Macro throw_span_err 

macro_rules! throw_span_err {
    ($span:expr, $msg:expr) => { ... };
    ($span:expr, $msg:expr, $f:expr) => { ... };
}

Emit a diagnostic on span $span with msg $msg (optionally performing additional decoration using the FnOnce passed in diag) and return Err(ErrorHandled).

For methods that return a Result<_, DiagnosticDeriveError>: