Function report_msg 

fn report_msg<'tcx>(
    diag_level: DiagLevel,
    title: String,
    main_span_labels: Vec<String>,
    extras: Vec<(ExtraKind, Option<Span>, String)>,
    stacktrace: &[FrameInfo<'tcx>],
    thread: Option<ThreadId>,
    machine: &MiriMachine<'tcx>,
)

Report an error or note (depending on the error argument) with the given stacktrace. Also emits a full stacktrace of the interpreter stack. We want to present a multi-line span message for some errors. Diagnostics do not support this directly, so we pass the lines as a Vec<String> and display each line after the first with an additional span_label or note call.