pub fn suggest_new_region_bound(
    tcx: TyCtxt<'_>,
    err: &mut DiagnosticBuilder<'_>,
    fn_returns: Vec<&Ty<'_>>,
    lifetime_name: String,
    arg: Option<String>,
    captures: String,
    param: Option<(Span, String)>,
    scope_def_id: Option<LocalDefId>
)