Support throwing initializers

[NiwakaDev]

This PR implements throwing initializers. See: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/errorhandling/.

Here's an example of using this feature:

// Rust
#[swift_bridge::bridge]
mod ffi {
    enum ResultTransparentEnum {
        NamedField { data: i32 },
        UnnamedFields(u8, String),
        NoFields,
    }
    extern "Rust" {
        type ThrowingInitializer;
        #[swift_bridge(init)]
        fn new(succeed: bool) -> Result<ThrowingInitializer, ResultTransparentEnum>;
        fn val(&self) -> i32;
    }
}

// Swift
do {
    let throwingInitializer = try ThrowingInitializer(false)
} catch let error as ResultTransparentEnum {
    //...
} catch {
    //...
}

[NiwakaDev]

I guess that it's not suitable because the convert_ffi_value_to_swift_value method returns a expression.

[chinedufn]

I'm not sure what you're communicating?

[chinedufn]

I think you're asking whether it's okay to return two statements here instead of a single expression?

This should be fine since the ThrowingInit code here is only ever used inside of an initializer and isn't ever embedded inside of another expression.

[NiwakaDev]

I'm not sure what you're communicating?

The other return values of convert_ffi_value_to_swift_value return closures, but this case doesn't return a closure. This means a simple statement, not expression.

For examples:

swift-bridge/crates/swift-bridge-ir/src/bridged_type/bridgeable_result.rs

Line 286 in 495611b

return format!("try {{ let val = {expression}; if val != nil {{ throw {err} }} else {{ return{ok} }} }}()", expression = expression, err = err, ok = ok);

swift-bridge/crates/swift-bridge-ir/src/bridged_type/bridgeable_result.rs

Line 247 in 495611b

"try {{ let val = {expression}; switch val.tag {{ case {c_ok_name}: return{ok_swift_type} case {c_err_name}: throw {err_swift_type} default: fatalError() }} }}()",

The above examples return closures.

[chinedufn]

Let's document #[swift_bridge(init)] in the Function Attributes section in the book

swift-bridge/book/src/bridge-module/functions/README.md

Line 66 in bdbb028

## Function Attributes

Can add a #[swift_bridge(init)] section.

Can include examples of:

• regular init
• throwing init
• option init

[chinedufn]

I'm not sure what you're communicating?

[chinedufn]

use a Swift class "create a Swift class"

[chinedufn]

Let's use an if statement instead of a switch so that we don't need this default: fatalError()

The default: fatalError makes it seem like this can fail, even though it can't.

[NiwakaDev]

Done. However, I don't address the others yet.

For example:

swift-bridge/crates/swift-bridge-ir/src/bridged_type/bridgeable_result.rs

Line 247 in 495611b

"try {{ let val = {expression}; switch val.tag {{ case {c_ok_name}: return{ok_swift_type} case {c_err_name}: throw {err_swift_type} default: fatalError() }} }}()",

I'll address this in the next PR, or we can open it as good first issue.

[chinedufn]

Need a test for FailableInitializerType::Throwing

[chinedufn]

Great work on this. Looks good. Left some minor feedback.

[NiwakaDev]

@chinedufn
Done.

[chinedufn]

Looks great thanks.