Unable to bridge function that receives &str and returns result

[sax]

When given the following bridge definition:

#[swift_bridge::bridge]
mod ffi {
  extern "Rust" {
    type RustApp;

    #[swift_bridge(init)]
    fn new() -> RustApp;

    #[swift_bridge(swift_name = "resultTest")]
    fn result_test(&self, receives: &str) -> Result<(), String>;
  }
}

The generated swift code produces a compiler error:

Invalid conversion from throwing function of type '(RustStr) throws -> ()' to non-throwing function type '(RustStr) -> ()'

extension RustAppRef {
  public func resultTest<GenericToRustStr: ToRustStr>(_ receives: GenericToRustStr) throws {
    return receives.toRustStr({ receivesAsRustStr in      // <-------------- compiler error here
      try {
        let val = __swift_bridge__$RustApp$resultTest(ptr, receivesAsRustStr)
        if val != nil { throw RustString(ptr: val!) } else { return }
      }()
    })
  }
}

I have included the following in my project:

extension RustString: @unchecked Sendable {}
extension RustString: Error {}

extension RustStr: @unchecked Sendable {}
extension RustStr: Error {}

If instead my bridge function is defined as follows, then the project compiles fine:

    #[swift_bridge(swift_name = "resultTest")]
    fn result_test(&self, receives: String) -> Result<(), String>;

That generates the following Swift:

public func resultTest<GenericIntoRustString: IntoRustString>(_ receives: GenericIntoRustString) throws {
    try {
      let val = __swift_bridge__$RustApp$result_test(
        ptr,
        {
          let rustString = receives.intoRustString()
          rustString.isOwned = false
          return rustString.ptr
        }())
      if val != nil { throw RustString(ptr: val!) } else { return }
    }()
  }

I'm not sure if this is expected behavior or not. So far I've been able to send Swift String values into Rust as &str fine for any function that does not return a result.

For my project, it's fine for it to use owned strings, but I did lose a bit of time before I tried switching the inputs. Even if there is no resulting change to swift-bridge, though, maybe at least having it documented in an issue will save others some heartache!

[chinedufn]

Thanks for taking the time to make a detailed and easy to follow issue.

Glad you have a workaround. I'll explain the problem and solution in case someone needs the signature to work in the future.

---

Looks like the problem is that we're using ToRustStr which takes a closure of type (RustStr) -> T.

public protocol ToRustStr {
    func toRustStr<T> (_ withUnsafeRustStr: (RustStr) -> T) -> T;
}

swift-bridge/crates/swift-bridge-build/src/generate_core/string.swift

Lines 71 to 82 in 7fc3d3c

	/// Used to safely get a pointer to a sequence of utf8 bytes, represented as a `RustStr`.
	///
	/// For example, the Swift `String` implementation of the `ToRustStr` protocol does the following:
	/// 1. Use Swift's `String.utf8.withUnsafeBufferPointer` to get a pointer to the strings underlying
	/// utf8 bytes.
	/// 2. Construct a `RustStr` that points to these utf8 bytes. This is safe because `withUnsafeBufferPointer`
	/// guarantees that the buffer pointer will be valid for the duration of the `withUnsafeBufferPointer`
	/// callback.
	/// 3. Pass the `RustStr` to the closure that was passed into `RustStr.toRustStr`.
	public protocol ToRustStr {
	func toRustStr<T> (_ withUnsafeRustStr: (RustStr) -> T) -> T;
	}

But the codegen is using a closure of that can throw, meaning it has type (RustStr) throws -> T.

    // The closure that is being passed into `toRustStr` can throw an exception,
    // but `toRustStr` is defined to take a closure that does not throw an exception.
    return receives.toRustStr({ receivesAsRustStr in
      try {
        let val = __swift_bridge__$RustApp$resultTest(ptr, receivesAsRustStr)
        if val != nil { throw RustString(ptr: val!) } else { return }
      }()
    })

Potential Solution

Here's a potential solution in case someone in the future needs this signature to work.
It boils down to making a toRustStrThrows function and calling that instead of toRustStr.

