
	#[export_name = "__swift_bridge__$some_function$_free$param0"]
	pub extern "C" fn free_some_function_param0(some_function_callback: *mut Box<dyn FnOnce() -> super::ARustType>) {
	let _ = unsafe { Box::from_raw(some_function_callback) };
	}
	},
	quote! {
	#[link_name = "__swift_bridge__$some_function"]
	fn __swift_bridge__some_function(callback: *mut Box<dyn FnOnce() -> super::ARustType>);
	},
	])
	}

swift-bridge/crates/swift-integration-tests/src/boxed_functions.rs

Lines 37 to 53 in af962ca

	// TODO
	// extern "Rust" {
	// fn rust_takes_callback_fnonce_no_args_no_return(arg: Box<dyn FnOnce() -> ()>);
	// fn rust_takes_callback_fnonce_primitive(doubling_fn: Box<dyn FnOnce(u8) -> u8>);
	// fn rust_takes_callback_fnonce_opaque_rust(
	// doubling_fn: Box<dyn FnOnce(CallbackTestOpaqueRustType) -> CallbackTestOpaqueRustType>,
	// );
	//
	// fn rust_takes_callback_fnonce_two_params(
	// arg: Box<dyn FnOnce(i16, CallbackTestOpaqueRustType)>,
	// );
	//
	// fn rust_takes_two_callbacks_fnonce_noop(
	// arg1: Box<dyn FnOnce()>,
	// arg2: Box<dyn FnOnce() -> ()>,
	// );
	// }

Please feel free to ask more questions as you go!

[daytime-em]

Right now we support passing an Box from Rust -> Swift. As in, Rust creates the callback and Swift calls the callback.

I think this was the source of my confusion. I was attempting to send a Closure from Swift to Rust, and call it in Rust. I edited the issue title to reflect this.

Thank you for the implementation guidance! If I have more questions can I just tag you here?

[chinedufn]

If I have more questions can I just tag you here?

Absolutely!