gistfile1.txt

// Have one error class that exposed by UniFFI, defined in your `.udl` file
// and with a correspondong struct definition in `lib.rs`. It's flat, because
// UniFFI doesn't pass nice nested structure in errors (yet).
//
// Instances of this error will only ever appear on the boundary of your API,
// where they get handed off to the foreign-language code.
//
pub enum LoginsError {
    UnexpectedLoginsStorageError(String),
    SyncAuthInvalid(String),
    MismatchedLock(&'static str),
    // A separate case for each invalid login reason.
    InvalidRecordEmptyOrigin(String),
    InvalidRecordEmptyPassword(String),
    // ...etc...
 }
 
 
// Have a separate Error class for internal use with nice rich structure.
// Doesn't actually have to be called "internal" :-)
//
pub enum InternalLoginsError {
    UnexpectedLoginsStorageError(String),
    SyncAuthInvalid(String),
    MismatchedLock(&'static str),
    // InvalidLoginReason can be a nested structure here,
    // since that's nicer to work with in the Rust code.
    InvalidRecord(String, InvalidLoginReason),
 }
 
 
// Make a `From` impl to convert the rich internal errors to pass over the FFI.
//
impl From<InternalLoginsError> for LoginsError {
  fn from(e: LoginsStorageError) -> Self {
    // The logic here matches the `get_code` logic from the hand-written bindings,
    // recursing into the `InvalidRecord()` options and flattening them out.
  }
}

// Any public API functions should throw a `LoginsError`.
// I'm pretty sure that the `?` operator will automatically find and use the `From` impl
// to do the conversion.

// On the Kotlin side, you may like to do the reverse of your `From` impl,
// and map the flat list of errors back into a richer structured error.
// That would have to be all in hand-written Kotlin code for now though.

The .udl for the error would then need to look like:

[Error]
enum LoginsError {
          "UnexpectedLoginsStorageError",
          "SyncAuthInvalid",
          "MismatchedLock",
          "InvalidRecordEmptyOrigin"
          "InvalidRecordEmptyPassword",
          // ..etc...
}