1. Review the documentation for supporting a new signature https://github.com/chinedufn/swift-bridge/blob/master/book/src/contributing/adding-support-for-a-signature/README.md

2. Add a func toRustStrThrows<T> (_ withUnsafeRustStr: (RustStr) throws -> T) to the ToRustStr protocol

3. Add a fn rust_fn_accepts_ref_str_returns_result(str: &str) -> Result<String, String>.

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

   Line 6 in 2724644

   fn create_string(str: &str) -> String;

4. Add an implementation for rust_fn_accepts_ref_str_returns_result

   fn rust_fn_accepts_ref_str_returns_result(str: &str) -> Result<String, String> {
   	if str == "should succeed" {
           return Ok("ok".to_string())
   	} else {
           return Err("error".to_string())
       }
   }

5. Add an integration test to StringTests.swift that calls rust_fn_accepts_ref_str_returns_result with "should_succeed" and confirms that it does not throw, then calls it with "fail" and confirms that it throws

• Tests to use as inspiration:
  • Calling rust_fn_accepts_ref_str_returns_result

    swift-bridge/SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunnerTests/StringTests.swift

    Lines 24 to 30 in 2724644

    	/// Verify that we can get a RustString's length
    	func testRustStringLen() throws {
    	let string = " hello "
    	let rustString: RustString = create_string(string)
    	XCTAssertEqual(rustString.len(), 7)
    	}

  • Testing that a function throws:

    swift-bridge/SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunnerTests/ResultTests.swift

    Lines 56 to 66 in de05902

    	/// Verify that we can receive a Result<UnitStruct, OpaqueRust> from Rust
    	func testSwiftCallRustResultUnitStructOpaqueRust() throws {
    	try! rust_func_return_result_unit_struct_opaque_rust(true)
    	do {
    	try rust_func_return_result_unit_struct_opaque_rust(false)
    	XCTFail("The function should have returned an error.")
    	} catch let error as ResultTestOpaqueRustType {
    	XCTAssertEqual(error.val(), 222)
    	}
    	}

  • Testing that a functiond does not throw:

    swift-bridge/SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunnerTests/ResultTests.swift

    Lines 68 to 77 in de05902

    	/// Verify that we can receive a Result<OpaqueRust, TransparentEnum> from Rust
    	func testResultOpaqueRustTransparentEnum() throws {
    	XCTContext.runActivity(named: "Should return a ResultTestOpaqueRustType") {
    	_ in
    	do {
    	let _ :ResultTestOpaqueRustType = try rust_func_return_result_opaque_rust_transparent_enum(true)
    	} catch {
    	XCTFail()
    	}
    	}

6. Add a codegen test to string_codegen_tests.rs with a function that accepts a string arg and returns a result. For example, fn some_function (arg: &str) -> Result<RustType, RustType>.

• The generated Swift code should call toRustStrThrows instead of toRustStr
• inspiration:

  • swift-bridge/crates/swift-bridge-ir/src/codegen/codegen_tests/string_codegen_tests.rs

