Potential approach to support external errors

[Hinton]

This is a bit of a stab in the dark attempt at supporting external errors in uniffi.

It currently seems to behave well for swift but before I continue diving into the uniffi internals I would appreciate some guidance if this is the appropriate way to address this as the internals of uniffi is a large mystery to me.

Remaining tasks:

• Support kotlin
• Fix any broken tests.
• Write new tests

[mhammond]

I'm mildly surprised we don't support this, but it looks like we do not! A very quick look implies this seems sane for Swift as it's what we did for other uses for external types - however, we'd obviously want tests here. I'd suggest maybe reusing the ext-types fixture - eg, there's a GuidError in the guid crate which you might be able to have a top-level function throw.

[Hinton]

I wrote up a test case for the proc-macro which works as expected. Unfortunately I ran into a few issues with udl. With the latest commit I get the following error message:

error[E0277]: the trait bound `GuidError: LowerError<UniFfiTag>` is not satisfied
   --> /target/debug/build/uniffi-fixture-ext-types-dac13a3e9f364522/out/ext-types-lib.uniffi.rs:253:1
    |
253 | #[::uniffi::export_for_udl]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | |
    | the trait `LowerError<UniFfiTag>` is not implemented for `GuidError`, which is required by `Result<(), GuidError>: LowerReturn<UniFfiTag>`
    | in this procedural macro expansion
    |
   ::: /uniffi_macros/src/lib.rs:203:1
    |
203 | pub fn export_for_udl(attrs: TokenStream, input: TokenStream) -> TokenStream {
    | ---------------------------------------------------------------------------- in this expansion of `#[::uniffi::export_for_udl]`
    |
    = help: the trait `LowerError<ext_types_custom::UniFfiTag>` is implemented for `GuidError`
    = help: for that trait implementation, expected `ext_types_custom::UniFfiTag`, found `UniFfiTag`
    = note: required for `Result<(), GuidError>` to implement `LowerReturn<UniFfiTag>

My suspicion from debugging the expanded udl file is that it's missing a ffi_converter_forward for GuidError. Unfortunately I'm a bit outside my knowledge depth, but I suspect we need to wire up some logic in uniffi_udl/src/attributes.rs to ensure external errors are appropriately imported?

---

I also experimented slightly with Kotlin which in some ways seems to also be quite straightforward as we should only need to add an import to the Exception. Unfortunately my initial attempt of adding a add_import function call inside uniffi_bindgen/src/bindings/kotlin/templates/macros.kt failed with no method named `add_import` found for reference `&KotlinWrapper<'a>` in the current scope.

Curious if you have any quick thoughts on a better approach.

[mhammond]

My suspicion from debugging the expanded udl file is that it's missing a ffi_converter_forward for GuidError

Yeah - I suspect you need ::uniffi::ffi_converter_forward!(GuidError, ext_types_custom::UniFfiTag, crate::UniFfiTag); where it's imported.

uniffi_bindgen/src/bindings/kotlin/templates/macros.kt failed with no method named `add_import` found for reference `&KotlinWrapper<'a>` in the current scope.

The templating system is tricky, but that should work if you move the call out of macros.kt

A quick look though shows you seem to be on the right track!