comprehensive-rust/src/chromium/interoperability-with-cpp/error-handling.md

# CXX error handling

CXX's [support for `Result<T,E>`][0] relies on C++ exceptions, so we can't use
that in Chromium. Alternatives:

* The `T` part of `Result<T, E>` can be:
    - Returned via out parameters (e.g. via `&mut T`).  This requires that `T`
      can be passed across the FFI boundary - for example `T` has to be:
      - A primitive type (like `u32` or `usize`)
      - A type natively supported by `cxx` (like `UniquePtr<T>`) that has a
        suitable default value to use in a failure case (*unlike* `Box<T>`).
    - Retained on the Rust side, and exposed via reference.  This may be needed
      when `T` is a Rust type, which cannot be passed across the FFI boundary,
      and cannot be stored in `UniquePtr<T>`.

* The `E` part of `Result<T, E>` can be:
    - Returned as a boolean (e.g. `true` representing success, and `false`
      representing failure)
    - Preserving error details is in theory possible, but so far hasn't been
      needed in practice.

[0]: https://cxx.rs/binding/result.html