It seems like it ought to be possible to use Iterator::map (and other iterator combinators) to avoid a lot of extra typing when defining a ToSocketAddrs impl, something like this:

use std::io; use std::iter; use std::net::{IpAddr, SocketAddr, ToSocketAddrs}; use std::range::{Range, RangeIter}; pub struct PortRange { addr: IpAddr, range: Range<u16>, } impl PortRange { pub fn new(addr: IpAddr, range: Range<u16>) -> PortRange { PortRange { addr, range } } } impl ToSocketAddrs for PortRange { type Iter = iter::Map<RangeIter<u16>, _>; // <-- problem here fn to_socket_addrs(&self) -> io::Result<Self::Iter> { Ok(self .range .iter() .map(|port| SocketAddr::new(self.addr, port))) } }

(Playground)

The problem is that the second type argument to iter::Map needs to be the type of the closure, or something close to it, and I haven't been able to find a way to name that type. If you compile the above as is you get

error[E0121]: the placeholder `_` is not allowed within types on item signatures for associated types

I tried changing the placeholder to FnMut(u16)->SocketAddr and impl FnMut(u16)->SocketAddr but those just gave me different errors.

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