rustc::traits::SelectionResult

type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;

Unstable (rustc_private)

When performing resolution, it is typically the case that there can be one of three outcomes:

• Ok(Some(r)): success occurred with result r
• Ok(None): could not definitely determine anything, usually due to inconclusive type inference.
• Err(e): error e occurred