Fix parsing of IPV6 URLs in client example

Currently, when trying to run the client example with an IPV6 address
URL, such as by running:

    cargo run --example client https://[::1]:4433/Cargo.toml --host localhost

A "failed to lookup address information: Name or service not known"
error is raised. This is because `url.host_str()` is `"[::1]"`, which
is wrapped in brackets. These brackets, specified by by RFC 2732, are
part of the URL syntax, not the IP address syntax.

Although this code succeeds, because the standard library treats this
like a URL:

    use std::net::ToSocketAddrs;
    "[::1]:4433".to_socket_addrs()

This code does not:

    use std::net::Ipv6Addr;
    "[::1]".parse::<Ipv6Addr>()

As the stdlib expects to just receive "::1". Consequentially, this
does not succeed, counterintuitively:

    use std::net::ToSocketAddrs;
    ("[::1]", 4433).to_socket_addrs()

This code fixes the client example's URL parsing behavior by
stripping out such brackets in the same way as [is done in
tokio-tungstenite][1].

[1]: https://github.com/snapview/tokio-tungstenite/blob/052d085aff4708b924b92becaa83923b15045841/src/lib.rs#L404

quinn/examples/client.rs

@@ -63,7 +63,8 @@ fn main() {
 #[tokio::main]
 async fn run(options: Opt) -> Result<()> {
     let url = options.url;
-    let remote = (url.host_str().unwrap(), url.port().unwrap_or(4433))
+    let url_host = strip_ipv6_brackets(url.host_str().unwrap());
+    let remote = (url_host, url.port().unwrap_or(4433))
         .to_socket_addrs()?
         .next()
         .ok_or_else(|| anyhow!("couldn't resolve to an address"))?;
@@ -102,11 +103,7 @@ async fn run(options: Opt) -> Result<()> {
     let request = format!("GET {}\r\n", url.path());
     let start = Instant::now();
     let rebind = options.rebind;
-    let host = options
-        .host
-        .as_ref()
-        .map_or_else(|| url.host_str(), |x| Some(x))
-        .ok_or_else(|| anyhow!("no hostname specified"))?;
+    let host = options.host.as_deref().unwrap_or(url_host);
 
     eprintln!("connecting to {host} at {remote}");
     let conn = endpoint
@@ -153,6 +150,16 @@ async fn run(options: Opt) -> Result<()> {
     Ok(())
 }
 
+fn strip_ipv6_brackets(host: &str) -> &str {
+    // An ipv6 url looks like eg https://[::1]:4433/Cargo.toml, wherein the host [::1] is the
+    // ipv6 address ::1 wrapped in brackets, per RFC 2732. This strips those.
+    if host.starts_with('[') && host.ends_with(']') {
+        &host[1..host.len() - 1]
+    } else {
+        host
+    }
+}
+
 fn duration_secs(x: &Duration) -> f32 {
     x.as_secs() as f32 + x.subsec_nanos() as f32 * 1e-9
 }