After reading the changelog for the 0.30.0 release I tried to clean up a long standing issue of ours in our uniffi codebase, errors. Enabling the Display trait is a great feature because we can then expose both the fields important to the errors as well as provide a clear message back to the developers reading these errors/exceptions (we previously had to choose between having the field or having the relevant message).

However there appears to be a bug on the Kotlin side for exporting Display. I'm opening this issue to report it.

Consider the following 3 examples:

#[derive(uniffi::Error, Debug)]
#[uniffi::export(Display)]
pub enum Example1Enum {
    Blue { id: u32 },
    Green { issue: String },
}

impl Display for Example1Enum {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Display for Example1Enum {:?}", self)
    }
}

#[derive(uniffi::Enum, Debug)]
#[uniffi::export(Display)]
pub enum Example2Enum {
    Blue,
    Green,
}

impl Display for Example2Enum {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Display for Example2Enum {:?}", self)
    }
}

#[derive(uniffi::Enum, Debug)]
#[uniffi::export(Display)]
pub enum Example3Enum {
    Blue { id: u32 },
    Green { issue: String },
}

impl Display for Example3Enum {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Display for Example3Enum {:?}", self)
    }
}

All three enums will produce the description field in Swift with the comment // The local Rust Display implementation.

Only the 3rd example, which is expressed as a Sealed Class in Kotlin, will get a toString() method implemented. The uniffi::Error and the simple Kotlin enum will not.

Note that in Python it's also a bit different than both Kotlin and Swift:

• Example1Enum (the error) gets a __repr__ method but no __str__
• Example2Enum (the straight enum) gets nothing (like Kotlin)
• Example3Enum gets a __str__ method