if_chain! {
    if let ExprKind::Repeat(value, length) = expr.kind;
    if let ExprKind::Lit(ref lit) = value.kind;
    if let LitKind::Int(1, LitIntType::Unsigned(UintTy::U8)) = lit.node;
    let expr1 = &cx.tcx.hir().body(length.body).value;
    if let ExprKind::Lit(ref lit1) = expr1.kind;
    if let LitKind::Int(5, LitIntType::Unsuffixed) = lit1.node;
    then {
        // report your lint here
    }
}