External trait interface isn't compiled (UDL mode)

[Voronar]

I'm trying to reuse uniffi trait interface (https://mozilla.github.io/uniffi-rs/foreign_traits.html) from an external crate living in my own workspace.

main_lib:

use std::sync::Arc;
use shared_lib::*;

pub fn use_common_trait(common_trait: Arc<dyn CommonTrait>) {
    common_trait.foo();
}

uniffi::include_scaffolding!("main_lib");

main_lib UDL:

namespace main_lib {
  void use_common_trait(CommonTrait common_trait);
};

[ExternalInterface="shared_lib"]
typedef extern CommonTrait;

shared_lib:

pub trait CommonTrait: Send + Sync {
    fn foo(&self);
}

uniffi::include_scaffolding!("shared_lib");

shared_lib UDL:

namespace shared_lib {};

[Trait]
interface CommonTrait {
    void foo();
};

Error:

error[E0782]: trait objects must include the `dyn` keyword
  --> uniffi_external_trait_interface/target/debug/build/main_lib-5098ef40e334e5d0/out/main_lib.uniffi.rs:34:38
   |
34 |     r#common_trait: ::std::sync::Arc<r#CommonTrait>,
   |                                      ^^^^^^^^^^^^^
   |
help: add `dyn` keyword before this trait
   |
34 |     r#common_trait: ::std::sync::Arc<dyn r#CommonTrait>,
   |                                      +++

error[E0782]: trait objects must include the `dyn` keyword
  --> uniffi_external_trait_interface/target/debug/build/main_lib-5098ef40e334e5d0/out/main_lib.uniffi.rs:53:38
   |
53 | ::uniffi::ffi_converter_arc_forward!(r#CommonTrait, ::r#shared_lib::UniFfiTag, crate::UniFfiTag);
   |                                      ^^^^^^^^^^^^^
   |
help: add `dyn` keyword before this trait
   |
53 | ::uniffi::ffi_converter_arc_forward!(dyn r#CommonTrait, ::r#shared_lib::UniFfiTag, crate::UniFfiTag);

Reproducible example: https://github.com/Voronar/uniffi_external_trait_interface
uniffi version: 0.25

[badboy]

Can you provide the actual error you're running into?

[Voronar]

As we see the dyn somehow isn't generated during codegen phase in case of external trait interface type.

[Voronar]

Maybe the problem lies here https://github.com/mozilla/uniffi-rs/blob/main/uniffi_bindgen/src/scaffolding/mod.rs#L66. In case of the external type it generates just Arc<T> without dyn modifier.

[mhammond]

yeah - the problem here is that the trait is "external". I don't think an external type even knows if it is a trait or not - it probably needs to though.

[mhammond]

this should actually work ok using procmacros I think. There are a few options for UDL though, but all have some work.

[Voronar]

It compiles with proc-macros, but crashes at runtime for example with Kotlin script test:

thread '<unnamed>' panicked at /Users/user/.cargo/git/checkouts/uniffi-rs-6f89edd2a1ffa4bd/659d5d6/uniffi_core/src/ffi/foreigncallbacks.rs:102:1:
Bug: callback not set.  This is likely a uniffi bug.

Reproducible example: https://github.com/Voronar/uniffi_external_trait_interface/tree/external_trait_proc_macro_kotlin_script_crash

[mhammond]

The problem here is still UDL - main_lib's UDL can't reference the trait - so if you remove the trait and function from the UDL and use #[uniffi::export] on the function in lib.rs things seem to work. It would be possible to make UDL work here but it would probably require a contributor to implement it.

[mhammond]

See also #1854 and #1855

[Voronar]

The problem here is still UDL...

I made reproducible example (mentioned in my previous message) without any UDL and it crashes at runtime (at least on my machine).

Reproducible example: https://github.com/Voronar/uniffi_external_trait_interface/tree/external_trait_proc_macro_kotlin_script_crash