Support bridging `Option<SwiftClass>` in `extern "Rust"` function

[chinedufn]

It is not currently possible to bridge Option<OpaqueSwiftType> in an extern "Rust" function.

For example, the following signature does not compile:

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        fn some_function(arg: Option<SwiftType>) -> Option<SwiftType>;
    }

    extern "Swift" {
        type SwiftType;
    }
}

Potential Solution

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 fn rust_reflect_option_swift_type(arg: Option<OptTestOpaqueSwiftType>) -> Option<OptTestOpaqueSwiftType> to the Option integration tests https://github.com/chinedufn/swift-bridge/blob/58f4a40f96bb050607c746376422ab3c62e0e771/crates/swift-integration-tests/src/option.rs#L1-L144

   • Can define OptTestOpaqueSwiftType in here https://github.com/chinedufn/swift-bridge/blob/58f4a40f96bb050607c746376422ab3c62e0e771/SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunner/Option.swift#L1-L68

3. Add a test that calls rust_reflect_option_swift_type

   • inspiration: https://github.com/chinedufn/swift-bridge/blob/53b118d17f2f1a2922e969de528b99a2ffbc7dde/SwiftRustIntegrationTestRunner/SwiftRustIntegrationTestRunnerTests/OptionTests.swift#L93-L99

4. Add a codegen test for a Rust fn accepting an Option<SwiftType> arg to option_codegen_tests.rs

   • inspiration:
   • https://github.com/chinedufn/swift-bridge/blob/58f4a40f96bb050607c746376422ab3c62e0e771/crates/swift-bridge-ir/src/codegen/codegen_tests/option_codegen_tests.rs#L773-L833
   • https://github.com/chinedufn/swift-bridge/blob/dd5bef56af28db4f1a1244d86115def6abede931/crates/swift-bridge-ir/src/codegen/codegen_tests/opaque_swift_type_codegen_tests.rs#L5-L67

5. Add a codegen test for a Rust fn returning an Option<SwiftType> in option_codegen_tests.rs

   • inspiration:
   • https://github.com/chinedufn/swift-bridge/blob/58f4a40f96bb050607c746376422ab3c62e0e771/crates/swift-bridge-ir/src/codegen/codegen_tests/option_codegen_tests.rs#L597-L653
   • can move the contents of this file to opaque_swift_type_codegen_tests.rs: https://github.com/chinedufn/swift-bridge/blob/54b9c09337701dd7580010d1d5ef6527b855ee98/crates/swift-bridge-ir/src/codegen/codegen_tests/extern_swift_function_opaque_swift_type_return_codegen_tests.rs#L5-L61

6. Get tests passing