url::Url example says the trait `FfiConverterArc<UniFfiTag>` is not implemented for `url::Url` | about uniffi-rs

[mhammond]

You don't want Arc<Url>, just plain Url - Arc<> is only used when passing objects/interfaces, but in this example it's a plain old string.

[linabutler]

Hi @KronicDeth, thanks for the issue! It's the Arc that's tripping it up: Socket::url returns an Arc<Url>, but UniffiCustomTypeConverter is implemented for Url, not Arc<Url>.

If you change Socket::url to return a Url:

/// The `url` passed to [Socket::spawn]
pub fn url(&self) -> Url {
    (*self.url).clone()
}

I think it should work!

Gory details: I don't think it's possible to safely implement UniffiCustomTypeConverter for an Arc<T>. An Arc is a pointer to a specific location in memory, so the type converter would need to roundtrip that pointer through foreign code—not just the value it points to. UniFFI does this in its generated bindings for interface / object types, using the unsafe Arc::{into_raw, from_raw} on the Rust side, and a wrapper class on the foreign side, but we can't do that in a safe and generic way for custom types.

[linabutler]

Oops, sorry, I saw @mhammond already replied as I was typing my comment! 😅

[KronicDeth]

@linabutler the pre-uniffi code returned Arc<Url> to try to be memory efficient, but you and @mhammond make a good point that it has to make a String over the FFI even if we use java.net.Url in Kotlin or URL in Swift, so it really doesn't matter being efficient with Arc<Url>.