    Lines 60 to 115 in 58f4a40

    	/// Test code generation for Rust function that takes an &str argument.
    	mod extern_rust_fn_with_str_argument {
    	use super::*;
    	fn bridge_module_tokens() -> TokenStream {
    	quote! {
    	mod foo {
    	extern "Rust" {
    	fn some_function (arg: &str);
    	}
    	}
    	}
    	}
    	fn expected_rust_tokens() -> ExpectedRustTokens {
    	ExpectedRustTokens::Contains(quote! {
    	#[export_name = "__swift_bridge__$some_function"]
    	pub extern "C" fn __swift_bridge__some_function(
    	arg: swift_bridge::string::RustStr
    	) {
    	super::some_function(arg.to_str())
    	}
    	})
    	}
    	fn expected_swift_code() -> ExpectedSwiftCode {
    	ExpectedSwiftCode::ContainsAfterTrim(
    	r#"
    	func some_function<GenericToRustStr: ToRustStr>(_ arg: GenericToRustStr) {
    	arg.toRustStr({ argAsRustStr in
    	__swift_bridge__$some_function(argAsRustStr)
    	})
    	}
    	"#,
    	)
    	}
    	fn expected_c_header() -> ExpectedCHeader {
    	ExpectedCHeader::ExactAfterTrim(
    	r#"
    	void __swift_bridge__$some_function(struct RustStr arg);
    	"#,
    	)
    	}
    	#[test]
    	fn extern_rust_fn_with_str_argument() {
    	CodegenTest {
    	bridge_module: bridge_module_tokens().into(),
    	expected_rust_tokens: expected_rust_tokens(),
    	expected_swift_code: expected_swift_code(),
    	expected_c_header: expected_c_header(),
    	}
    	.test();
    	}
    	}

  • swift-bridge/crates/swift-bridge-ir/src/codegen/codegen_tests/result_codegen_tests.rs

    Lines 129 to 200 in 852ece7

    	/// Test code generation for Rust function that accepts a Result<T, E> where T and E are
    	/// opaque Rust types.
    	mod extern_rust_fn_return_result_opaque_rust {
    	use super::*;
    	fn bridge_module_tokens() -> TokenStream {
    	quote! {
    	mod ffi {
    	extern "Rust" {
    	type SomeType;
    	fn some_function () -> Result<SomeType, SomeType>;
    	}
    	}
    	}
    	}
    	fn expected_rust_tokens() -> ExpectedRustTokens {
    	ExpectedRustTokens::Contains(quote! {
    	#[export_name = "__swift_bridge__$some_function"]
    	pub extern "C" fn __swift_bridge__some_function() -> swift_bridge::result::ResultPtrAndPtr {
    	match super::some_function() {
    	Ok(ok) => {
    	swift_bridge::result::ResultPtrAndPtr {
    	is_ok: true,
    	ok_or_err: Box::into_raw(Box::new({
    	let val: super::SomeType = ok;
    	val
    	})) as *mut super::SomeType as *mut std::ffi::c_void
    	}
    	}
    	Err(err) => {
    	swift_bridge::result::ResultPtrAndPtr {
    	is_ok: false,
    	ok_or_err: Box::into_raw(Box::new({
    	let val: super::SomeType = err;
    	val
    	})) as *mut super::SomeType as *mut std::ffi::c_void
    	}
    	}
    	}
    	}
    	})
    	}
    	fn expected_swift_code() -> ExpectedSwiftCode {
    	ExpectedSwiftCode::ContainsAfterTrim(
    	r#"
    	public func some_function() throws -> SomeType {
    	try { let val = __swift_bridge__$some_function(); if val.is_ok { return SomeType(ptr: val.ok_or_err!) } else { throw SomeType(ptr: val.ok_or_err!) } }()
    	}
    	"#,
    	)
    	}
    	const EXPECTED_C_HEADER: ExpectedCHeader = ExpectedCHeader::ContainsAfterTrim(
    	r#"
    	struct __private__ResultPtrAndPtr __swift_bridge__$some_function(void);
    	"#,
    	);
    	#[test]
    	fn extern_rust_fn_return_result_opaque_rust() {
    	CodegenTest {
    	bridge_module: bridge_module_tokens().into(),
    	expected_rust_tokens: expected_rust_tokens(),
    	expected_swift_code: expected_swift_code(),
    	expected_c_header: EXPECTED_C_HEADER,
    	}
    	.test();
    	}
    	}

7. Get tests passing

[sassman]

I also ran into this. The change from having a &str to String on the arguments of the function that returns the Result<> did the trick for me.