Function unwrap_array_expr 

pub(crate) fn unwrap_array_expr<'tcx>(
    tcx: TyCtxt<'tcx>,
    def_id: DefId,
    expr: &Expr,
) -> Option<Ty<'tcx>>

Extract the inner type from an Expr::Array (the [T] notation in SplitTransmute([T], [U])), then resolve it via parse_type.