UniFFI fails to generate Swift bindings in my project

[bes]

Hello, I am using a staticlib-only iOS project with UniFFI, and I am having some trouble generating Swift bindings.

I have a small proof-of-concept here https://github.com/bes/uniffi-2024-11-poc

When I run the build.sh script, the output is:

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
+ cargo run --features=uniffi/cli --bin uniffi-bindgen generate --library ./target/aarch64-apple-ios-sim/debug/libuniffi_2024_11_poc.a --language swift --out-dir generated/uniffi
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s
     Running `target/debug/uniffi-bindgen generate --library ./target/aarch64-apple-ios-sim/debug/libuniffi_2024_11_poc.a --language swift --out-dir generated/uniffi`
Failed to extract data from archive member `uniffi_2024_11_poc-e43440458ed975e7.988kinrgws14mi3ykd8qm9h0t.rcgu.o`

Rust:

rustc --version
rustc 1.82.0 (f6e511eec 2024-10-15)

[bes]

Downgrading to 0.27.3 and compiling reveals that it doesn't like that the constructor returns Option<Arc<Self>>. Changing the constructor to return Arc<Self> instead fixes the problem.

Does NOT work on 0.27.3 or 0.28.3

    #[uniffi::constructor]
    pub fn new(
        id: Option<String>,
        id2: String,
        id3: Option<String>,
        loaded: bool,
    ) -> Option<Arc<Self>> {
        todo!();
    }

Works on both

    #[uniffi::constructor]
    pub fn new(
        id: Option<String>,
        id2: String,
        id3: Option<String>,
        loaded: bool,
    ) -> Arc<Self> {
        todo!();
    }

But at least 0.27.3 complains, which makes it possible to temp fix.

I would really like the constructor to be able to return Option :)

[bes]

Workaround

It's possible to work around by adding another layer of indirection, by storing the optional result in another object and then using a regular self-method to fetch it.

[bendk]

We've had a bunch of philosophical discussions over the years on what exactly a "constructor" is. IMO, a constructor should not be able to return None, since that means it didn't construct anything. On a practical level, I think what you want is:

• A better UniFFI error message (let's use this issue for that)
• Switching your code to a failible constructor: one that returns a Result where the error values represent failure to construct. See the discussion in Support constructors returning Option<>? #1882.
• Static method support (static method support #1074). In the meantime, you can define a top-level function that returns Option<YourClass>

[bes]

As long as the problem is solvable (easily solvable as soon as I understood the problem) I'm fine with whatever you propose.

I must say, I converted my code from swift-bridge to uniffi, and the experience was smooth, I converted a lot of code and it mostly just worked, with better interfaces (and safer to boot!). Well done!