How can I coerce a String into a &str for the purposes of implementing the ToSocketAddrs trait?

Question

When a string is stored as a String rather than a &str it fails to implement the trait ToSocketAddrs. The closest possible one that does is impl<'a> ToSocketAddrs for (&'a str, u16).

use std::net::TcpStream;

fn main() {
    let url = "www.google.com".to_string(); // String
    let url2 = "www.google.com";            // &'static str
    let port = 80;

    // Does not work
    let tcp = TcpStream::connect((url, port));

    // Works
    let tcp2 = TcpStream::connect((url2, port));
}

This fails with:

error[E0277]: the trait bound `(std::string::String, {integer}): std::net::ToSocketAddrs` is not satisfied
 --> src/main.rs:9:11
  |
9 | let tcp = TcpStream::connect((url, port));
  |           ^^^^^^^^^^^^^^^^^^ the trait `std::net::ToSocketAddrs` is not implemented for `(std::string::String, {integer})`
  |
  = help: the following implementations were found:
            <(std::net::Ipv6Addr, u16) as std::net::ToSocketAddrs>
            <(std::net::Ipv4Addr, u16) as std::net::ToSocketAddrs>
            <(&'a str, u16) as std::net::ToSocketAddrs>
            <(std::net::IpAddr, u16) as std::net::ToSocketAddrs>
  = note: required by `std::net::TcpStream::connect`

How can I coerce a String into a &str for the purposes of implementing the ToSocketAddrs trait? From the documentation for Rust 1.0, I thought that String would automatically move to &str.

Answer 1

hauleth's answer will work for you, but it's not quite the whole story. In particular, your String isn't coercing to a &str because auto deref coercion does not kick in when trait matching, as per RFC 0401. Therefore, using a plain &url won't work in this case because auto deref won't be applied to it. Instead of getting an &str, you'll just get a &String, which doesn't have a matching impl for ToSocketAddrs. However, you can explicitly cause deref to happen with the dereference operator. In particular, &*url should work. (&url[..] also works because [..] is the syntax for "take a slice over everything", but it's a bit more verbose.)

Answer 2

As std::net::ToSocketAddrs is implemented only on (&str, _) you need to take a slice of of the string using the &url[..] slice syntax:

let tcp = TcpStream::connect((&url[..], port));