Function fmt_diag 

pub fn fmt_diag<'a, T>(f: &mut dyn Write, container: &'a T) -> Result
where
    T: ErrorDiagnostic + AsError,
    ResultType: From<&'a T::Target>,

Formats a full diagnostic view of an error for logging and tracing.

For ServiceError types, this includes debug representations of all nested errors, and a backtrace when available.