TryStream on rule get prevent the use of stream combinators

[Stormshield-robinc]

Hi,

I stumble on a annoying problem while trying to list rules for both ipv4 and ipv6.
I wanted to merge both requests into the same Stream with a futures::stream::select, but rust cannot figure out that the resulting Stream is also a TryStream

use futures::TryStreamExt;

pub async fn get_rules() {
    let (connection, handle, _) = rtnetlink::new_connection().unwrap();

    let rules4 = handle.rule().get(rtnetlink::IpVersion::V4).execute();
    let rules6 = handle.rule().get(rtnetlink::IpVersion::V6).execute();

    let mut rules = futures::stream::select(rules4, rules6);

    while let Some(rule) = rules.try_next().await.unwrap() {
    }
}

error[E0599]: the method `try_next` exists for struct `Select<impl TryStream<Ok = RuleMessage, Error = Error>, ...>`, but its trait bounds were not satisfied
  --> src/lib.rs:11:34
   |
11 |       while let Some(rule) = rules.try_next().await.unwrap() {
   |                                    ^^^^^^^^
   |
  ::: /home/Stormshield-robinc/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.31/src/stream/select.rs:8:1
   |
8  | / pin_project! {
9  | |     /// Stream for the [`select()`] function.
10 | |     #[derive(Debug)]
11 | |     #[must_use = "streams do nothing unless polled"]
...  |
15 | |     }
16 | | }
   | |_- doesn't satisfy `_: TryStreamExt` or `_: TryStream`
   |
   = note: the full type name has been written to '/home/Stormshield-robinc/bin/rust_bin/target/debug/deps/rust_bin-1fde713015a894f5.long-type-8387156802266828194.txt'
   = note: consider using `--verbose` to print the full type name to the console
   = note: the following trait bounds were not satisfied:
           `futures::stream::Select<impl TryStream<Ok = netlink_packet_route::rule::message::RuleMessage, Error = rtnetlink::Error>, impl TryStream<Ok = netlink_packet_route::rule::message::RuleMessage, Error = rtnetlink::Error>>: TryStream`
           which is required by `futures::stream::Select<impl TryStream<Ok = netlink_packet_route::rule::message::RuleMessage, Error = rtnetlink::Error>, impl TryStream<Ok = netlink_packet_route::rule::message::RuleMessage, Error = rtnetlink::Error>>: TryStreamExt`

Since the rule get signature is

pub fn execute(self) -> impl TryStream<Ok = RuleMessage, Error = Error>

it does not constrain the Stream::Item to a Result, which later on wont let rust understand that the Select is also compatbile with a TrySelect.

I think that changing the signature to Stream would fix this issue without breaking the interface. I can submit a PR if this is ok for you

[cathay4t]

I need proof on API stability on changing impl TryStream to impl Stream. Otherwise, please add new functions as:

impl RuleGetRequest {
    pub fn execute_stream(self) -> impl Stream<Item=Result<RuleMessage, Error>> {
    }
}

Another approach would be find a way to allow select on two TryStream, maybe asking in https://github.com/rust-lang/futures-rs ?