Return a concrete type from Endpoint::watch_addr

[TheButlah]

Describe the bug
The current impl T on Endpoint::watch_addr makes it impossible to store the watcher in a struct. This is undesirable.,

Relevant Logs

Expected behavior
Name a concrete type.

Iroh

Version:
1.0.3

lolwut

• This issue was created by a human that thought critically about
  the issue being reported and wrote this as concisely and clearly
  as they could. Taking full responsibility for the issue being
  accurate.
• This issue isn't slop, and is backed up by evidence.

[flub]

I'm confused, there is no generic. But you probably would like an explicit struct rather than an -> impl Watcher and then you'd be able to store it.

But why can't you box it?

[TheButlah]

Currently an anonymous impl Watcher is returned instead of a generic or a concrete type, it's impossible to store it without a Box<dyn Watcher>. However, watcher is not dyn-compatible so it cannot be boxed in that way. The only solution is to spawn a new tokio task to drain the watcher and push the new addresses into a tokio::watch or similar, which is undesirable machinery just to get a concrete type.

Adjusting the function sig to use a concrete type that implements watcher (instead of an anonymous impl Watcher) would solve it. I believe that can be done in a semver-compatible way. If you want to safeguard against semver hazards, you can even newtype the underlying struct to avoid leaking any implementation details and firewall your api against future changes to the underlying watcher.

[flub]

fwiw this works and might be a short-term workaround:

#[tokio::main]
async fn main() -> Result<()> {
    tracing_subscriber::fmt::init();

    let ep = iroh::Endpoint::builder(presets::N0).bind().await?;
    let watcher = SomeStruct {
        addrs: ep.watch_addr(),
    };

    Ok(())
}

struct SomeStruct<T> {
    addrs: T,
}

[TheButlah]

Because the type cannot be named, I cannot make a function which returns a SomeStruct<impl Watcher> afaik

Edit: I guess it works? https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=713ccbf19fb327b9f648a4ae6751b702