refactor(hickory-dns): async new_resolver (#84)

examples/headers_order.rs

@@ -7,7 +7,11 @@ async fn main() -> Result<(), Box<dyn Error>> {
     // Build a client to mimic Edge127
     let client = rquest::Client::builder()
         .impersonate(Impersonate::Edge127)
-        .header_order(vec![header::HOST, HeaderName::from_static("priority"), header::COOKIE])
+        .header_order(vec![
+            header::HOST,
+            HeaderName::from_static("priority"),
+            header::COOKIE,
+        ])
         .build()?;
 
     // Use the API you're already familiar with

src/blocking/client.rs

@@ -207,7 +207,7 @@ impl ClientBuilder {
     ///
     /// The host header needs to be manually inserted if you want to modify its order.
     /// Otherwise it will be inserted by hyper after sorting.
-    pub fn header_order(mut self, order: Vec<HeaderName>) -> ClientBuilder {
+    pub fn header_order(self, order: Vec<HeaderName>) -> ClientBuilder {
         self.with_inner(|inner| inner.header_order(order))
     }
 

src/dns/hickory.rs

@@ -24,10 +24,7 @@ impl Resolve for HickoryDnsResolver {
     fn resolve(&self, name: Name) -> Resolving {
         let resolver = self.clone();
         Box::pin(async move {
-            let resolver = resolver
-                .state
-                .get_or_try_init(|| async { new_resolver() })
-                .await?;
+            let resolver = resolver.state.get_or_try_init(new_resolver).await?;
 
             let lookup = resolver.lookup_ip(name.as_str()).await?;
             let addrs: Addrs = Box::new(SocketAddrs {
@@ -48,7 +45,7 @@ impl Iterator for SocketAddrs {
 
 /// Create a new resolver with the default configuration,
 /// which reads from `/etc/resolve.conf`.
-fn new_resolver() -> io::Result<TokioAsyncResolver> {
+async fn new_resolver() -> io::Result<TokioAsyncResolver> {
     let (config, opts) = system_conf::read_system_conf().map_err(|e| {
         io::Error::new(
             io::ErrorKind::Other,